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galaxy evolution

GRAZING ENCOUNTER COLLIDING GALAXIES
The galaxies NGC 2207 and
IC 2163 have become distorted by Relative to their size, galaxies are packed much more tightly
a close encounter. NGC 2207, on than are stars, so violent encounters between them are common.
the left, has torn a long streamer During collisions, the interacting galaxies’ gas clouds are forced
of stars from the smaller galaxy. together, triggering bursts of star formation.The clouds can be
heated so much that they “boil” away from the galaxy in a
Violent events such as this process called pressure stripping. Stars themselves rarely collide.
are a key influence on They are so small relative to their spacing that they pass like the
the evolution of members of marching bands.Whole spiral arms, however, can be
galaxies. flung free of the galaxy by gravity. Computer models of galaxy

collisions have recently been combined with observations
of the early Universe, during which collisions were even
more frequent.This research suggests that galaxy
interactions are responsible for changing galaxies
from one type to another over billions of years.

HIDDEN ACTIVITY
The peculiar double-lobed galaxy Arp 220 is host to a
burst of star formation triggered by an intergalactic
collision, only visible in infrared. Many more such
luminous infrared galaxies are now coming to light,
suggesting that galaxy collisions may be even more
frequent than was previously thought.

dense background gas

EVOLUTION

BY MERGING

The most popular model of

galaxy evolution today suggests

galaxies change through a series

of collisions and interactions.

A key element of the theory

is that the free hydrogen in SPIRAL GALAXY (NGC 3370)

intergalactic space is steadily diminishing spiral galaxies spiral galaxy
as it is absorbed into galaxies. In the merge to form develops from
early, gas-rich era, the first galaxies to elliptical galaxy elliptical (or small
irregular) galaxy

form were small irregulars or ellipticals.

As these pulled in more material and background gas BEYOND THE MILKY WAY

sometimes merged, they developed more new disc forming
internal structure, becoming the first from in-falling gas
spiral galaxies.When spirals merge, the

gas from their discs is stripped away and rejuvenated spiral DUSTY ELLIPTICAL (NGC 1316)
their stars are thrown into chaotic orbits galaxy with star
– they become elliptical galaxies. If there birth in spiral arms

is enough intergalactic hydrogen, it falls

into the elliptical galaxy, forming a new

disc of gassy and dusty material that may sparse background gas THE MERGER MODEL
According to recent thinking, galaxy
eventually develop its own spiral arms. mergers tend to form ellipticals, but
in-falling gas can rejuvenate them,
By this time, the only stars remaining from the original spiral forming new discs and star-forming
spiral arms. The way a galaxy develops
are the old red and yellow stars, which explains the dominance depends on the time between mergers,
and the amount of background gas.
of old red and yellow stars in the hubs of old spirals. In regions There is a general trend towards large,
gas-poor elliptical galaxies.
where many such events have happened (for instance, within

old, dense galaxy clusters), the available free gas is reduced until

eventually the galaxies are all ellipticals, merging occasionally to

form ever-larger systems such as cD galaxies (see p.316). ELLIPTICAL GALAXY (M87)

GALAXIES

Astronomers are drawn naturally to the brightest, the most beautiful, and the most intriguing

galaxies. However, of the 100 billion galaxies in the observable Universe, only a minority are

spectacular spirals and giant ellipticals. Astronomers are beginning to understand that most galaxies

are relatively small and faint – diffuse balls and irregular clouds of stars.The faintest and commonest

BIG AND BRIGHT galaxies are dwarf ellipticals, which are like oversized globular star
Spirals such as Bode’s Galaxy, M81, may be
clusters of only a few million stars.These feeble galaxies are visible

the most attractive type of galaxy, but they only if they lie nearby in intergalactic terms.The most brilliant
are far from the most common. Making up are the giant ellipticals, which can be 20 times as luminous as
less than 30 per cent of all galaxies, they are the Milky Way.
outnumbered by smaller, fainter galaxies.

DWARF ELLIPTICAL GALAXY galactic neighbour. It was not found STAR DENSITY
until 1994 and was supplanted only SagDEG’s existence came
SagDEG by the discovery of the even closer to light only when a
Canis Major Dwarf in 2004. SagDEG survey of Sagittarius found
SAGITTARIUS CATALOGUE NUMBER remained hidden for so long because, regions of increased star
like all dwarf ellipticals, it is a very density – the bright
None faint scattering of stars. It is also well patches in this image.
disguised by its position behind the
DISTANCE great Sagittarius star clouds that mark SagDEG’s existence so
our galaxy’s centre. SagDEG is small close to our galaxy is
88,000 light-years and obscure, but it has at least four a puzzle. It orbits the
orbiting globular clusters, which are Milky Way in less than
DIAMETER brighter and more obvious. One of a billion years and so must have gone
these, M54, was discovered by Charles through several close encounters that
10,000 light-years Messier more than 200 years before should have ripped it apart and
the parent galaxy was found. scattered its stars through the galactic
MAGNITUDE 7.6 for M54
star cluster in SagDEG

The Sagittarius Dwarf Elliptical halo. It has survived only due to
Galaxy, often called SagDEG, was a large amount of dark matter,
until recently our closest known producing more gravity than
SagDEG’s visible stars.

IRREGULAR GALAXY CLOUD DETAILS
In this image, the LMC’s main “bar”
Large Magellanic appears as the yellowish star cloud on
Cloud the left, outshone by the pink star-birth
regions of the Tarantula Nebula.
CATALOGUE NUMBER
to be a distinctive, isolated region
None of the Milky Way, some 10 degrees
across, with its own areas of
DISTANCE nebulosity and star clusters.

179,000 light-years The LMC is in fact an
irregular galaxy, orbiting the
DIAMETER Milky Way roughly once every
1.5 billion years on a path that
20,000 light-years brought it to within 120,000
light-years of our galaxy at
MAGNITUDE 0.1 its closest approach around
250 million years ago. Although
DORADO the LMC is irregular and is
being distorted by the gravity
The Large Magellanic Cloud (LMC) of the Milky Way, it shows some
bears the name of 16th-century signs of basic structure. Many
explorer Ferdinand Magellan (see of its stars are concentrated in a
panel, opposite). However, cultures central bar-like nucleus, curved
native to the southern hemisphere at one end. Some astronomers
have recognized its existence since have likened the LMC to a
prehistoric times. Like its smaller barred spiral with just one arm.
counterpart, the Small Magellanic
Cloud, the LMC appears from Earth Like all irregular galaxies, the
LMC is rich in gas, dust, and
BEYOND THE MILKY WAY RADIO MAP young stars, including some of the
This false-colour radio image of the LMC is largest known regions of star birth.
centred on the Tarantula Nebula. It shows One such region is the magnificent
intense radiation as red and black, indicating Tarantula Nebula, also known as
ionized hydrogen and star formation. 30 Doradus. It is so brilliant that,
if transported to the location of the
Orion Nebula (see p.239) – only
1,500 light-years away in the Milky
Way – it would be bright enough
to cast shadows on Earth at night.

In recent times, the LMC was host
to the only bright supernova since the
invention of the telescope. Supernova
1987A (see p.262) was observed by
astronomers around the world both
during and after its explosion, and it
has taught astronomers a lot about
the final stages of the stellar life cycle.

galaxies

IRREGULAR GALAXY EXPLORING SPACE

Small Magellanic MAGELLAN’S
Cloud DISCOVERY

CATALOGUE NUMBER The southernmost sky was not
visible to Europeans until they
NGC 292 visited the southern hemisphere.
The Portuguese explorer Ferdinand
DISTANCE Magellan was among the first to
do so during his round-the-world
210,000 light-years voyage of 1519–21. He was the
first European to record two
DIAMETER isolated patches of the Milky Way,
which were later named after him.
10,000 light-years
FERDINAND MAGELLAN
MAGNITUDE 2.3

TUCANA

Like the Large Magellanic Cloud, the is far from CLOUD OF STARS
Small Magellanic Cloud (SMC) is proven. It has The SMC forms a distinctive wedge-shaped
an irregular galaxy in orbit around one known globular cloud in southern skies. The pinkish areas in
the Milky Way. It was in the SMC cluster in orbit, but the this optical photograph show the galaxy’s
that Henrietta Leavitt discovered the SMC lies deceptively close in major star-forming regions.
Cepheid variable stars that were to the sky to one of the Milky Way’s
unlock the secrets of the galactic largest globulars – 47 Tucanae. previous encounters.This “Magellanic
distance scale (see pp.278, 340).Thanks Stream” has allowed astronomers to
to her discovery, astronomers know Both the Magellanic Clouds are trace and refine their models for the
that the SMC is both more distant ultimately doomed to be torn to orbits of the clouds.
and genuinely smaller than the LMC, shreds and absorbed into our own
with around one-tenth of the larger galaxy.They have survived several
cloud’s mass. Like the LMC, the small close passes of the Milky Way, but
cloud is also undergoing intense star now share their orbit with a trail of
formation. Some astronomers argue gas, dust, and stars torn away during
that the SMC also shows signs of a
central bar-like structure, but the case

SUPERNOVA ECHO Sc SPIRAL GALAXY FLOCCULENT SPIRAL
This unique image shows reflected M33 is an example of
light from the LMC’s supernova of Triangulum Galaxy a flocculent spiral – a
1987. The rings are created by light galaxy with arms that
from the explosion bouncing off sheets CATALOGUE NUMBERS divide like split ends
of dust close to the supernova and and separate into
being deflected onto a path towards M33, NGC 598 patches. The clumpy
Earth. The reflected light reaches us star clouds are
years after light that took the direct DISTANCE thought to form due
route. The “light echo” was made to localized changes
visible only by subtracting an image 3 million light-years in density.
of the region before the supernova
from a recent picture. Unchanged DIAMETER the larger and brighter Andromeda
stars therefore appear black. Galaxy (M31), and the two lie close to
50,000 light-years each other in the sky. M33 is affected
TARANTULA NEBULA by its larger neighbour’s gravity, and it
Massive stars run through their MAGNITUDE 5.7 may even be in a long, slow orbit
entire life cycle inside the stellar around the giant Andromeda spiral.
nursery of the Tarantula Nebula. TRIANGULUM
This image shows a new open Seen from Earth, M33 is fainter
cluster, Hodge 301, whose After the Andromeda Galaxy and the and more diffuse than M31 – partly
biggest stars have already gone Milky Way, the Triangulum Galaxy because it is closer to face-on than
supernova. As the shock waves (M33) is the third major member
spread across space, they ripple of the Local Group of galaxies. edge-on, and partly because it really
the nearby gas clouds, triggering It is slightly more is less spectacular. However, the
further star formation. distant than Triangulum Galaxy is more
typical of spiral galaxies than
its unusually bright BEYOND THE MILKY WAY
companions. As with
several Local Group
galaxies, M33 is large
and bright enough in
the sky for its features
to be catalogued, and
several of them have
NGC numbers. Most
prominent is the star-
forming region NGC
604, the largest emission
nebula known. At 1,500
light-years across, it dwarfs
anything in our own galaxy.

NEBULA NGC 604
This emission nebula’s gas glows as it is
excited by ultraviolet light from a central
star cluster. The stars are so massive and
bright that they emit most of their light
in ultraviolet, and so are not prominent in
visible-light photographs such as this.

BEYOND THE MILKY WAY Sb SPIRAL GALAXY astronomers as typical a
thought that M31 and spiral galaxy as it
Andromeda Galaxy other “spiral nebulae” appears. For example,
might be solar systems in the despite its huge size, it
CATALOGUE NUMBERS process of formation, while others appears to be less massive
guessed rightly that they were than the Milky Way, with a sparse
M31, NGC 224 independent systems of many stars. halo of dark matter. Despite this,
It was in the early 20th century that astrophysicists calculate that M31’s
DISTANCE Edwin Hubble (see p.43) revealed the central black hole has the mass of 30
true nature of M31, at a stroke hugely million Suns, almost ten times more
2.5 million light-years increasing estimates of the size of than the Milky Way’s central black
the Universe (see panel, opposite). hole.The huge mass of M31’s black
DIAMETER Astronomers now know that M31, hole is surprising, because a galaxy’s
like the Milky Way, is a huge galaxy black hole is thought generally to
250,000 light-years attended by a cluster of smaller reflect the mass of its parent galaxy.
MAGNITUDE 3.4 orbiting galaxies, which occasionally Furthermore, studies at different
ANDROMEDA fall inwards under M31’s gravity wavelengths have revealed disruption
and are torn apart. in the galaxy’s disc, possibly caused by
The Andromeda Galaxy (M31) is an encounter with one of its satellite
the closest major galaxy to the Milky Despite being intensively studied, galaxies in the past few million years.
Way and the largest member of the the Andromeda Galaxy still holds
Local Group of galaxies. Its disc is many mysteries, and it may not be M31 and the Milky Way are
twice as wide as our galaxy’s. moving towards each other, and they
should collide and begin to coalesce
M31’s brightness and size mean it in around 5 billion years’ time.
has been studied for longer than any
other galaxy. First identified as a “little CENTRAL BLACK HOLE
cloud” by Persian astronomer Al-Sufi This X-ray image shows M31’s central black
(see p.405) in the tenth century, it was hole as a blue dot – it is cool and inactive
for centuries assumed to be a nebula, compared to the galaxy’s other X-ray sources
at a similar distance to other objects (yellow dots).
in the sky. Improved telescopes
revealed that this “nebula”, like many
others, had a spiral structure. Some

DOUBLE CORE
Analysis of optical images
(above) reveals that M31
has two cores. The galaxy
may have collided with a
smaller galaxy millions of
years ago. The double core
could be evidence of the
smaller galaxy.

GALACTIC NEIGHBOURS
Dark dust lanes are silhouetted against
glowing gas and stars in this view of the
Andromeda Galaxy and its two close
companions, the dwarf elliptical galaxies
M32 (upper left) and M110 (bottom).

EXPLORING SPACE a Cepheid BEYOND THE MILKY WAY
variable star
INTERGALACTIC DISTANCE at its faintest

The study of M31 played a key role in the discovery the same star
that galaxies exist beyond our own. Although the at its brightest
spectra of galaxies suggested they shone with the
light of countless stars, no one could measure their
immense distance. In 1923, Edwin
Hubble (see p.43) proved that M31
lay outside our galaxy. He found the
true distance of M31 by calculating
the luminosities of its Cepheid
variable stars (see pp.278–79), and
relating their true brightness to
their apparent magnitude.

galaxies

Sb SPIRAL GALAXY core

Bode’s Galaxy X-RAY SOURCES
A Chandra X-ray image shows a
CATALOGUE NUMBERS strong X-ray source at the galaxy’s
core, surrounded by smaller sources,
M81, NGC 3031 probably X-ray binary stars.

DISTANCE millions of years.The near miss
created tidal forces that enhanced the
10.5 million light-years density waves (see p.295) in M81.
The rate of star birth around the
DIAMETER density waves increased, highlighting
the spiral arms. A long, straight dust
95,000 light-years lane along one side of the core could

MAGNITUDE 6.9 also have been created
in the encounter.
URSA MAJOR
By measuring the
Bode’s Galaxy, also known as M81, Doppler shifts of light
is one of the brightest spiral galaxies from either side of the
visible from the northern hemisphere. core, astronomers have
It is the dominant member of a galaxy found that the outer
group lying near to the Local Group. regions rotate more
The galaxy is named after Johann slowly than in most
Elert Bode, a German astronomer galaxies.This suggests
who found it in 1774. that M81 has little of
the dark matter that
Bode’s Galaxy has creates higher rotation
had a close encounter rates in other galaxies.
with M82, the Cigar
Galaxy (see below), in
the past few tens of

CLUSTERS REVEALED M81’S COLOURS
This combined visible and An enhanced image emphasizes the colour
ultraviolet image shows difference between the old red and yellow
the hottest and brightest stars of Bode’s Galaxy’s core and the young
star clusters (blue and blue stars of the disc and spiral arms.
white blobs), lying in the
core and spiral arms.

IRREGULAR DISC GALAXY Sb SPIRAL GALAXY

Cigar Galaxy Black Eye Galaxy

CATALOGUE NUMBERS CATALOGUE NUMBERS

M82, NGC 3034 M64, NGC 4826

DISTANCE DISTANCE

12 million light-years 19 million light-years

DIAMETER DIAMETER

40,000 light-years 51,000 light-years

MAGNITUDE 8.9 MAGNITUDE 8.5

URSA MAJOR COMA BERENICES

BEYOND THE MILKY WAY The brightest and most spectacular This distinctive galaxy has a dark dust
example of a “starburst galaxy”, the lane, running in front of its core, from
Cigar Galaxy (M82) is an irregularly which it gets its name.The dust lane
shaped cloud of stars that looks like a is unusual because it arcs above the
cigar from Earth. It is undergoing a galaxy’s core in an orbit of its own.
period of intense star birth as a result Because it has not yet settled into the
of a close encounter with Bode’s plane of the galaxy’s rotation, it must
Galaxy (M81).The near miss has have a recent origin and probably
disrupted the galaxy’s centre, creating dates from the galaxy’s absorption of a
smaller galaxy that strayed too close.
the dark dust lanes that obscure Another bizarre feature of the Black
much of the core and triggering Eye Galaxy is that its outer regions
the creation of many massive, are rotating in the opposite direction
brilliant star clusters in an to the inner regions.This could be
area a few thousand light- another effect of the collision.
years across. At infrared
GAS STREAMERS wavelengths, M82 is the
M82’s most spectacular brightest galaxy in the sky,
features can be observed and it is also a strong radio
only at the radio wavelength source.The infrared light
emitted by ionized hydrogen, comes from disturbed gas
here represented as and dust around the core.
magenta. This wavelength
reveals a huge envelope of X-RAY VIEW STARBURST GALAXY M64’S CENTRAL REGION AND DUST LANE
gas above and below the The intense activity in M82’s
core, blown out in long cluster of active core is luminous at optical and
streamers by fierce radiation black holes X-ray wavelengths. The young
from the central star clusters. stars illuminate the nebulae
OPTICAL with visible light, while those
IMAGE that have rapidly completed
their life cycle form active
black holes, emitting X-rays.

galaxies

Sc SPIRAL AND IRREGULAR GALAXIES LIGHT INTENSITY WILLIAM PARSONS
Plotting the intensity of light
Whirlpool Galaxy from different regions of M51 William Parsons (1800-67) was
reveals the brightness of the an Irish nobleman who used his
CATALOGUE NUMBERS two galactic cores (the twin great wealth to build the largest
peaks on the graph). telescope of his time and made the
M51, NGC 5194, first detailed studies of nebulae. In
NGC 5195 1845, he made detailed drawings
and noticed the spiral structure of
DISTANCE NGC 5195 NGC 5194 some “nebulae”, as galaxies were
thought to be at the time.This was
31 million light-years an important step to discovering

DIAMETER foreground that galaxies were
star not nebulae
100,000 light-years but separate
star systems.
MAGNITUDE 8.4
PARSONS’S
CANES VENATICI SKETCH
OF M51
Discovered by Charles Messier (see THE MISSING LINK brighter overall because of the large
p.69) in 1773, the Whirlpool Galaxy Combining a visible-light image of M51 young star clusters in its spiral arms.
is now known to be a pair of galaxies (green) with one in hydrogen-alpha radio It is thought to be the dominant
that is interacting – the brightest waves (blue) reveals the trail of gas member of a small group of galaxies,
and clearest example of such a pair linking the two galaxies. called simply the M51 group, which
visible from Earth.The individual also includes the galaxy M63.
components are a spiral galaxy viewed undergoing a burst of star formation,
face-on (NGC 5194) and a smaller which explains its unusual brightness,
irregular galaxy (NGC 5195). In while NGC 5194’s core is also much
visible light, the connection between brighter than expected. It is even
them cannot be seen, but images at classified by some astronomers as an
other wavelengths reveal an envelope active Seyfert Galaxy (see p.310).
of gas connecting the two. One effect
of the interaction is to enhance the The Whirlpool is very bright
density wave in the larger galaxy, despite its distance, indicating that
triggering increased star formation it is large and luminous – it
and making its spiral arms stand out is similar in size to the
very clearly.The Whirlpool was in Milky Way, but
fact the first “nebula” in which spiral
structure was recognized, by William
Parsons (see panel, right).

The interaction has also triggered
increased activity in the cores of both
of the galaxies – NGC 5195 is

LUMINOUS WHIRLPOOL BEYOND THE MILKY WAY
This Hubble image combines data from
different filters to reveal detail in M51,
such as dark dust behind each spiral arm
and bright pink regions of star birth.

galaxies

Sc SPIRAL GALAXY RELATIVE RED SHIFT
This computer image shows
Pinwheel Galaxy the red shift and blue shift
of objects within M101,
CATALOGUE NUMBERS revealing its rotation.
Yellow and red regions are
M101, NGC 5457 moving away, green and
blue parts are approaching.
DISTANCE
lopsided, spiral-arm
27 million light-years system, giving the
appearance that the core is offset
DIAMETER from the galaxy’s true centre. M101
is one of the largest spirals known –
170,000 light-years its visible diameter is more than
twice that of our own galaxy. Its large
MAGNITUDE 7.9 angular size in the sky (larger than the
size of the full Moon) makes it one
URSA MAJOR of the few galaxies whose individual
regions can be isolated for study.
Catalogued by Charles Messier (see
p.69) as M101, the Pinwheel Galaxy ASYMMETRICAL DISC
is a bright, nearby spiral galaxy, but M101’s lopsided shape is thought to be
one that reveals its nature only when caused by uneven distribution of mass
studied with powerful telescopes or in the disc affecting the orbit of its stars.
seen on long-exposure photographs.
Because it lies face-on to Earth, most
of the Pinwheel’s light is spread out
across its disc, and a casual glance
reveals only the bright central core.
Detailed photographs show that
M101 has an extensive, though rather

DUST LANE
The thick dust lane around the Sombrero
Galaxy is silhouetted against its bright disc
in this Hubble Space Telescope image.

BEYOND THE MILKY WAY Sa SPIRAL GALAXY although its core is LIGHT CONTOURS was William Herschel (see p.90), who
unusually large and Computer image was the first to note the dark dust
Sombrero Galaxy bright. Another manipulation reveals lanes that are M104’s most distinctive
odd feature is variations in the feature. More recently, the Sombrero
CATALOGUE NUMBERS the dense swarm intensity of light within provided some of the first evidence
of globular star the Sombrero Galaxy. for objects lying far beyond our own
M104, NGC 4594 clusters orbiting the galaxy (see panel, below).
galaxy. More than
DISTANCE 2,000 have been counted – ten times VESTO SLIPHER
more than orbit the Milky Way.
50 million light-years US astronomer Vesto Slipher
In the galaxy’s core is a disc of (1875–1969) was one of the first to
DIAMETER bright material tilted relative to the suggest that the Universe is bigger
galaxy’s plane. It is probably the than our galaxy. In 1912, at Lowell
50,000 light-years accretion disc of a central supermassive Observatory in Flagstaff, Arizona, he
black hole. X-ray emission from the identified red-shifted lines in M104’s
MAGNITUDE 8.0 region suggests some material is still spectrum.The lines told him the
being absorbed by the hole. galaxy was receding at 3.6 million
VIRGO kph (2.25 million mph) – too fast
M104 was a late addition to for it to reside within the Milky Way.
The dark dust lane and bulbous core Messier’s catalogue of celestial objects.
of the Sombrero Galaxy (M104) He added it by hand to his copy of
give it a likeness to the traditional the catalogue after discovering it in
Mexican hat after which it is named. 1781. Several other astronomers also
From Earth, we see the Sombrero found it independently. One of these
Galaxy from just six degrees above its
equatorial plane – an ideal angle to
provide a clear view of the core while
also revealing the spiral arms. It is
usually classified as an Sa or Sb spiral,

galaxies

S0 LENTICULAR GALAXY The Spindle (NGC 5866) is an E2 ELLIPTICAL GALAXY larger volume. It probably has a mass
attractive galaxy orientated edge-on of several trillion suns, and is orbited
Spindle Galaxy to observers on Earth. It is usually M60 by thousands of globular clusters.
classified as a lenticular galaxy – a disc Using the Hubble Space Telescope to
CATALOGUE NUMBERS of stars, gas, and dust with a typical CATALOGUE NUMBERS measure the motions of M60’s stars,
bulging core, but with no sign of true astronomers have discovered that a
M102 (not confirmed), spiral arms. However, spiral structure M60, NGC 4649 black hole of 2 billion solar masses
NGC 5866 is hard to detect in an edge-on galaxy. lies at the galaxy’s heart.
DISTANCE
DISTANCE The Spindle Galaxy is the major CLOSE NEIGHBOURS
member of the NGC 5866 Group, a 58 million light-years M60 lies very close to the spiral M59 (upper
40 million light-years small cluster of galaxies. Astronomers right), and the two galaxies are thought to
have measured the way these galaxies DIAMETER be interacting. In 1 billion years’ time, M60
DIAMETER move and have found that the Spindle may even swallow
must contain an enormous mass of 120,000 light-years M59 entirely.
60,000 light-years material – up to 1 billion solar
masses, or 30 to 50 per cent more MAGNITUDE 8.8
MAGNITUDE 9.9 than the Milky Way.
DRACO VIRGO
The Spindle Galaxy could be
the mysterious entry number 102 M60 is one of several giant elliptical
in Charles Messier’s catalogue of galaxies in the Virgo galaxy cluster
astronomical features. Messier included (see p.319), the central cluster in our
the object at first without a location, own Local Supercluster of galaxies.
then later gave coordinates that did The galaxy and its neighbour, M59,
not match any feasible object. Some were discovered in 1779 by German
believe that Messier had listed the astronomer Johann Köhler, who
Pinwheel Galaxy, M101, twice. More was observing a comet that
likely, however, is that M102 was the passed close by. Charles
Spindle, and he added 5 degrees to Messier (see p.69) found
his measurements in error. them a few nights later,
and added them to his
MASSIVE SPINDLE catalogue of objects
From Earth we see the Spindle Galaxy edge- that might confuse
on, giving it a cigar-shaped appearance with comet hunters.
a fine silhouetted dust lane.
M60 is similar in
diameter to many
spiral galaxies but, as
an E2 elliptical, it is
very nearly spherical,
containing a much

DISRUPTED SPIRAL GALAXIES The Antennae have been STAR NURSERIES
studied for what they can tell This enhanced optical
Antennae Galaxies us about galaxy collisions. image highlights, in
Detailed images of the central patches of red and
CATALOGUE NUMBERS region show that it is lit by yellow, the Antennae’s
hundreds of bright, intense star most intense star-
NGC 4038, NGC 4039 clusters.These are thought to birth regions. These
be forming as gas clouds in the patches are mostly in
DISTANCE galaxies become compressed by the compressed dust
the collision, triggering cloud between the
63 million light-years starbursts (see the Cigar Galaxy, two galactic cores.
p.304). Astronomers can use
DIAMETER 360,000 the clusters’ redness to estimate
light-years (total) their age – older clusters emit
redder light because the brighter
MAGNITUDE 10.5 blue stars are the most massive
and therefore the first to die.
CORVUS
COLLIDING CORES
The Antennae Galaxies, NGC 4038 Turbulent clouds of dust and brilliant
and 4039, are among the sky’s most star clusters are revealed in this
spectacular interacting galaxies. Seen Hubble Space Telescope view.
from Earth, they appear as a central
bright double-knot of material, with BEYOND THE MILKY WAY
two long streamers of stars stretching
in opposite directions, resembling an
insect’s antennae. However, powerful
telescopes reveal that each streamer is
in fact a spiral arm, uncurled from its
parent galaxy by the tremendous
gravitational forces of an intergalactic
collision that began around 700
million years ago and continues today.

THE BIGGER PICTURE
A wide-field view of the Antennae taken
from Earth reveals both the bright, distorted
cores and the long, faint streamers formed
by the disrupted spiral arms.

galaxies Despite being referred to only by a the collision is still going on, and the gas than in the stars, so it is usually
number rather than a name (its long dust lane is the “ghost” of a galaxy most obvious at radio wavelengths.
DISRUPTED SPIRAL GALAXY designation comes from the European that ESO 510-G13 has swallowed – as Our near neighbour M31 (see
Southern Observatory’s catalogue), seen in the active galaxy Centaurus A pp.302–303) has such a distortion, and
ESO 510-G13 ESO 510-G13 is one of the most (see p.312). Alternatively, the disc the Milky Way seems to have one, too
intriguing galaxies in the sky. It is an might have been warped by the – perhaps caused by interaction with
CATALOGUE NUMBER ESO 510-G13 edge-on spiral with a clear dust lane gravity of a nearby galaxy.The galaxy its own family of smaller neighbours.
DISTANCE 150 million light-years marking its central plane.The dust responsible might be a small neighbour
lane has an obvious twist. or a more distant but larger member WARPED DISC
DIAMETER of the same group. As their techniques The bright core of ESO 510-G13 silhouettes
The most obvious explanation for and instruments improve, astronomers the galaxy’s warped dust lane in this image.
105,000 the kink is that ESO 510-G13 has are finding this kind of distortion is The blue glow on the right is a huge area of
light-years had a close encounter or collision common in spirals – although it often bright young stars – evidence perhaps of a
with another galaxy in its recent past. shows up more in the distribution of collision in the galaxy’s recent history.
MAGNITUDE Some astronomers have suggested that

13.3

HYDRA

SB0 BARRED SPIRAL GALAXY DISRUPTED SPIRAL GALAXIES EXPLORING SPACE

NGC 6782 The Mice SIMULATING GALAXY COLLISIONS

CATALOGUE NUMBER CATALOGUE NUMBER The great challenge for
astronomers studying
NGC 6782 NGC 4676 colliding galaxies is that
they can only ever see
DISTANCE DISTANCE one stage in a story that
unfolds over millions
183 million light-years 300 million light-years of years. Fortunately,
today’s supercomputers 0 MY
DIAMETER DIAMETER can help to speed things up. By building
“model” galaxies with simplified star
82,000 light-years 300,000 light-years clouds, gas, dust, and dark matter, then
smashing them into each other in a
MAGNITUDE 12.7 MAGNITUDE 14.7 computer, astronomers can measure how
gravity affects the fate of the galaxies.
PAVO COMA BERENICES 400 MY 650 MY
SPIRAL COLLISION SIMULATION 1,000 MY
The Hubble Space Telescope imaged The object classified as NGC 4676 This computer simulation shows two spiral galaxies
BEYOND THE MILKY WAY the apparently normal barred spiral is in fact a pair of colliding galaxies – interacting and merging to form a large, irregular
galaxy NGC 6782 in 2001. Using known as the Mice because they galaxy. Time is measured in millions of years (My).
ultraviolet detectors, it studied the appear to have white bodies and long,
pattern of the galaxy’s hottest material. narrow tails. As with the Antennae DESTINED TO UNITE
The image (see below) showed, in Galaxies, the long streamers are the Although currently moving apart from
pale blue, two rings of stars so brilliant result of the spiral arms “unwinding” a close encounter, the Mice are
and hot that they emit most of their during the collision – though in this gravitationally locked together
light as ultraviolet.The inner ring lies case one of the arms lies edge-on to and doomed eventually to
in the galaxy’s bar and could have us and so appears to be long and merge, perhaps resulting
been ignited by tidal forces between straight, despite being strongly curved in the formation of
the bar and the rest of the galaxy.The away from us. Knots of bright blue a new giant
outer star ring is at the galaxy’s edge. stars in the streamers and the main elliptical galaxy.
bodies of the galaxies show where
bursts of star formation are taking
place. Computer simulations of the
collision (see panel, right) suggest that
the galaxies are now separating after a
closest approach 160 million years ago.

ULTRAVIOLET STAR RINGS HIDDEN EXTENT
Image processing
allows astronomers
to amplify faint light
from the outlying
parts of the Mice,
revealing their true
shape and extent.

galaxies

DISRUPTED SPIRAL GALAXY through each other at high speed formation. An inward-travelling shock disrupted shape and vigorous star
while orientated at right angles to wave is probably responsible for the formation, while a yellow galaxy
Cartwheel Galaxy each other.The rotating density wave core’s unusual “bull’s-eye” appearance. could have been stripped of its
that is normally responsible for the star-forming gas in the encounter.
CATALOGUE NUMBER spiral arms was disrupted in this case, For years, most astronomers However, recent radio observations
resulting in the disappearance of the suspected that one of the Cartwheel’s have shown a telltale stream of gas
ESO 350-G40 spiral structure. Meanwhile a shock two immediate neighbours was leading from the Cartwheel towards
wave spread to the outer edge of the responsible for the collision. Both another small galaxy, a quarter of a
DISTANCE galaxy, creating a ring of vigorous star showed signs of being the culprit – million light-years away.
a nearby small, blue galaxy has a
500 million light-years

DIAMETER

150,000 light-years

MAGNITUDE 19.3

SCULPTOR

If the Cartwheel Galaxy looks unusual,
it’s because it is the victim of an
intergalactic “hit-and-run”.The
Cartwheel was once a normal spiral
galaxy. As we see the galaxy, it is
recovering from a head-on collision
with a smaller runaway galaxy many
millions of years earlier in its history.
Such events are rare in the cosmos –
galactic collisions usually involve
grazing encounters or a slow dance
towards an eventual merger.The
Cartwheel shows what happens when
two galaxies pass

CLOUDS IN THE CORE SPIRAL REGENERATION
So-called “comet The “spokes” of the Cartwheel Galaxy
clouds”, each a (on the left) are the ghostly outlines
thousand light-years of returning spiral arms.
long, are found in the
Cartwheel’s core. They
are thought to arise
as hot, fast-moving
gas set in motion by
the collision ploughs
through denser,
slower-moving matter.

RING GALAXY LOW-SURFACE-BRIGHTNESS GALAXY

Hoag’s Object Malin 1

CATALOGUE NUMBER CATALOGUE NUMBER

PGC 54559 None

DISTANCE DISTANCE

500 million light-years 1 billion light-years

DIAMETER DIAMETER

120,000 light-years 600,000 light-years

MAGNITUDE 15.0 MAGNITUDE 25.7

SERPENS COMA BERENICES

Hoag’s Object is one of the most Despite its dull appearance, Malin 1 BEYOND THE MILKY WAY
bizarre galaxies in the sky. Although is an extremely important galaxy.
its ring structure suggests parallels Discovered by accident in 1987, it is
to the Cartwheel Galaxy (a an enormous but faint spiral that is
spiral disrupted by a head-on for some reason poor at forming
collision, see above), there are no stars. It seems that such low-surface-
nearby galaxies that could have brightness galaxies could account
caused an impact. One of two for up to half the galaxies in the
theories might account for the Universe, though Malin 1 is one
shape of Hoag’s Object and of the largest of the type.
that of similar ring galaxies.
The galaxies may be members
of an unusual class of spiral in
which the two arms develop
into a circle. Alternatively, they
may be former elliptical galaxies
that have each swallowed another
galaxy, creating a surrounding
ring of star-forming material.

SEE-THROUGH GALAXY MALIN 1 IN A NEGATIVE IMAGE
The gap between Hoag’s Object’s core and
its ring is truly transparent – a background
galaxy can be seen through it near the top
of this image. However, the gap could still
contain large numbers of faint stars.

active galaxies

ACTIVE GALAXIES material blasted from the
nucleus expands into a
lobe as it is slowed by
the intergalactic medium

28–31 Matter MANY GALAXIES ACROSS THE UNIVERSE show surprising
32–35 Radiation features that mark them as out of the ordinary. Although
38–41 Space and time there are several types of these strange galaxies, their
226–29 The Milky Way unusual behaviour can always be traced back to powerful
294–97 Types of galaxy activity in their nucleus – it seems that there is an underlying
298–99 Galaxy evolution similarity between them, and for this reason they are often
studied together under the term “active galaxies”.

WHAT ARE ACTIVE GALAXIES?

Astronomers think that the features of active galaxies are linked to their central giant

black holes. Most, if not all, galaxies have black holes with the mass of many millions

jet of particles of suns, known as supermassive black holes, at their nuclei (see p.297), but most such
shooting from black
hole’s magnetic pole black holes are dormant – all material in these galaxies is in a stable orbit around the
star being
ripped apart by black hole. In active galaxies, matter is still falling inwards, and as it falls it is heated by
intense gravity
intense gravity, generating a brilliant blast of radiation. As the black hole “engine” pulls
location of
black hole matter in, the superheated material forms a spiralling accretion disc.The hot disc

torus of dust, emits X-rays and other fierce, high-energy magnetic field line
typically 10 light- radiation. Around the outer edge of the electron
years across
jet expands into disc, a dense torus (doughnut-shape) photon of radio-
lobe thousands of dust and gas forms.The intense wavelength
of light-years long
magnetic field surrounding the radiation

black hole also catches some of the

infalling material, firing it out as

two narrow beams at the poles, at

spinning right-angles to the plane of the

accretion disc accretion disc.These jets shine with

of heated gas radio-wavelength radiation, due to the

synchrotron mechanism (right).

BLACK-HOLE ENGINE SYNCHROTRON RADIATION
The black hole of an active galactic nucleus is As electrons from the black-hole jets move through the
surrounded by a bright accretion disc and an black hole’s magnetic field, they are forced into spiral
outer dust cloud. Jets of material flow paths, releasing synchrotron radiation – a type of EM
outwards from the black hole’s poles. radiation that is most intense at long radio wavelengths.

ACTIVE TYPES RADIO GALAXY QUASAR
In a radio galaxy such In quasars, Earth-
Astronomers distinguish between four major as NGC 383, the bound observers can
types of active galaxy. Each displays its own set central region of the see over the dust
of active features, and in each case these features nucleus is hidden by ring, and brilliant
are evidence of the violent activity at the the edge-on dust ring, light from the nucleus
nucleus. Radio galaxies are the most intense and observers on and disc drowns out
natural sources of radio waves in the sky.The Earth see only the the light of the
emissions typically come from two huge lobes radio jets and lobes. surrounding galaxy.
on either side of an apparently innocuous parent
BEYOND THE MILKY WAY galaxy (and often linked to it by narrow jets). radio jet SEYFERT GALAXY
Seyfert galaxies are relatively normal spirals with dust ring In Seyfert galaxies
a compact, luminous nucleus that may vary in such as M106, the
brightness over just a few days. Quasars appear RADIO SOURCE 3C31 QUASAR PG 0052+251 nucleus and accretion
as starlike points of light that show similar but (RADIO GALAXY NGC 383) disc are exposed to
more extreme variability. Red-shifted lines in BLAZAR our view, as in a
their spectra reveal that they are extremely distant BLAZAR 3C 279 Blazars are active quasar, but the
galaxies – powerful modern telescopes can galaxies aligned so that activity is weak.
resolve them as galaxies with incredibly brilliant observers on Earth look
cores.They are more powerful and more distant straight down the black-
cousins of the Seyfert galaxies. Finally, blazars hole jet onto the nucleus.
(also known as BL Lacertae objects) are starlike The galaxy is hidden by
variable points similar to quasars, but with no the brilliant light, but
significant lines in their spectra.The standard radio lobes can
model of the black-hole engine (above) can sometimes be detected,
explain the major features of each type – how as in blazar 3C 279.
the galaxy appears depends on the intensity of
its activity, and the angle at which we see it. SEYFERT GALAXY M106

THE HISTORY OF NUDGED BACK INTO LIFE
Optical images of Centaurus A

ACTIVE GALAXIES clearly show the dark dust
lane of a spiral colliding with

The distribution of different types of this elliptical galaxy. The
active galaxies in the Universe provides overlaid radio map shows the
burst of activity – the jets and

clues about how they evolve. Quasars and plumes – triggered by this event.

blazars are never seen close to Earth.They

are always faint and distant, with red shifts indicating that they lie

billions of light-years from Earth – we are seeing them as they were in

much earlier times. Radio and Seyfert galaxies, in contrast, are scattered

throughout the nearby Universe, and radio jets are linked to both spiral

and elliptical galaxies. So what happened to the quasars and blazars? It

seems likely that they represent a brief phase in a galaxy’s evolution, soon false-colour
after its birth. At this time, material in the central regions would have radio image
had chaotic orbits, and the central black hole engine would have been of jet of
particles
fuelled by a continuous supply of infalling stars, gas and dust. As the

black hole swept up the available matter, objects with stable orbits

at a safe distance remained. Starved of fuel, the engine would have

petered out, and the quasar became dormant – a normal

galaxy such as the Milky Way.Today, such galaxies can

become active again if they are involved in collisions dust lane

that send new material falling in towards the black (optical
image)
hole. Many nearby radio and Seyfert galaxies show
optical view of
evidence of recent collisions or close encounters, galaxy’s elliptical false-colour
and some of these galaxies are close enough for arrangement of stars radio image of
galaxy’s lobe
infrared telescopes to image

disc of the dust rings around their EXPLORING SPACE
spiral galaxy
cores directly (see p.313).
jet of particles SUPERLUMINAL JETS
emitting radio However, levels of recent

waves activity are restrained – Year Some quasars and blazars appear to defy the laws
even the most spectacular 1992 of physics. Image sequences, taken years apart,
active nucleus of radio galaxies generate
galaxy, containing show jets of material blasting away from the
an active black hole little energy compared to
nucleus, apparently travelling faster than the
surrounded by a
bright accretion quasars, while Seyferts 1994 speed of light.This apparent motion is called
disc and a dust ring are the feeblest type “superluminal”. In reality, it is an illusion, created

of active galaxy. when jets travelling at very high speeds, of up to
1996 99 per cent of the speed of light, happen to be
ACTIVE GALAXY
This idealized active galaxy is a spiral with a bright pointing almost directly towards us.

nucleus, which hides an active black hole. From the 1998 TIME-LAPSE SEQUENCE
black hole’s poles blast two jets of particles, leaving at These images show jet emissions from blazar 3C 279,
close to light speed, only slowing and billowing out 20 40 60 80 taken at intervals of almost two years, and showing
into lobes many thousands of light-years away, as Distance (light-years) motion apparently five times the speed of light.
the particles hit the intergalactic medium.

IS THE MILKY WAY ACTIVE? ANTIMATTER FOUNTAIN BEYOND THE MILKY WAY
This gamma-ray image traces positrons
The Milky Way galaxy, like any galaxy with a central black hole, has the (antielectrons) around the Milky Way.
potential to be active, and there is intriguing evidence that it might
have burst into activity in the recent past. In 1997, scientists discovered The horizontal feature is the plane of the
Galaxy, with the fountain above it.
a huge cloud of gamma-ray emission above the galactic centre.The
radiation has a distinctive frequency, suggesting it is the result
of electrons encountering positrons – their

antimatter equivalent (see p.31) – and
annihilating in a burst of energy.The
positrons might have been generated

by activity at the core – perhaps an
infall of matter into the black hole –
and are now meeting scattered

electrons in the outer galaxy and
mutually annihilating to produce
the distinctive glow. Since the clouds
lie just 3,000 light-years from the

galactic centre, the activity must
have occurred recently.

ACTIVITY AT THE CORE
This Chandra X-ray image of the
Milky Way’s nucleus shows brilliant high-
mass stars and a supernova remnant around

the centre. Material from this crowded
region may occasionally feed the black hole.

ACTIVE GALAXIES

There are no simple rules governing the appearance of active galaxies. Some have a disrupted

structure, seen either in visible light or at other wavelengths, while others appear normal at

first, but radiate unusually large amounts of energy at certain wavelengths. In fact, the majority

of galaxies show activity of one kind or another. However, a smaller proportion of galaxies

have particularly active nuclei, powered by matter spiralling into their central black hole.These

JET FROM AN ACTIVE GALAXY include Seyfert galaxies, radio galaxies, quasars, and blazars.The vast
Pictured in radio waves and false colours, majority of known active galaxies are distant quasars. Objects lying
this jet of particles blasted from the core nearer to the Milky Way, although less spectacularly violent, are at
of the galaxy M87 is a typical feature of least close enough for astronomers to study in detail.
active galaxies with black-hole engines.

TYPE-II SEYFERT GALAXY for so long partly because it lies just the galaxy, around 1,300 light-
4 degrees below the plane of the Milky years across, where great bursts
Circinus Galaxy Way and is obscured by star clouds. of star formation are occurring.
The full extent of the Circinus Galaxy’s Finally, Hubble showed a cone-
CIRCINUS CATALOGUE NUMBER extraordinary nature was revealed shaped cloud billowing above
only when it was observed by the the plane of the galaxy.This is
ESO 97-G13 Hubble Space Telescope in 1999.The matter ejected by the magnetic
galaxy is a Seyfert (see p.310) – a spiral fields of the black hole and glows
SHAPE Sb spiral with an unusually bright, compact as it is heated by the ultraviolet
region at its core, thought to result radiation from the nucleus.
DISTANCE from material slowly drifting onto a
massive central black hole. Hubble’s CONE OF MATTER
13 million light-years infrared camera revealed how the The pinkish-white region near the core
galaxy’s gas is concentrated in a central of the Circinus Galaxy shows where
DIAMETER ring, just 250 light-years in diameter, matter is being flung out, in a cone
around the black hole. Also apparent shape, from the central black hole
37,000 light-years is a loose outer ring in the plane of into the gas cloud above the galaxy.

MAGNITUDE 11.0

Although the spiral galaxy in Circinus
is probably the nearest active galaxy to
Earth, it went undiscovered until just
a few decades ago. It remained hidden

RADIO CONTINUUM COMPOSITE PICTURE
Astronomers have
captured images of
Centaurus A in radio,
optical, and X-ray
radiation, combining
them to make the
composite at far left.

RADIO (21CM WAVELENGTH)

jet

OPTICAL WAVELENGTHS X-RAY WAVELENGTHS

BEYOND THE MILKY WAY DUSTY DISC
This Hubble Space Telescope close-
up of Centaurus A (right) reveals
dark interstellar dust, glowing
orange gas clouds, and brilliant
blue star clusters formed in the
collision between two galaxies.

RADIO GALAXY A ball of old yellow stars, NGC 5128 wavelengths. The
shows some features typical of an Hubble Space Telescope
Centaurus A elliptical galaxy, but its most striking looked through the dust
aspect is the dark dust lane that cuts lanes with its infrared camera
CENTAURUS CATALOGUE NUMBER across it, bisecting the uniform glow and found a huge accretion
of stars with a ragged silhouette.What disc at the centre – a sure sign
NGC 5128 is more, the galaxy is at the centre of of an active black hole pulling
a pair of vast radio lobes, 1 million in matter at Centaurus A’s core.
SHAPE Peculiar elliptical light-years across.The name of this It is now generally agreed
radio source, Centaurus A, is now the that NGC 5128 is an elliptical
DISTANCE most widely used name for the galaxy galaxy absorbing a spiral.The
itself. Astronomers have studied ghost of the spiral is shown by
15 million light-years Centaurus A in detail at a range of the dust lane and by the bright
star clusters that stud it – perhaps
DIAMETER generated by shock waves as the
two galaxies merge.
80,000 light-years

MAGNITUDE 7.0

ACTIVE galaxies

RADIO GALAXY Lying at the heart of the Virgo galaxy TYPE-II SEYFERT GALAXY CARL SEYFERT
cluster (see p.319), M87 is the closest
M87 example of a giant elliptical galaxy – a Fried Egg Galaxy US astronomer Carl Seyfert (1911–
class of galaxy often found at the cores 60) was the son of a pharmacist
VIRGO CATALOGUE NUMBERS old galaxy clusters.This huge ball of CATALOGUE NUMBER from Cleveland, Ohio. He studied
stars seems to have a diameter roughly at Harvard and went on to work
M87, NGC 4486 equivalent to that of the Milky Way, NGC 7742 at McDonald Observatory, then at
but, because its stars are distributed Mount Wilson in California. It was
SHAPE E1 giant across its spherical structure, it contains SHAPE Sb spiral here that he first identified the class
elliptical many more stars – probably several of galaxies with unusually bright
trillion. Long-exposure photographs DISTANCE nuclei that bear his name (see
DISTANCE have revealed that the galaxy also has p.310). In 1951, he also
an extensive halo of more loosely 72 million light-years discovered Seyfert’s
60 million light-years scattered stars, extending well beyond Sextet, an interesting,
the central region in a more elongated DIAMETER compact cluster
DIAMETER shape.The galaxy also has an unrivalled of galaxies
collection of globular star clusters in 36,000 light-years (see p.319).
120,000 light-years orbit – some astronomers estimate as
many as 15,000 such groups. PEGASUS MAGNITUDE 11.6
MAGNITUDE 8.6
What is more, M87 is an active The small spiral galaxy NGC 7742
galaxy – its location coincides with resembles a fried egg because of the
the Virgo A radio source, and with a intense yellow glow from its core.The
strong source of X-rays.There is even core is much brighter than is usual
a sign of this activity that is visible at for a galaxy of this size, because this
optical wavelengths, in the form of a is a Seyfert galaxy, with a moderately
long, narrow jet of material being active core. Seyferts emit radiation
blasted from its interior. across a broad band of wavelengths –
NGC 7742 is a Type-II – a galaxy
that is brightest
in infrared
light.

PARTICLE JET BALL OF STARS CELESTIAL EGG SEYFERT’S OBSERVATORY
The blue glow of M87’s jet results M87’s full extent is At Nashville, Seyfert found time to give
from synchrotron radiation – light shown in this wider public lectures as well as raising support
emitted by electrons spinning image. The elliptical and supervising the construction of the
through an intense magnetic field. galaxy is remarkably Arthur J. Dyer Observatory (above).
uniform, and the star-
like specks around it
are just a few of its
many thousands of
globular clusters.

RADIO GALAXY The elliptical galaxy NGC 4261 lies material in NGC 4261 come from? TYPE-I SEYFERT GALAXY
at the centre of two great lobes of The most likely answer is that the
NGC 4261 radio emission measuring 150,000 elliptical galaxy has merged with a NGC 5548
light-years from tip to tip. In many ways spiral in its relatively recent history.
CATALOGUE NUMBER a typical radio galaxy, it is also one of The spiral’s individual stars have now CATALOGUE NUMBER
the few active elliptical galaxies to become indistinguishable from the
NGC 4261 have revealed its internal structure to stars that were originally part of the NGC 5548
astronomers. Infrared images from the elliptical galaxy, but the ghostly outline
SHAPE E1 elliptical Hubble Space Telescope pierced the of the galaxy’s gas and dust remains. SHAPE Sb spiral
obscuring clouds of stars to reveal an
DISTANCE unexpectedly dense disc of dusty DUST WHIRLPOOL DISTANCE
material, apparently spiralling onto The Hubble Space Telescope’s close-up image
100 million light-years the galaxy’s central black hole. of the core reveals a dusty spiral of matter 220 million light-years
within a ring of glowing outer clouds. A distinct
DIAMETER Most elliptical galaxies are thought DIAMETER
to be relatively dust-free, cone shows where matter is being
60,000 light-years so where did the flung off from the active 100,000 light-years
galactic nucleus into
VIRGO MAGNITUDE 10.3 the radio lobes. BOÖTES MAGNITUDE 10.5

NGC 5548 is a Type-I Seyfert galaxy BEYOND THE MILKY WAY
– that is, a Seyfert that emits more
ultraviolet and X-ray radiation than
visible light. Like all Seyferts, it has a
bright, compact core, but, unlike the
Fried Egg Galaxy (see above), its core
is an intense blue-white. Using the
Chandra X-ray telescope, astronomers
have detected an envelope of warm
gas expanding around the core.The
gas eventually forms two lobes of weak
radio emission around the galaxy.

RADIATING PLUMES HUBBLE IMAGE OF NGC 5548
Combining optical and radio images of
NGC 4261 reveals its full extent. The
visible part of the galaxy is the white
blob in the centre, while the orange
plumes mark the radio-emitting regions.

RADIO GALAXY

NGC 1275

CATALOGUE NUMBER

NGC 1275

SHAPE Elliptical and
distorted spiral

DISTANCE

235 million light-years

PERSEUS DIAMETER

70,000 light-years

MAGNITUDE 11.6

COLLISION DEBRIS CLUSTERS IN THE NUCLEUS Despite being catalogued as a Seyfert
Young, blue star clusters in the dark The core of NGC 1275 offers galaxy by Carl Seyfert himself (see
dust of NGC 1275 show that the clues to the origin of globular p.313), NGC 1275 has remained a
galactic collision triggered star birth. clusters – numerous globular-like mystery. Recent observations have
clusters are found here, but they shown that there are two objects –
are composed of young blue,
rather than old yellow stars. one in front of the other.
A ghostly spiral galaxy,
revealed by its bright blue
star clusters, is responsible
for the dust lanes that cross
the bright central region,
but this brighter region is
in fact a separate galaxy.
Despite its Seyfert-like
core, it is an elliptical, not
a spiral.This galactic giant
lies at the heart of the
Perseus galaxy cluster, and
the foreground spiral is
racing towards it at 10.8
million kph (6.7 million
mph), its structure already
disrupted by the elliptical’s
gravity. Adding to the
complexity, the elliptical
galaxy is also a radio source, and some
astronomers have argued that it shows
blazar-like activity (see BL Lacertae,
opposite).Whatever the details, NGC
1275 displays many of the typical
features of an active galactic nucleus.

RADIO GALAXY

Cygnus A

CYGNUS CATALOGUE NUMBER

3C 405

SHAPE Pec (peculiar)

DISTANCE

600 million light-years

DIAMETER

120,000 light-years
(excluding radio lobes)

MAGNITUDE 15.0

BEYOND THE MILKY WAY The most spectacular and powerful (see p.312), which is thought to be at either end. Studies by the Chandra LOBES EMITTING RADIO WAVES
radio galaxy in the nearby Universe, an elliptical galaxy that has recently X-ray telescope have shown that This radio map of Cygnus A shows the
Cygnus A was discovered as soon as swallowed a spiral. Recent detection Cygnus A lies at the centre of a cloud galaxy’s extremely narrow jets blasting from
radio telescopes began operating in of a large cloud of red-shifted gas of hot but sparse gas.The jets have its core, the hot spots at the end its radio
the 1950s. It features two huge lobes moving through the Cygnus A galaxy blown out a rugby-ball-shaped cavity lobes, and the tendrils of hot gas falling
of material emitting radio waves.The suggests that a collision may indeed in the gas so vast that it dwarfs the back towards the central galaxy.
lobes are visibly linked to their origin be the root cause of the activity. central galaxy.Tendrils of gas, which
at the heart of a faint, central, elliptical are emitting X-rays and radio waves, drawn by its gravitational pull.The
galaxy by two long narrow jets. From Astronomers have also argued are also falling back down through the hot spots are apparently created where
lobe to lobe, the entire structure about the origin of the “hot spots”, cavity onto the poles of the galaxy, the outward blast of the jets collides
extends over half a million light-years. where the radio lobes glow brightest with the hot gas falling inwards.

Despite its prominence in the
radio sky, mysteries still surround
Cygnus A, largely because of its great
remoteness. Early observations led
astronomers to believe the central
galaxy was in fact a pair of colliding
galaxies. Hubble Space Telescope
images suggested a resemblance to
NGC 5128, the Centaurus A galaxy

active galaxies

BLAZAR (BL LAC OBJECT) changed. For a variable star, it was QUASAR quasars, as well as other galaxies close
very mysterious, showing rapid but to the quasars.The images showed
BL Lacertae completely unpredictable variations. PKS 2349 that in many cases quasars do not just
At the same time, it displayed a totally sit at the centres of their host galaxies,
CATALOGUE NUMBER featureless spectrum – it had neither CATALOGUE NUMBER but are involved in violent interactions
the dark absorption lines seen in stars, with neighbouring galaxies and other
BL Lac nor the bright emission lines found in PKS 2349 quasars. PKS 2349 was referred to as
galaxies (see p.33). It was not until a “smoking gun” because it showed
SHAPE Elliptical 1969, when BL Lac was found to be SHAPE Disrupted these interactions so clearly.The
a strong radio source, that astronomers quasar is surrounded by a ring of
DISTANCE realized it might be a new type of DISTANCE faint material that may mark the
active galaxy.Today it is seen as the outline of its host galaxy – though, if
1 billion light-years founder member of a class of active 1.5 billion light-years so, the quasar itself is remarkably “off-
galaxies called blazars or BL Lac centre”. A small companion galaxy,
DIAMETER objects. Blazars show many similarities DIAMETER about the size of the Large Magellanic
to quasars but also some differences, Cloud (see p.300), also lies nearby
Unknown most notably their featureless spectra. Unknown and seems doomed to collide with

LACERTA MAGNITUDE 12.4–17.2 The mystery of BL Lac was solved PISCES MAGNITUDE 15.3 the quasar itself.
in the 1970s, when two astronomers
BL Lacertae (BL Lac for short) was blocked out or “occulted” BL Lac’s The Hubble Space Telescope offered
first catalogued as an irregular variable bright core to study its surroundings. astronomers an unprecedented
star by German astronomer Cuno This revealed that it was embedded in chance to study quasars in detail
Hoffmeister in the 1920s. Since then, a faint elliptical galaxy, whose light during the 1990s. One of their most
astronomers’ understanding of the was normally drowned out. Red- intriguing subjects was the otherwise
object has shifted lines in the spectrum of this undistinguished quasar PKS 2349
galaxy confirmed BL Lac’s great (referred to by its designation
distance (see p.42).Today, blazars are in the catalogue of the
accepted as rare cases in which Earth’s Australian Parkes radio
position happens to align directly telescope). For the first
with the jet of material blasting out time, astronomers
of an active galactic nucleus, with no were able to see the
obscuring material in the way. faint host galaxies
surrounding
MAP OF A BLAZAR
This radio map of BL Lacertae shows the QUASAR CLOSE-UP
intensity of radiation (contour lines) and In Hubble’s image of
also its polarization (colour) – an indication PKS 2349, the
of magnetic field strength. The red object quasar is the bright
at the top is the galaxy’s nucleus, while central object, the
the lower regions are parts of a radio jet. companion galaxy is
the smaller bright
region above it, and
the supposed host
galaxy is the fainter
ring extending from
the quasar.

QUASAR used an occultation by the Moon (see QUASAR FIRST QUASAR
p.65) to precisely establish its position, At first, 3C 48 is
3C 273 linking the radio source to what 3C 48 indistinguishable
appeared to be an irregular variable from foreground
CATALOGUE NUMBERS star.The star’s spectrum had a forest CATALOGUE NUMBERS stars. It was only
of unidentifiable dark emission lines its unpredictable
3C 273, PKS 1226+02 (see p.33). Astronomers finally realized 3C 48, PKS 0134+029 variability and
that the lines could have been formed radio emission that
SHAPE E4 elliptical by hydrogen, oxygen, and magnesium SHAPE SB interacting marked it out as
if the light was heavily red-shifted and something special.
DISTANCE its source was racing away from us at DISTANCE
16 per cent of the speed of light, or could not have been emitted by any
2.1 billion light-years 173 million kph (107 million mph). 2.8 billion light-years known element. Studies of similar
We now know that the object is not lines in the optical counterpart of
DIAMETER 160,000 light- a star, but a distant active galaxy. DIAMETER 3C 273 (left) suggested that the lines
years (excluding jet) of 3C 48 were hydrogen lines with a
100,000 light-years huge red shift, suggesting the object
was extremely distant and receding at
VIRGO MAGNITUDE 12.8 TRIANGULUM MAGNITUDE 16.2 great speed. 3C 48 was therefore the
first quasi-stellar object, or quasar, to
The brightest quasar in the sky, 3C The radio source 3C 48 has a unique be discovered. BEYOND THE MILKY WAY
273 was the second to be discovered. place in the history of the study of
The existence of this radio active galaxies. It was detected in the
source was already known 1950s, and in 1960 Allan Sandage
when, in 1963, Australian (see panel, below) confirmed that it
astronomer Cyril Hazard coincided with a faint, blue, star-like
object.The object’s spectrum revealed
HOST GALAXY strange emission lines (see p.33) that
By blocking the light from 3C 273’s
nucleus, the Hubble Space Telescope ALLAN SANDAGE
was able to photograph detail (right)
in the fainter surrounding galaxy.

RADIO JET Beginning his astronomical career
This long-wavelength radio image shows as a student under Edwin Hubble
a jet shooting out 1.2 million light-years himself (see p.43), Allan Sandage
from 3C 273’s core (top left of the image). (b.1926) has had a great influence on
The end of the jet is as bright as the core. our understanding of the Universe’s
evolution. Sandage’s studies have
focused on detecting Cepheid
variable stars in distant galaxies,
for use in measuring cosmological
expansion. His many quasar
discoveries were a natural offshoot
from his studies of deep space.

316 galaxy clusters

GALAXY CLUSTERS DENSE CLUSTER
The massive galaxy cluster Abell 1689 lies
22–23 The scale of the Universe GALAXIES ARE NATURALLY GREGARIOUS. 2.2 billion light-years away. The yellow elliptical
24–27 Celestial objects Pulled together by their enormous gravity, galaxies are surrounded by arcs of light, which
36–37 Gravity, motion, and orbits they cluster tightly, sometimes orbiting one are images of more distant galaxies
38–41 Space and time another, often colliding. As galaxies slowly distorted by the cluster’s
42–43 Expanding space move within a cluster, the cluster’s gravitational lensing.
294–97 Types of galaxy structure changes.The evolution of
clusters can tell astronomers about

dark matter, and clusters can even be used as cosmic

“lenses” to peer back into the early Universe.

TYPES OF CLUSTERS

Some galaxy clusters are sparse, loose collections of galaxies.

The smallest clusters are usually termed “groups”.The Local

Group (see p.318), of which the Milky Way is a member, is

one such cluster. Other clusters, such as the nearby Virgo

Cluster (see p.319), are denser, containing many hundreds of

galaxies in a chaotic distribution. Yet other clusters, such as

the Coma Cluster (see p.320), are even more dense, with

galaxies settled into a neat, spherical pattern around a

centre dominated by giant elliptical galaxies. Although

clusters differ in density, the volume of space they occupy

is generally the same – a few million light-years across.

Not all galaxies exist in clusters – there are more isolated

“field galaxies” than there are cluster galaxies. Some

galaxy types do not exist outside clusters, however. Giant

ellipticals (see p.296) always lie near the centre of large

clusters, as do vast, diffuse cD galaxies (below right).The

most numerous cluster components may be invisible,

including faint, diffuse dwarf elliptical galaxies and proposed

“dark galaxies”. A dark galaxy would consist of hydrogen

gas and material too thin to condense and ignite stars.The

Andromeda Galaxy (M31) first such galaxy
may have been

found, in the Virgo

Cluster, in early 2005.

SPARSE CLUSTER
This sparse cluster, or group, of
galaxies is in fact the Local Group,

containing the Milky Way and its
galactic neighbours. Most

Milky of the galaxies are orbiting
Way either the Milky Way or the

Andromeda Galaxy (M31).

BEYOND THE MILKY WAY dense core of cluster containing DWARF ELLIPTICAL cD GALAXY
many large galaxies Most galaxies in the Local cD galaxies are similar
Group, including the Sculptor to giant ellipticals but
IDEAL DENSE CLUSTER Dwarf, are dwarf ellipticals. have extensive, sparse
A dense cluster occupies the same They are invisible in distant outer haloes of stars.
volume as a sparse cluster such as clusters, but must be present. They sometimes have
the Local Group, but the galaxies hints of multiple cores,
are mainly elliptical and have a suggesting the merger
roughly spherical distribution of several smaller
around the cluster’s centre. ellipticals. NGC 4889
(left) is a cD galaxy at
the heart of the dense
Coma Cluster.

galaxy clusters 317

ABELL 2029 THE INTERGALACTIC MEDIUM
This visible-light image
of Abell 2029 shows Astronomers can estimate the overall mass of a galaxy cluster from the way in which its galaxies
that it is an old, regular,
spherical cluster full are moving, but also through the phenomenon of gravitational lensing – an effect of general
of elliptical galaxies.
relativity (see pp.40–41).When a compact cluster lies in front of more distant galaxies, its mass

bends the light passing close to it and deflects distorted images of the distant galaxies towards

Earth. By measuring the strength of this effect, it is possible to measure the mass of the cluster

and model how it is distributed. Galaxy clusters contain far more mass than the visible galaxies

can account for and most of it is in the matter that permeates the space between galaxies.This

intergalactic medium is distributed around the cluster’s centre, rather than around the galaxies.

X-ray satellites such as Chandra have revealed the nature of part

INTERGALACTIC GAS of this material – large galaxy clusters often contain huge clouds of

An X-ray image of cluster Abell sparse, hot gas, glowing at X-ray wavelengths. Most is hydrogen, but
2029 shows the hot gas cloud heavier elements are present. It is thought to originate in the cluster
around its centre. If not for the galaxies, and to be stripped away during encounters and collisions.
gravity of the cluster’s dark Most of a cluster’s mass is not gas, however, but dark matter.
matter, this gas would escape.

apparent position path of light without GRAVITATIONAL LENSING
and distorted gravitational lensing Light leaves a distant galaxy in all
shape of multiple directions. As it passes close to a
galaxy images light bent towards massive cluster of galaxies, it is
observer by lens deflected from its path, due to the way
mass distorts space. Light paths arrive
at Earth apparently from different

directions, creating multiple,
distorted images of
the galaxy.

actual position
and shape of
galaxy

PERFECT ARC galaxy cluster acting
This striking example of lensing is created as a gravitational lens
by the cluster CL-2244-02. The lensed
galaxy, unlike the cluster galaxies, is blue, observer in the Milky Way
so it must be a spiral or an irregular.

CLUSTER EVOLUTION VIOLENT MERGER BEYOND THE MILKY WAY
An optical image (left) of
Astronomers have built a picture of cluster development cluster Abell 400 shows
that complements their models of galaxy evolution (see two galaxies merging at
p.299). According to this thinking, galaxy clusters start as its centre to form a giant
loose collections of gas-rich spirals, irregulars, and small elliptical. Radio images
ellipticals. Because of their proximity and huge gravity, the (below) reveal that they
spirals tend to merge, regenerating as spirals or forming are both active radio
ellipticals. Each interaction drives off more of the galaxies’ galaxies. Such an event
free gas into the intergalactic medium.The high temperature is typical of those that
and speed of atoms in this medium prevents their recapture shape galaxy clusters.
by the cluster’s galaxies. At this stage, the cluster is irregular,
IRREGULAR AND RELAXED CLUSTERS or “unrelaxed”, and the pattern of galaxies and intergalactic
The central regions of the Virgo Cluster gas is irregular and chaotic. However, as galaxies swing
(above) and the Coma Cluster (below) show round each other, their random motions are eliminated and they
the difference between an irregular and a settle into a stable, spherical distribution around the cluster’s centre.
more spherical (relaxed) pattern of galaxies. Eventually even the largest elliptical galaxies begin to merge,
forming giant ellipticals and cD galaxies.The hot
gas, freed from ties to individual galaxies,
sinks into the centre of the cluster,
where it lies evenly around the
cluster’s major elliptical
galaxies.What remains is an
old, spherical, “relaxed”
cluster full of ellipticals.

GALAXY CLUSTERS

The shape and size of galaxy clusters are thought to be linked to their evolution. Clusters

range from small groups comprising young, gas-rich irregular and spiral galaxies, to highly

evolved clusters dominated by giant ellipticals, with a central cloud of gas so hot that it

emits X-rays. Astronomers can study details in nearby clusters that are too faint to see in

STEPHAN’S QUINTET distant clusters. Earth’s neighbouring clusters do not offer
This elegant group of five galaxies shows a spectacle to stargazers, however, because clusters are so

that clusters are constantly changing – two vast that their members are widely scattered across the sky.
of its spiral galaxies are colliding, while a To appreciate clusters in a single picture, it is necessary to
third is being distorted by their gravity and peer tens of millions of light-years into deep space.
is doomed to collide with them one day.

Andromeda Galaxy, M31 IRREGULAR CLUSTER the other of these large spirals.The spiral.The Local Group appears to be
third large spiral in the group, M33 relatively young. Its major galaxies are
Triangulum Galaxy, M33 Local Group (see p.301), may also be trapped in all spirals, and there is little matter in
LOCAL GROUP MEMBERS a long orbit around M31. the space between galaxies – most of
DISTANCE the cluster’s gas is still trapped in the
Since Earth is in the midst of the Local Outnumbering these spirals is a spirals. It is in an early state of cluster
Group, the galaxies are scattered around 0–5 million light-years host of dwarf elliptical and irregular evolution.The Milky Way is currently
the sky. However, two large members, galaxies. Examples include SagDEG colliding with the Magellanic Clouds,
M33 and M31, are near enough in the NUMBER OF GALAXIES 46 and the two Magellanic clouds (see and is heading inexorably towards an
sky to appear in the same frame. pp.300–301), as well as M110 and ultimate merger with M31.
BRIGHTEST MEMBERS M32, both ellipticals orbiting the M31

Milky Way;
M31 (magnitude 3.5)

ANDROMEDA AND TRIANGULUM BARNARD’S GALAXY
This small, irregular galaxy (right), catalogued
The Local Group is the small galaxy as NGC 6822, lies 1.7 million light-years away
cluster of which the Milky Way is a within the Local Group. It is rich in gas and
member. From Earth, its members dust, with many pinkish star-birth regions.
appear dispersed throughout the sky,
but some of its galaxies are grouped in
the constellations of Andromeda and
Triangulum. In space, the core of the
group comprises about 30 members
in a region just over 3 million light-
years across. It is dominated by the
Andromeda Galaxy (M31; see pp.302–
303), and the Milky Way. Most of the
smaller galaxies orbit close to one or

FORNAX DWARF GALAXY
This dwarf spheroidal galaxy (left) has
no obvious nucleus. Such faint and
diffuse galaxies are easily missed in
more distant galaxy clusters, but they
are probably the most numerous.

BEYOND THE MILKY WAY THE MILKY WAY GALAXY
A major member of the Local group is
the Milky Way galaxy. Earth is within
the galaxy’s disc, so our view is edge-
on and stretched acoss the sky.

galaxy clusters

IRREGULAR CLUSTER the Local Group. It is also a young IRREGULAR CLUSTER “groups” is striking – the Virgo Cluster
cluster of irregular and spiral galaxies, contains around 160 major spiral and
Sculptor Group with no major ellipticals. It is possible Virgo Cluster elliptical galaxies crammed into a
that this group, the Local Group, and volume little larger than that of the
ALTERNATIVE NAME another group called Maffei 1 were ALTERNATIVE NAME Local Group, along with more than
once part of the same larger cluster. 2,000 smaller galaxies. At its heart lie
South Polar Group Virgo I Cluster the giant ellipticals M87 (see p.313),
The nearest member to Earth is M84, and M86, which are thought to
DISTANCE 9 million NGC 55, an irregular galaxy that, DISTANCE 52 million have formed from the collisions of
light-years to centre like the Large Magellanic Cloud (see light-years to centre spirals over billions of years. Each
p.300), shows enough structure for giant elliptical seems to be at the
NUMBER OF GALAXIES some astronomers to consider it a NUMBER OF GALAXIES centre of its own subgroup of galaxies
single-armed spiral.The dominant – the cluster has not yet settled to
19 (6 major) galaxy, however, is NGC 253.This 2000 (160 major) become uniform.The cluster’s gravity
large spiral is the same size as the influences a huge region, extending as
SCULPTOR BRIGHTEST MEMBER Milky Way and more than twice the VIRGO BRIGHTEST MEMBER far as the Local Group and beyond –
size of any other galaxy in the group. the Milky Way and its neighbours are
NGC 253 (8.2) M49 (9.3) falling towards the Virgo Cluster at
GALAXY NGC 253 1.4 million kph (900,000 mph).
Lying just beyond the gravitational This large spiral dominates the Sculptor The Virgo Cluster is the nearest
boundaries of the Local Group, the Group in this wide-field image. Most galaxy cluster worthy of the name; it
Sculptor Group is similar in size to of the other galaxies are is a dense collection of galaxies at
too faint to be seen the heart of the larger supercluster to
without powerful which the Local Group also belongs.
telescopes. The contrast with smaller galaxy

STARBURST GALAXY CENTRE OF THE CLUSTER
NGC 253 is a spiral starburst galaxy – The Virgo Cluster’s core has a high density
a galaxy undergoing a surge of star of large galaxies. The two bright galaxies
formation. The surge may have been on the right are the ellipticals M84 and M86.
triggered by a series of supernovae.

EXPLORING SPACE REGULAR CLUSTER mostly ellipticals, distributed evenly COMPACT GROUP
around the giant elliptical NGC
X-RAY IMAGING Fornax Cluster 1399. Dwarf galaxies lying between Seyfert’s Sextet
the major ones are also mostly small
AND CLUSTER GAS FORNAX CATALOGUE NUMBER ellipticals, suggesting that the cluster SERPENS CATALOGUE NUMBERS
formed long ago and that interactions
Many galaxy clusters are strong Abell S 373 between its galaxies have had time to NGC 6027 and
sources of X-rays, and orbiting strip away most of their star-forming NGC 6027A–C
X-ray telescopes can reveal features DISTANCE 65 million gas (see p.317).This account of the
that remain hidden in visible-light light-years to centre cluster’s evolution has recently been DISTANCE
images.While some X-ray sources confirmed by the orbiting Chandra
are located at the centres of the NUMBER OF GALAXIES X-ray observatory (see panel, left). 190 million light-years
cluster galaxies, the majority of
radiation often comes from diffuse 54 major galaxies NUMBER OF GALAXIES 4
gas clouds, independent of the
individual galaxies.The process BRIGHTEST MEMBER BRIGHTEST MEMBER
that strips gas out of the cluster
galaxies (see p.317) also heats it NGC 1316 (9.8) NGC 6027 (14.7)
to generate the X-rays. The
distribution of gas offers clues Fornax is home to a relatively nearby Seyfert’s Sextet actually contains just
to a cluster’s age and history. four members – each a misshapen
galaxy cluster, centred at around the spiral galaxy locked to the others in
a gravitational waltz within a region
same distance as the Virgo Cluster. of space no larger than the Milky
Way.The sextet, as seen from Earth, is
However, the Fornax Cluster is at a completed by a small face-on spiral
that happens to lie in the background,
later stage of evolution and by a distorted star cloud (at lower
right in the image below).
than the younger GALAXY NGC 1365 BEYOND THE MILKY WAY
Virgo group. Here, One of the Fornax

spiral galaxies are Cluster’s few spirals,

rare – the cluster’s NGC 1365 has a dust

major galaxies are bar through its core.

FORNAX IN X-RAYS CLUSTER CORE
This image of the Fornax cluster shows X-ray- In the Fornax Cluster’s
emitting gas in blue. Both central galaxies central region lie NGC 1399
have trailing plumes of gas, suggesting the (upper left of centre) and
entire cluster is moving through sparser clouds. NGC 1365 (bottom right).
As a rule, elliptical
galaxies predominate. QUARTET PLUS TWO

galaxy clusters

REGULAR CLUSTER COMPACT GROUP

Hydra Cluster Stephan’s Quintet

CATALOGUE NUMBER Abell 1060 CATALOGUE NUMBER
DISTANCE 160 million light-years
Hickson 92
NUMBER OF
DISTANCE
GALAXIES
340 million light-years
1,000+ (NGC 7320: 41 million
light-years)
HYDRA BRIGHTEST PEGASUS
NUMBER OF GALAXIES 4/5
MEMBER
BRIGHTEST MEMBER
NGC 3311
(11.6) NGC 7320 (13.6)

The Hydra Cluster is similar in size to First observed by French astronomer QUINTET CLOSE-UP
the huge Virgo Cluster (see p.319). It E. M. Stephan at the University of This detailed Hubble Space
is the closest example of a “relaxed” Marseilles in 1877, Stephan’s Quintet Telescope view of Stephan’s
cluster (see p.317) of mainly elliptical appears to be a remarkably compact Quintet shows chains of
galaxies in a spherical distribution. Its cluster of five galaxies.The galaxies stars linking several of its
hot X-ray gas also forms a spherical are a mixture of spirals, barred spirals, interacting galaxies.
cloud around the core.The cluster is and ellipticals and show clear signs
BEYOND THE MILKY WAY centred on two giant elliptical galaxies of disruption from interactions.The
and an edge-on spiral, each 150,000 largest galaxy as seen from Earth,
light-years across.These galaxies are NGC 7320, is probably a foreground
interacting – the ellipticals’ gravity object lying in front of a quartet of
has warped the spiral, while both interacting galaxies.The spectral red
ellipticals have distorted outer haloes. shift (see p.33) of NGC 7320 is much
The cluster is the major member of smaller than those of the other four
the Hydra Supercluster, which adjoins galaxies, and instead matches that of
the Local Supercluster (see p.324). several other galaxies close to it in the
sky. Since it also appears physically
HEART OF THE HYDRA CLUSTER different from the quartet, it seems
In this image, the central giant ellipticals likely that NGC 7320 is much closer
NGC 3309 and 3311 lie below the large, and the unusual red shift is a normal
blue spiral NGC 3312. The two bright result of the expansion of space (see
objects on either side are foreground stars. p.42). However, a few astronomers
claim that trails of material link NGC
SPIRAL SILHOUETTE 7320 to other Quintet galaxies. If this
NGC 3314, an unusual is the case, then the red shift suggests
case of one spiral that the galaxy is moving very fast
galaxy silhouetted relative to its neighbours and towards
against another, is Earth, therefore reducing its overall
one of Hydra’s most speed of recession and its red
beautiful objects. shift. Or perhaps the red shift
does not originate from its
motion at all.These competing
theories have turned Stephan’s
Quintet into a battleground
for the small minority of
astronomers who think that
red shifts are not all caused by
the expansion of the space,
and that Hubble’s Law (see
p.42) does not always apply.

FOUR OR FIVE?
The quintet consists of a quartet of
yellow galaxies beside the white
spiral NGC 7320. The contrasting
appearance of NGC 7320 suggests
it lies in front of the other galaxies.

REGULAR CLUSTER nebulae” in 1785, this is one of the the cluster’s centre lie the giant elliptical
nearest highly evolved or “relaxed” NGC 4889 and the lenticular galaxy NGC
Coma Cluster galaxy clusters (see p.317). It is very 4874. Most of the spirals and irregulars are in
dense, with over 3,000 galaxies, and is the outer regions. X-ray images show two
CATALOGUE NUMBER dominated by elliptical and lenticular distinct patches of cluster gas, suggesting that
galaxies. Because it is near the north the cluster is absorbing a smaller
Abell 1656 galactic pole (and therefore free of the cluster of galaxies. Like the
dense star fields of the Milky Way), it Virgo and Hydra clusters,
DISTANCE is well studied. Swiss-American Coma forms the core of its
astronomer Fritz Zwicky used Coma own galaxy supercluster.
300 million light-years when he made the first measurements
of galaxy movements within a cluster COMA ELLIPTICAL
NUMBER OF GALAXIES in the 1930s. He found the cluster This image is dominated
contained many times more mass than by the Coma Cluster
3,000+ its visible galaxies suggested – an idea elliptical NGC 4881
that was not accepted until the 1970s. and a nearby spiral.
COMA BERENICES BRIGHTEST MEMBER Overall, the cluster is moving away at The other galaxies
25 million kph (16 million mph). At are far more distant.
NGC 4889 (13.2)

Although it lies near the Virgo Cluster
in the sky (see p.319), the Coma
Cluster is much farther away. First
recognized by William Herschel (see
p.90) as a concentration of “fine

galaxy clusters

IRREGULAR CLUSTER stage of development. In keeping with REGULAR CLUSTER GEORGE ABELL
the best models of such clusters’
Hercules Cluster formation (see p.317), it shows little Abell 1689 George Abell (1927–1983) was a
sign of structure.Within the cluster, career astronomer and popularizer
HERCULES CATALOGUE NUMBER several pairs or groups of galaxies VIRGO CATALOGUE NUMBER of science who carried out the
seem to be merging or interacting – first, and most influential, survey
Abell 2151 encounters that will transform them Abell 1689 of galaxy clusters. After working
into different kinds of galaxies and on the Palomar Sky Survey during
DISTANCE reduce their random movements until DISTANCE the 1940s and 1950s, using the
they become more evenly distributed. powerful Palomar Schmidt
500 million light-years The most prominent of these mergers 2.2 billion light-years telescope, he turned his attention
is NGC 6050, a pair of interlocking to analysing the results, developing
NUMBER OF GALAXIES spiral galaxies near the cluster’s centre NUMBER OF GALAXIES methods for distinguishing galaxy
that may eventually form the core of clusters from isolated field galaxies,
100+ a giant elliptical, such as those found 3,000+ and classifying clusters into types.
in more evolved clusters.
BRIGHTEST MEMBER BRIGHTEST MEMBER

NGC 6041A (14.4) Unnamed galaxy (17.0)

The small Hercules Cluster is Abell 1689 is one of the densest galaxy
dominated by spiral and irregular clusters known, with thousands of
galaxies, suggesting that it is in an early galaxies packed into a volume of space
only 2 million light-years across. Its ball
shape makes it a fine gravitational lens,
bending the images of distant galaxies
into arcs. By noting the lensing power
throughout the cluster, astronomers
have worked out the distribution of
the cluster’s dark matter.

HERCULES FIELD
This wide-field view captures most of the
bright galaxies in Hercules and shows
their irregular, “unrelaxed” distribution.

LENSING IN CLUSTER ABELL 1689

REGULAR CLUSTER IRREGULAR CLUSTER X-ray images reveal what optical ones them through the intergalactic
cannot – that the cluster is forming medium. A fainter cloud of almost
Abell 2065 Abell 2125 from the merger of several smaller equal size, enveloping hundreds more
clusters.The most intense cloud of galaxies, has remarkably few heavy
CATALOGUE NUMBER CATALOGUE NUMBER X-ray emitting gas shows “clumpiness”, elements, suggesting that the gas-
which indicates it has recently come stripping process becomes more
Abell 2065 Abell 2125 together. Spectra reveal that the cloud powerful and thorough over time,
is enriched with heavy elements such and that the cloud is much younger
DISTANCE DISTANCE as iron, and close-up images show gas than its fainter neighbour.
actively being stripped away from
1 billion light-years 3 billion light-years galaxies such as C153.With it, the gas Since X-ray evidence shows so
carries atoms of heavy metals created much activity within the cluster,
NUMBER OF GALAXIES NUMBER OF GALAXIES in supernova explosions, distributing astronomers have also imaged it at
other wavelengths. Infrared telescopes,
1,000+ 1,000+
for example, have revealed
BRIGHTEST MEMBER URSA MINOR BRIGHTEST MEMBER enormous bursts of star
formation going on in galaxies
CORONA BOREALIS PGC 54876 (16.0) Magnitude 17.0 far from the cluster centre.
One possible explanation is
Abell 2065, also known as the Corona Abell 2125 has been the subject of that, even at distances of up to BEYOND THE MILKY WAY
Borealis Cluster, contains 400 or more intense scrutiny from the orbiting 1 million light-years, the tidal
large galaxies. A highly evolved cluster Chandra X-ray Observatory.The forces from the centre of a
like the Coma Cluster (opposite), it cluster lies close enough to Earth to large cluster are enough to
emits X-rays from a diffuse cloud of see detail, but so far away that images disrupt nearby galaxies and
hot gas. However, X-ray observations reaching Earth show an early and still trigger starbursts.
have found two distinct X-ray cores, active phase of its evolution, 3 billion
suggesting that Abell 2065 may be years ago. Abell 2125 is therefore ideal
two already ancient clusters merging for testing ideas on cluster formation.
together.The cluster lies at the centre
of the Corona Borealis Supercluster.

THE CORONA BOREALIS CLUSTER ZOOMING IN ON C153
This sequence of Chandra
X-ray images zooms into the
hot gas cloud at the core of
Abell 2125, showing how
gas is being stripped from
galaxy C153 (right).

BEYOND THE MILKY WAY REGULAR CLUSTER The cluster has taught astronomers The galaxies beyond Abell 2218 lie
much about galaxy clusters, and about much farther away, and therefore, their
Abell 2218 galaxies themselves.The cluster’s images come from a much earlier
density is so great that it affects the time. Most of the lensed galaxies are
CATALOGUE NUMBER shape of the surrounding space, as blue-white, suggesting they are young
predicted by Einstein’s theory of irregulars and spirals very different
Abell 2218 general relativity (see p.40). Many from Abell 2218’s own aged ellipticals.
more distant galaxies lie directly Some of the lensed galaxies align with
DISTANCE behind the cluster, and as light rays X-ray sources, suggesting they are
from these objects pass close to Abell active galaxies. Recent studies yielded
2 billion light-years 2218 their paths are deflected and images of a galaxy so far beyond Abell
focused towards Earth, in the same 2218 that all its light has been red-
NUMBER OF GALAXIES way that a magnifying lens focuses
sunlight.This gravitational lensing HOLE IN THE COSMIC BACKGROUND
250 or more (see p.317) brightens the images of In this composite image of Abell 2218, yellow
galaxies that would otherwise be too and red depict the X-ray-emitting gas around
DRACO BRIGHTEST MEMBER far away to detect. It results in a series its core. The gas scatters the cosmic
of distorted images of distant galaxies microwave background radiation, creating
Unnamed galaxy (17.0) ringing the centre of Abell 2218. a hole, outlined here by contours.

Abell 2218 is a spectacular example of
a highly evolved and extremely dense
galaxy cluster. It contains more than
250 mostly elliptical galaxies in a
volume of space roughly 1 million
light-years across.

DISTORTED BY GRAVITY
Most of the bright objects in this image
are galaxies in the Abell 2218 cluster. The
arcs are much more remote galaxies, their
images distorted by Abell 2218’s gravity.

EXPLORING SPACE BEYOND THE MILKY WAY

MAPPING THE MISSING MASS

shifted into the infrared part of the Astronomers have now begun to use spikes The total mass of a cluster can be up to five
spectrum. At the time, it was the most Abell 2218 to probe the origins of coincide times that of its visible galaxies, but the
distant galaxy known, at 13 billion the Universe. A phenomenon called with distribution of the other, dark matter was
light-years from Earth. It must have the Sunyaev–Zel’dovich effect (see galaxies a mystery until recently. Gravitational
formed shortly after the first stars, in caption, opposite) creates holes and lensing now allows astronomers to
the aftermath of the Big Bang. ripples in the cosmic microwave measure the missing mass in clusters.
background radiation shining through By analysing images of lensed
Gravitational lensing can also the cluster.This happens because gas galaxies, astronomers can
reveal hidden properties of Abell 2218 around Abell 2218’s core scatters pinpoint concentrations
itself. Because the strength of lensing photons of microwave radiation, just of mass distorting
depends on the cluster’s density, it as Earth’s atmosphere scatters light. the light as it
offers a measure of the distribution of The strength of these ripples passes through
all matter in the cluster – including can be used to estimate the the cluster.
the dark matter. Abell 2218 is one of true diameter of the cluster’s core,
the few galaxy clusters in which the and therefore its distance from Earth, cluster gas MAP OF CLUSTER CL0024+1654
pattern of visible matter (galaxies and independently of its red shift.The red and dark matter This mass map shows the difference
X-ray-emitting gas) and the calculated shift and distance can then be used appear as a broad hump in distributions of visible and dark
distribution of dark matter do not together to find the expansion rate around the cluster’s core matter in a mature galaxy cluster.
match, suggesting the cluster is not as of the Universe (see p.42).
uniform as it appears in visible light.

324 galaxy superclusters

GALAXY SUPERCLUSTERS

22–23 The scale of the Universe THE LARGEST SCALE OF STRUCTURE in the Universe is
24–27 Celestial objects that of galaxy superclusters – collections of neighbouring
28–31 Matter galaxy clusters bunched together in chains and sheets
32–35 Radiation that stretch across the cosmos.This structure is an echo
316–17 Galaxy clusters of the Big Bang itself. By studying the Universe at these
enormous scales, astronomers can learn about our place
within it and the way in which it formed.

GALAXY SUPERCLUSTERS

Just as galaxies are bound by gravity into clusters, galaxy clusters are linked to form
even larger structures called superclusters. A chain of galaxy clusters links the Local
Group of galaxies, containing the Milky Way, to the Virgo Cluster, 52 million light-
years away.This is the heart of the Local Supercluster, or Virgo Supercluster. As in
other superclusters, the galaxy clusters are not discrete, but merge at their edges.
The entire supercluster stretches over 200 million light-years, itself merging with
others at the edges.Within a supercluster, each cluster is an evolving, collapsing
knot of gravity. Even on this vast scale, the gravity of superclusters resists the
background cosmological expansion of space (see pp.42–43).The Virgo
Cluster’s gravity is counteracting cosmic expansion, pulling other clusters
towards it at 5.4 million kph (3.4 million mph).

Sculptor Group Local Group Virgo Cluster Virgo III groups

Fornax Cluster Maffei group circle is 200 million Leo II groups THE LOCAL SUPERCLUSTER EXPLORING SPACE
light-years across This map of the Virgo
Supercluster is centred on the MAPPING THE
Local Group and shows groups
and clusters of galaxies linked UNIVERSE
into a curved, branched chain.
Each point denotes a major A project, currently underway, is
galaxy – there are thousands charting the large-scale structure
of smaller ones not pictured. of the Universe to unprecedented
depth.The Sloan Digital Sky
PLOT OF GALAXIES THE NEARBY UNIVERSE Survey uses a telescope (below)
This deep-sky plot, made with a wide field of view ideal for
BEYOND THE MILKY WAY in 1986, shows a wedge Astronomers map the Universe by measuring red shifts in imaging large areas of the sky at
of sky out to a distance once. In total, it will measure the
of 1 billion light-years. the spectral lines of millions of galaxies (see p.33). If the red positions and red shifts of one
It shows how galaxies million nearby galaxies and
cluster on the largest shifts are due to galaxies moving apart as space expands (see 100,000 quasars,
scale. The “Stickman” building up a
figure is in fact the p.42), they can be used as a measure of distance. Astronomers new map of
Coma Supercluster. the cosmos.
often use red-shift values instead of light-years for vast

distances.They adjust the red-shift distance estimates if they

know of large-scale motions of superclusters and clusters not

due to cosmological expansion.The entire Virgo Supercluster,

for example, is falling towards a mysterious concentration

of mass called the Great Attractor at 2.2 million kph (1.4

THE GREAT ATTRACTOR million mph). Maps of the Universe
This view of the sky in
out to a few hundred million light-

Centaurus looks towards years reveal that galaxies cluster into

the “Great Attractor”, strands, and holes appear between the

which is probably a strands. As the maps reach further,
massive supercluster these features turn out to be repeated
centred on the Norma across the entire cosmos, appearing
Cluster but hidden by the same in all directions.
the southern Milky Way.

galaxy superclusters 325

LARGE-SCALE STRUCTURE FILAMENTS AND VOIDS
This is a computer model of the large-scale structure
of the Universe at an age of 2 billion years, showing At the largest scales measured, the Universe reveals a clear general pattern.
the concentration of matter into filaments. Matter
in the filaments is dense enough to form galaxies, Galaxy superclusters join to form string-like “filaments” or flat “sheets”,

within which stars ignite. Clusters of galaxies surrounding enormous and apparently empty regions called “voids”.While
are concentrated at the nodes, where
filaments meet, but chainlike structure up to the level of clusters can be explained by the action of gravity
superclusters are strung out
along the filaments. Yellow since the Big Bang, the present age of the Universe (13.7 billion years) is not
denotes the densest
matter, followed by nearly long enough for gravity alone to have organised the Universe on the
red, then blue,
and then black. scale of filaments and voids. Studies of the cosmic background radiation

(CMBR; see p.50) suggest that filaments and voids are an expanded echo of

features from the earliest times.The CMBR shows ripples in the temperature

and matter distribution of the Universe when it was just 300,000 years old –

irregularities that were seeded in the Big Bang itself.The ripples of matter

created “rivers” of gravity into which

matter fell, forming filaments when

the Universe was still very small.

Superclusters condensed out of the

densest parts of the filaments, while

clusters and galaxies

formed within them dense filament
of galaxies
from the “bottom

up” (see p.298). Earth

LOOKING DEEPER
The first results from the
Sloan Digital Sky Survey
plot the position of more
than 200,000 galaxies in
two wedge-shapes (far
right) centred on Earth.
The inset picture shows the
location of an individual
galaxy cluster.

GALAXY CLUSTER

BETWEEN THE SUPERCLUSTERS

For decades, astronomers have speculated that the voids between galaxy

superclusters might not be empty.The voids may hold matter that is

both transparent and non-luminous, including some of the Universe’s

mysterious dark matter. Astronomers have now discovered invisible

clouds of hydrogen, detectable only by their effect on the spectra of

distant quasars. As light travelling from these quasars passes through

hydrogen clouds, each cloud imprints an absorption line (called a Lyman

Alpha line) onto the spectrum at a characteristic wavelength.The

spectrum of light shifts to the red as it travels through expanding space

before it enters the next cloud.The result is a series of lines red-shifted

across the spectrum – the “Lyman Alpha

BETWEEN THE CLUSTERS forest”. By decoding the red shift of the BEYOND THE MILKY WAY
This computer model shows hydrogen lines, astronomers can compute the distance
clouds between galaxy clusters in the of the cloud that produced each one, and
early Universe. The cube is 30 million perhaps map out the distribution of clouds
light-years along each side. The clouds are between the galaxy clusters.
thought to be more sparsely distributed today.

QUASAR INTERGALACTIC CLOUD photons INTERGALACTIC CLOUD EARTH LYMAN ALPHA
wavelength
intensity peak radiation red-shifted peak FOREST
from quasar absorption by cloud introduces line Light travelling from a
distant quasar passes
red-shifted line through a series of
absorption by cloud introduces another line hydrogen clouds. Each
superimposes an
absorption line onto
the quasar’s spectrum,
but red shifts mean the
lines do not overlap.
The result is a series of
red-shifted lines called
a Lyman Alpha forest.

red-shifted lines
building up into a forest



the night sky

THE NIGHT SKY

“Why did not somebody teach me the constellations,
and make me at home in the starry heavens,
which are always overhead, and which I don’t
half know to this day?”

Thomas Carlyle

THE HUMAN EYE HAS ALWAYS seen patterns
among the stars. Ancient peoples traced the figures
of gods, heroes, and mythical animals onto the
skies and used the relationship between these
constellations to illustrate their myths and legends.
In most cases, stars within a constellation lie in the
same region of sky merely by chance, however, and
are not related. Despite the apparent permanence
of the skies, these patterns are not fixed, because
all the stars are moving relative to the Earth. Over
time, the shape of all the constellations will change,
and hundreds of thousands of years from now, they
will be unrecognizable. Future generations will
need to invent constellations of their own. But for
now, 88 constellations fill our sky, interlocking like
pieces of an immense jigsaw puzzle. Some are large,
others small, some richly stocked with objects of
note, others faint and seemingly barren. All are
featured in the following pages.

PATTERNS IN THE SKY
As darkness falls, a stargazer scans the sky with
binoculars. The familiar shape of the Plough
looms overhead, part of the constellation Ursa
Major, the Great Bear. The north pole star,
Polaris, can be seen high up on the right.

THE CONSTELLATIONS

the constellations

THE HISTORY OF CONSTELLATIONS

7 66–67 Star motion and patterns THE FIRST CONSTELLATIONS were patterns of stars that ancient
72–73 Naked-eye astronomy peoples employed for navigation, timekeeping, and storytelling.
82–83 Ancient astronomy Recently, the pictorial aspect of constellations has become less
significant, and they have become simply delineated regions of
8 85 Arabic astronomy the sky, although the attraction of the myths and legends remains.

EARLY CONSTELLATION LORE ANTICANIS
This page from a 9th-century edition of
The constellation system used today stems from patterns recognized by the star myths of Hyginus shows the
ancient Greek and Roman civilization.The earliest surviving account constellation Canis Minor, here termed
of ancient Greek constellations comes from the poet Aratus of Soli Anticanis. Hyginus’s words, in Latin, form
(c.315–c.245 BC). His poem, the Phaenomena, written around 275 BC, the shape of the dog’s body.
describes the sky in storybook fashion and identifies 47 constellations.
It is based on a lost book of the same name by the
Greek astronomer Eudoxus (c.390–c.340 BC).
Eudoxus reputedly introduced the constellations to
the Greeks after learning them from priests in
Egypt.These constellations had been adopted from
Babylonian culture; they were originally created by
the Sumerians around 2,000 BC. However, the
Greeks attached their own myths to the
constellations detailed by Eudoxus, and Aratus’s
storybook of the stars proved immensely popular.
Sometime in the 2nd century AD, it was joined by a
more elaborate work of constellation lore called
Poetic Astronomy, written by the Roman author
Hyginus. Many editions of both these works were
produced and translated over the centuries.

FILLING THE HEAVENLY SPHERE

The oldest surviving star catalogue dates from the 2nd century AD

and is contained in a book called the Almagest, written by the Greek

astronomer and geographer Ptolemy (see p.85). It records the

positions and brightnesses of one thousand stars, arranged into

48 constellations, based on an earlier catalogue by Hipparchus

of Nicaea (c.190–c.120 BC). In the 10th century AD, an Arab

astronomer, al-Sufi (see p.405), updated the Almagest in his Book

of Fixed Stars, which included Arabic names for many stars.These

Arabic names are still used today, although often in corrupted form.

No more constellations were introduced until the end

of the 16th century, when Dutch explorers sailed

to the East Indies. From there, they could observe

the southern sky that was below the European

horizon.Two navigators, Pieter Dirkszoon

Keyser and Frederick de Houtman (see

p.400), catalogued nearly 200 new

southern stars, from which they

and their mentor, Petrus

Plancius (see p.342), a leading

Dutch cartographer, created 12

new constellations. Plancius also

THE NIGHT SKY created other northern

constellations, forming them

between those listed by Ptolemy.

Nearly a century later, Johannes

Hevelius (see p.368), a Polish

astronomer, filled the remaining GLOBAL COVERAGE
This beautiful celestial globe
gaps in the northern sky, and in was made around 1625 by
Arnold van Langren, a celebrated
POCKET GLOBE the mid-18th century, the French Dutch globe maker. As with all
This pocket globe from the National Maritime astronomer Nicolas Louis de celestial globes, the figures are
Museum, England, positions the Earth within a Lacaille (see p.406) introduced shown reversed in comparison to
shell that represents the surrounding celestial another 14 constellations in the their appearance in the sky.
sphere. On the inside of the open shell are the southern sky.
constellations, painted as mirror images.

THE history of CONSTELLATIONS

STAR CHARTS AND ATLASES

The first printed star chart was produced in 1515 by the great

German artist Albrecht Dürer. Like a celestial globe,

Dürer’s chart depicted the constellations in reverse,

showing the sky as it would be seen from an

imaginary position outside the celestial

sphere, but before long, charts were being

made that could be compared directly

with the sky.The finest early star atlas

was Uranometria, produced in 1603

by the German astronomer

Johann Bayer (see panel, below). SKETCHY FIGURES

This atlas remains one of the Leo, the Lion, an easily recognizable
most beautiful examples of constellation of the zodiac, is here depicted
the celestial cartographer’s on the Atlas Coelestis, by English astronomer
art. Shortly after its John Flamsteed, published in 1729.

publication, astronomy was revolutionized by the invention of

the telescope.The first major star catalogue and atlas of this

new era was produced by England’s first Astronomer Royal,

John Flamsteed (1646-1719). Atlas Coelestis shows the

Ptolemaic constellations visible from Greenwich, England,

based on Flamsteed’s own painstaking observations.The

pinnacle of celestial mapping came in 1801 when

Johann Bode, a German astronomer, published an

atlas called Uranographia. Covering the entire sky,

this atlas depicted over 100 constellations, some

invented by Bode himself. Finally, in 1922,

a list of 88 constellations was agreed upon

by the International Astronomical Union,

astronomy’s governing body, which

also defined the boundaries of each

constellation. On modern star charts,

the only sign of the traditional

pictorial charts are the few lines that

link the main stars, suggesting the

overall shape of each constellation.

JOHANN BAYER THE NIGHT SKY

Johann Bayer (1572–1625) was a
German lawyer and amateur
astronomer who produced the first
major printed star atlas, called
Uranometria, in 1603. As well as
Ptolemy’s 48 constellations, it
included 12 recently introduced
southern constellations. It was the
first atlas to include the entire sky.
On his charts, Bayer introduced
the convention of labelling the
brightest stars in each constellation
with Greek letters.These are now
known as Bayer letters (see p.68).

BAYER’S BEASTS
Cygnus, the swan, is seen flying along the
Milky Way on this hand-coloured chart
from Bayer’s Uranometria.

HEAVENLY
PICTURE BOOK
Ancient people
imagined gods, heroes,
and beasts among the stars,
and these figures were
depicted on star charts until the
19th century. These charts, from John
Flamsteed’s Atlas Coelestis (1729), show
those 48 constellations known to the
ancient Greeks depicted on the northern and
southern halves of the sky.

THE CONSTELLATIONS

MAPPING THE SKY celestial
coordinates

constellation

The following pages divide the celestial sphere into six parts – two border

polar regions and four equatorial regions – which show the deep-sky

location of the 88 constellations. Each constellation is then profiled object

in the following section. Each entry places the constellation and its linking lines join
up constellation
main features into the context of the rest of the sky. figure

VISIBILITY MAPS CONSTELLATION CHARTS

Not visible 80°N Each of the 88 constellation entries has its own chart, DEEP-SKY OBJECTS
60°N centred around the constellation area. These charts
Partially show all stars brighter than magnitude 6.5. Within the Galaxy
visible 40°N constellation borders, every star brighter than magnitude 5 is
20°N labelled. Deep-sky objects are represented by an icon. Globular cluster
Visible 0°
20°S KEY TO STAR MAGNITUDES Open cluster
40°S
60°S -1.5–0 0–0.9 1.0–1.9 2.0–2.9 3.0–3.9 4.0–4.9 5.0–5.9 6.0–6.9 Diffuse nebula

The entry for each constellation contains a map showing Planetary nebula or
the parts of the world from which it can be seen. The entire supernova remnant
constellation can be seen from the area shaded black, part is
visible from the area shaded grey, and it cannot be seen from the Black hole or
area shaded white. Exact latitudes for full visibility are given X-ray binary
in the accompanying dataset.

THE NORTH
POLAR SKY

THE NIGHT SKY Almost in the
centre of this chart
is the star Polaris, in
Ursa Minor, which
lies less than 1° from
the north celestial
pole. For observers
in the northern
hemisphere, the stars
around the pole never
set – they are circumpolar.
The viewer’s latitude will
determine how much of the
sky is circumpolar: the farther
north, the larger the circumpolar
area.This chart shows the sky from
declinations 90° to 50°.

STAR MAGNITUDES

-1 0 1 2 3 4 5 Variable star

Star magnitudes shown here are for the equatorial and polar sky charts

Mapping the sky

THE GREEK ALPHABET VISIBILITY ICONS

On most star charts, Alpha α Eta η Nu ν Tau τ Beside every photograph is an icon indicating the kind 2 Naked eye
bright stars are identified Beta β Theta θ Xi ξ Upsilon υ of view it illustrates. Some photographs show the star 1 Binoculars
by Greek letters Gamma γ Iota ι Omicron ο Phi ϕ or deep-sky object as it can be seen by the naked eye, 5 Telescope (amateur)
according to a system Delta δ Kappa κ Pi π Chi χ through binoculars, or through amateur telescopes. 4 CCD
invented by Johann Epsilon ε Lambda λ Rho ρ Psi ψ Others are the result of CCD photography or show the 3 Professional equipment
Bayer (see p.331). Zeta ζ Mu μ Sigma σ Omega ω view through professional observing equipment.

ALPHABETICAL INDEX OF THE 88 CONSTELLATIONS

The constellation entries are ordered by their position on Corona Borealis p.363 Hydra p.378 Norma p.398 Sagittarius p.384
the celestial sphere, beginning with Ursa Minor in the Corvus p.381 Hydrus p.403 Octans p.409
north and spiralling south in a clockwise direction, before Crater p.381 Indus p.400 Ophiuchus p.365 Scorpius p.386
finishing with Octans. This alphabetical list provides an Crux p.396 Lacerta p.353 Orion p.374
alternative way of locating constellation entries. Cygnus p.350 Leo p.361 Pavo p.408 Sculptor p.388
Delphinus p.369 Leo Minor p.360 Pegasus p.370
Dorado p.405 Lepus p.391 Perseus p.354 Scutum p.366
Draco p.339 Libra p.363 Phoenix p.401
Equuleus p.369 Lupus p.383 Pictor p.404 Serpens (Caput
Eridanus p.390 Lynx p.343 Pisces p.372
Fornax p.389 Lyra p.349 Piscis Austrinus p.388 and Cauda) p.364
Gemini p.358 Mensa p.406 Puppis p.393
Andromeda p.352 Canis Major p.376 Grus p.401 Microscopium p.387 Pyxis p.392 Sextans p.380
Antlia p.380 Canis Minor p.376 Hercules p.348 Monoceros p.377 Reticulum p.404
Apus p.407 Capricornus p.387 Horologium p.403 Musca p.397 Sagitta p.366 Taurus p.356
Aquarius p.371 Carina p.395
Aquila p.367 Cassiopeia p.341 Telescopium p.400
Ara p.399 Centaurus p.382
Aries p.355 Cepheus p.340 Triangulum p.353
Auriga p.343 Cetus p.373
Boötes p.347 Chamaeleon p.407 Triangulum Australe p.398
Caelum p.389 Circinus p.397
Camelopardalis p.342 Columba p.392 Tucana p.402
Cancer p.359 Coma Berenices p.360
Canes Venatici p.346 Corona Australis p.399 Ursa Major p.344

Ursa Minor p.338

Vela p.394

Virgo p.362

Volans p.406

Vulpecula p.368

THE SOUTH
POLAR SKY

There is no THE NIGHT SKY
southern equivalent
of Polaris, the north
pole star – in fact,
the area around the
south celestial pole is
remarkably barren.
This chart shows the
sky from declinations
-50° to -90°. Many of
the stars on this chart are
circumpolar for southern
observers – that is, the stars
never set and are always visible
in the night sky.The farther
south the viewer, the greater the
amount of sky that is circumpolar.

STAR MAGNITUDES

-1 0 1 2 3 4 5 Variable star

Star magnitudes shown here are for the equatorial and polar sky charts

the constellations

EQUATORIAL SKY CHART 1

This part of the sky is best placed for observation on evenings in September, October, and
November. It contains the vernal equinox, in Pisces, which is the point at which the Sun’s
path, the ecliptic, crosses the celestial equator into the northern half of the sky.The Sun
reaches this point in late March each year.The 0h line of right ascension also passes through
this point; this is the celestial equivalent of 0° longitude (the prime meridian) on Earth.The
most distinctive feature in this region of the night sky is the great Square of Pegasus –
although one star in the square actually belongs to neighbouring Andromeda.

THE NIGHT SKY STAR MAGNITUDES

-1 0 1 2 3 4 5 Variable star

Star magnitudes shown here are for the equatorial and polar sky charts

mapping the sky

EQUATORIAL SKY CHART 2

This area of sky is best placed for observation on evenings in June, July, and August. It contains
the point where the Sun reaches its most southerly declination each year, in Sagittarius.This
happens around 21 December, which is the longest day in the southern hemisphere and the
shortest day in the northern. Rich Milky Way star fields cross this region of sky, from Cygnus
in the north to Sagittarius and Scorpius in the south. Hercules and Ophiuchus, both
representing mythical giants, stand head to head in the north. Notable star patterns in the
south are the Teapot asterism in Sagittarius and the curving tail of Scorpius, the Scorpion.

STAR MAGNITUDES THE NIGHT SKY

-1 0 1 2 3 4 5 Variable star

Star magnitudes shown here are for the equatorial and polar sky charts

the constellations

EQUATORIAL SKY CHART 3

This region is best placed for observation on evenings in March, April, and May. It contains
the point at which the Sun moves across the celestial equator into the southern hemisphere
each year.This point lies in Virgo, and the Sun reaches it around 21 September. In the
northern constellation Boötes lies Arcturus, a notably orange-coloured star whose visibility
marks the arrival of northern spring. South of it is the zodiacal constellation of Virgo, whose
brightest star is the blue-white Spica. Adjoining Virgo is Leo, one of the few constellations
that genuinely resembles the animal it is said to represent – in this case, a crouching lion.

THE NIGHT SKY STAR MAGNITUDES

-1 0 1 2 3 4 5 Variable star

Star magnitudes shown here are for the equatorial and polar sky charts

mapping the sky

EQUATORIAL SKY CHART 4

This region is best placed for observation on December, January, and February evenings.
It contains the point at which the Sun is farthest north of the celestial equator, on the border
of Taurus with Gemini.This occurs around 21 June, when days are longest in the northern
hemisphere and shortest in the southern. Glittering stars and magnificent constellations
abound in this region of sky, including the brightest star of all, Sirius in Canis Major. A
distinctive line of three stars marks the belt of Orion, while in Taurus the bright star
Aldebaran glints like the eye of the bull, along with the Hyades and Pleiades star clusters.

STAR MAGNITUDES THE NIGHT SKY

-1 0 1 2 3 4 5 Variable star

Star magnitudes shown here are for the equatorial and polar sky charts

LONG-TAILED BEAR 2
The tail of the Little Bear curves away
from the north Pole Star, Polaris (upper
left). Unlike real bears, the celestial
bears Ursa Minor and Ursa Major both
have long tails.

THE NIGHT SKY THE LITTLE BEAR THE NORTH POLE STAR 5 THE
Seen through a small telescope, Polaris LITTLE BEAR
Ursa Minor appears to have a faint companion (right), but
this background star is unrelated. Its true MYTHS AND STORIES
SIZE RANKING 56 companion is seen here just below Polaris.
NURSING NYMPHS
BRIGHTEST STAR SPECIFIC FEATURES
Polaris, the north Pole Star, is a According to Greek mythology,
Polaris (α) 2.0 creamy white supergiant and a at his birth, the infant Zeus
Cepheid variable (see p.278), but its was hidden from his murderous
GENITIVE brightness changes are too slight to be father, Cronus, and taken to a
noticeable to the naked eye.With a cave on the island of Crete,
Ursae Minoris telescope, an unrelated 8th-magnitude where he was nursed by two
star can be seen nearby. nymphs, usually named as
ABBREVIATION UMi Adrastea and Ida. In
Two stars in the bowl of the Little gratitude, Zeus later placed
HIGHEST IN SKY AT 10PM Dipper – Gamma and Eta Ursae the nymphs in the sky as
Minoris – are both wide doubles. the Great Bear and the
May–July Gamma is the brighter of the two, at Little Bear, respectively.
magnitude 3.0, and its 5th-magnitude
FULLY VISIBLE companion, 11 Ursae Minoris, can THE PROTECTED CHILD
be seen with the naked eye or The infant Zeus is cared for by
90°N–0° binoculars. Eta – at magnitude 5.0 – nymphs and shepherds, in the
can also be seen with the naked eye. Feeding of Jupiter by the French
Ursa Minor is an ancient Greek It has a partner of magnitude 5.5, 19 artist Nicolas Poussin.
constellation, which is said to Ursae Minoris; both stars are easily
represent Ida, one of the nymphs who visible with binoculars. Each of the
nursed the god Zeus when he was an component stars in both Gamma and
infant (see panel, right). Ursa Minor Eta lie at different distances from the
contains the north celestial pole and Earth and, hence, are unrelated.
also its nearest naked-eye star, Polaris
or Alpha (α) Ursae Minoris (see
pp.274–75), which is currently less
than one degree from the north
celestial pole.The distance between
them is steadily decreasing due to
precession (see p.60).They will come
closest around 2100, when the
separation will be about 0.5 degrees.

The main stars of Ursa Minor
form a shape known as the Little
Dipper, reminiscent of the larger and
brighter Big Dipper in Ursa Major,
although its handle curves in the
opposite direction.The two brightest
stars in the bowl of the Little Dipper,
Gamma (γ) and Eta (η) Ursae
Minoris, are popularly known as the
Guardians of the Pole.

THE DRAGON and is considered to be among the BEAR AND DRAGON 2 the constellations
finest doubles visible with binoculars. The long body of Draco
Draco Psi (ψ) Draconis is a somewhat closer curls around the stars of MYTHS AND STORIES
pair, with components of 5th and 6th Ursa Minor, the Little Bear.
SIZE RANKING 8 magnitudes, and requires a small The head of the dragon is HERCULES AND
telescope to be divided. More easily identifiable. THE DRAGON
BRIGHTEST STAR challenging to discern is Mu (μ)
Draconis, with its two 6th-magnitude The dragon Ladon guarded the
Etamin (γ) 2.2 stars, which requires a telescope golden apples that grew on Mount
GENITIVE Draconis with high magnification to be seen Atlas in the garden of Hera, wife
as double. of Zeus. As his twelth labour, the
ABBREVIATION Dra hero Hercules was required to steal
The wide pair of stars 16 and 17 some apples.To get to them, he
HIGHEST IN SKY AT 10PM Draconis is easily spotted with killed the dragon with a poisoned
binoculars, and the brighter of the arrow. Hera placed the dragon in
April–August two – 17 Draconis – can be further the sky as the constellation Draco.
divided with a small telescope with
FULLY VISIBLE high magnification, turning this into DRAGON KILLER
a triple star. A similar triple is 39 In this 16th-century painting by the Italian
90°N–4°S Draconis; when viewed with a small artist Lorenzo dello Sciorina, Hercules is
telescope with low magnification, depicted slaying the dragon by hand.
One of the ancient Greek it appears a double but at higher
constellations, Draco represents the magnification the brighter star divides
dragon of Greek myth that was slain into a closer pair with components of
by Hercules (see panel, below).This magnitudes 5.0 and 8.0.Two more
large constellation winds for nearly doubles that can readily be seen with
180 degrees around the north celestial a small telescope are Omicron (ο)
pole. Despite its size, Draco is not Draconis, with stars of 5th and 8th
particularly easy to identify, apart from magnitudes, and 40 and 41 Draconis,
a lozenge shape marking the head. which are both 6th-magnitude
This is formed by four stars, including orange dwarfs.
the constellation’s brightest member,
Gamma (γ) Draconis, popularly In central Draco lies a planetary
known as Etamin or Eltanin. nebula made famous by a striking
Hubble Space Telescope image: NGC
SPECIFIC FEATURES 6543, or the Cat’s Eye Nebula (see
Double and multiple stars are a p.254). Processed in false colour, the
particular feature of Draco. Nu (ν) Hubble picture shows the nebula as
Draconis, the faintest of the four stars red, but when seen through a small
in the dragon’s head, is a readily telescope it appears blue-green, as
identifiable pair. It consists of identical do all planetary nebulae.
white components of 5th magnitude

THE CAT’S EYE NEBULA 54
This amateur CCD image of NGC 6543 shows
some of the colour and structure captured by the
Hubble Space Telescope, but visually
the nebula appears as a
blue-green
ellipse.

THE DRAGON THE NIGHT SKY

the constellations

CEPHEUS

Cepheus

SIZE RANKING 27

BRIGHTEST STAR

Alpha (α) 2.5
GENITIVE Cephei

ABBREVIATION Cep

HIGHEST IN SKY AT 10PM

September–October

FULLY VISIBLE

90°N–1°S

Cepheus lies in the far northern IC 1396 4
sky between Cassiopeia and Draco. The Garnet Star or Mu Cephei (top left) lies
Its main stars form a distorted tower on the edge of the large but faint nebula
or steeple shape, yet this ancient IC 1396. The nebula is centred on the 6th-
Greek constellation in fact represents magnitude multiple star Struve 2816.
the mythical King Cepheus of
Ethiopia, who was the husband of These changes can be followed with
Queen Cassiopeia and the father the naked eye. Delta (δ) Cephei is
of Andromeda. Cepheus is not a also a double star; its 6th-magnitude,
particularly prominent constellation. blue-white companion is visible
through a small telescope.
SPECIFIC FEATURES
The constellation’s most celebrated A significant variable star of a
star is Delta (δ) Cephei (see p.282), different kind is Mu (μ) Cephei,
from which all Cepheid variables take which is a red supergiant that ranges
their name. Just under 1,000 light- anywhere between magnitudes 3.4
years away, this yellow-coloured and 5.1 every two years or so.This
supergiant varies between magnitudes supergiant is also known as the
3.5 and 4.4 every five days nine hours. Garnet Star on account of its strong
red coloration.

Non-variable stars near Delta (δ)
and Mu (μ) Cephei can be used to
gauge the magnitude of these two
variable stars at any given time. For
example, they can be compared to
Zeta (ζ) at magnitude 3.4, Epsilon (ε)

at magnitude 4.2, or Lambda (λ)
Cephei at magnitude 5.1
(see chart, below).

THE KING 2 HENRIETTA LEAVITT
Shaped like a bishop’s mitre,
Cepheus is not easy to pick out Henrietta Swan Leavitt (1868-
in the sky. He is flanked by his 1921) worked at Harvard College
prominent wife, Cassiopeia, and Observatory in the early 20th
Draco, the dragon. century. Her study of variable stars
in the Small Magellanic Cloud led
CEPHEUS to the period-luminosity law.This
law links the variation period of a
DELTA (δ) AND MU (μ) CEPHEI MAGNITUDE KEY Cepheid variable to its intrinsic
brightness, which in turn can
THE NIGHT SKY 0.0–0.9 indicate distance. Her law remains
fundamental to our knowledge of
1.0–1.9 the scale of the Universe.

2.0–2.9 DETERMINATION
3.0–3.9 By painstakingly
4.0–4.9 measuring
5.0–5.9 photographic
6.0–6.9 plates, Henrietta
Leavitt discovered
2,400 variable
stars of all types.

the constellations

CASSIOPEIA Eta (η) Cassiopeiae is an M103 15 MYTHS AND STORIES
attractive stellar pair consisting M103’s main feature is a chain of three stars
Cassiopeia of a yellow and a red star. Its like a mini Orion’s belt. The northernmost THE VAIN QUEEN
components are of magnitudes member of the line (top) is not a true member
SIZE RANKING 25 3.5 and 7.5 and can be seen of the cluster but lies closer to Earth. Wife of Cepheus and mother of
through a small telescope.This Andromeda, Queen Cassiopeia was
BRIGHTEST STARS Shedir pair forms a true binary; the M52 15 notoriously vain. She enraged the
(α) 2.2, Gamma (γ) 2.2 fainter companion orbits the Through binoculars, this cluster appears as a Nereids, daughters of Poseidon, by
brighter star every 480 years. misty patch about one-third the diameter of boasting she was more beautiful. In
GENITIVE Cassiopeiae the full Moon. A telescope is needed to punishment, Poseidon sent a sea-
Cassiopeia contains a number of resolve its individual stars. monster to ravage her kingdom,
ABBREVIATION Cas open clusters within range of small which eventually led to the rescue
instruments. Chief among them is
HIGHEST IN SKY AT 10PM M52 (see p.286), near the border of Andromeda by
with Cepheus. It is visible through Perseus (see p.352).
October–December binoculars as a somewhat elongated
patch of light, and its individual stars ETERNAL VANITY
FULLY VISIBLE – including a bright orange giant at The boastful queen
one edge – can be seen through a is depicted sitting in
90°N–12°S small telescope. M103 is a small, a chair, fussing with
elongated group, best viewed through her hair. Cassiopeia
This distinctive constellation of the a small telescope. Nearby is a larger was condemned to
northern sky is found within the cluster, NGC 663, which is more circle the celestial
Milky Way between Perseus and suitable for binocular observation.
Cepheus and north of Andromeda. NGC 457 is a looser star cluster, pole, sometimes
The large W shape formed by its five containing the 5th-magnitude appearing to
main stars is easily recognizable. It is star Phi (ϕ) Cassiopeiae.This hang upside
an ancient Greek constellation, cluster’s appearance has been down in an
representing the mythical Queen likened to an owl – its two undignified
Cassiopeia of Ethiopia. brightest stars mark the manner.
owl’s eyes.
SPECIFIC FEATURES
Gamma (γ) Cassiopeiae (see p.281)
is a hot, rapidly rotating star that
occasionally throws off rings of gas
from its equator, which causes
unpredictable changes in its
brightness. It has ranged between
magnitudes 3.0 and 1.6, but it
currently lies at magnitude 2.2,
which makes it the equal-brightest
star in the constellation.

A variable with a more predictable
cycle is Rho (ρ) Cassiopeiae, an
intensely luminous, yellow-white
supergiant that fluctuates
between 4th and 6th
magnitudes every 10 or 11
months. It is estimated that
it lies more than 10,000
light-years away, which is
exceptionally distant for a
naked-eye star.

CASSIOPEIA

POLAR POINTER 2 THE NIGHT SKY
The distinctive W shape formed by the
main stars of Cassiopeia is easy to
locate in the sky. The centre of the W
points towards the north celestial pole.

the constellations

THE GIRAFFE SPECIFIC FEATURES KEMBLE'S CASCADE 1
The brightest star in the constellation, In an area five times the diameter of the full
Camelopardalis Beta (β) Camelopardalis, is a double Moon, the stars of Kemble’s Cascade seem
star whose fainter companion can be to tumble down the sky. The small star
SIZE RANKING 18 seen with a small telescope or even cluster NGC 1502 can be seen in the lower
powerful binoculars. South of Beta (β) left of the picture.
BRIGHTEST STAR is 11 and 12 Camelopardalis, a wide
double star with components of 5th
Beta (β) 4.0 and 6th magnitudes.

GENITIVE Within the giraffe’s hindquarters is
NGC 1502, a small open star cluster
Camelopardalis visible through binoculars or a small
ABBREVIATION Cam telescope. Binoculars also show a long
chain of faint stars called Kemble’s
HIGHEST IN SKY AT 10PM Cascade, which lead away from NGC
1502 towards Cassiopeia.This star
December–May feature is named after Lucian Kemble,
a Canadian amateur astronomer who
FULLY VISIBLE first drew attention to it in the late
1970s. None of the stars, however, are
90°N–3°S actually related.

This dim constellation of the far NGC 2403 is a 9th-magnitude
northern sky, representing a giraffe, spiral galaxy that looks like a comet
was introduced in the early 17th when seen through a small
century on a celestial globe created by telescope. It is one of the
the Dutch astronomer Petrus Plancius brightest and closest
(see panel, below).The giraffe’s long galaxies to the Earth,
neck can be visualized as stretching outside the Local Group.
around the north celestial pole
towards Ursa Minor and Draco.

THE GIRAFFE

THE NIGHT SKY PARTIAL VIEW 2 NGC 2403 54
It can be difficult to relate the figure of Colour images of this galaxy reveal the pink
a giraffe to the stars of Camelopardalis. glow of large emission nebulae in its spiral
Here, the stars of the giraffe’s legs are arms. It is about 11 million light-years away.
shown. The animal’s long neck would
stretch off the top of the picture. PETRUS PLANCIUS

This Dutch church minister was also
an expert geographer and astronomer.
Petrus Plancius (1552–1622) taught
the navigators on the first Dutch sea
voyages to the East Indies how to
measure star positions. In turn, they
produced for him a catalogue of the
southern stars divided into 12 new
constellations, which Plancius
depicted on his celestial globes. He
also invented several constellations,
such as Columba, Camelopardalis,
and Monoceros, using some of the
fainter stars visible from Europe.

THE CHARIOTEER Auriga also contains two THE
extraordinary eclipsing binaries CHARIOTEER
Auriga of long period. One is Zeta (ζ)
Aurigae, which is an
SIZE RANKING 21 orange giant orbited by
a smaller blue star that
BRIGHTEST STAR eclipses it every 2.7
years.This causes a 30
Capella (α) 0.1 per cent decrease in
brightness for six weeks,
GENITIVE from magnitude 3.7 to 4.0.
More remarkable, however, is
Aurigae Epsilon (ε) Aurigae (see p.277).This
intensely luminous supergiant is
ABBREVIATION Aur orbited by a mysterious dark partner
that eclipses it every 27 years – the
HIGHEST IN SKY AT 10PM longest interval of any eclipsing
binary. During the eclipse, Epsilon’s
December–February brightness is halved, from magnitude
3.0 to 3.8, and it remains dimmed for
FULLY VISIBLE more than a year. Astronomers think
that its companion is a close binary
90°N–34°S star enveloped in a disc of dust.The
next eclipse is due at the end of 2009.
Auriga is easily identified in the
northern sky by the presence of
Capella (α), the most northerly first-
magnitude star. Auriga lies in the
Milky Way between Gemini and
Perseus, to the north of Orion.The
constellation represents a charioteer.

SPECIFIC FEATURES SHARED STAR 2 THE FLAMING STAR NEBULA 54
Auriga’s outstanding feature is a chain Neighbouring Beta (β) Tauri AE Aurigae is a hot, massive star of
of three large and bright open star completes the charioteer figure. magnitude 6 that lights up the surrounding
clusters. All three will just fit within Auriga is usually identified as a cloud of gas and dust that is the Flaming
the same field of view in wide-angle king of Athens, Erichthonius. Star Nebula, IC 405.
binoculars. Of the trio, M38’s stars are
the most scattered and, when viewed
with a small telescope, seem to form
chains.The middle cluster is M36, the
smallest cluster but also the easiest to
spot, while M37 is the largest and
contains the most stars, but these are
faint. All three clusters lie about 4,000
light-years away.

The star-forming nebula IC 405 is
located nearby. Bright light from 6th-
magnitude AE Aurigae near its centre
lights up the surrounding gases.

THE LYNX SPECIFIC FEATURES double star is 38 Lyncis, with
Lynx contains many interesting components of 4th and 6th
Lynx double and multiple stars. For magnitudes. A telescope of 75mm
example, 12 Lyncis appears double (3in) aperture is required to
SIZE RANKING 28 with a small telescope, but with a separate the individual stars.
telescope of 75mm (3in) or larger
BRIGHTEST STAR aperture the brighter star divides into
two components of 5th and 6th
Alpha (α) 3.1 magnitudes, which have an orbital
period of about 700 years.
GENITIVE
An easier triple to identify is
Lyncis 19 Lyncis.This consists of two stars
of 6th and 7th magnitudes and a
ABBREVIATION Lyn wider 8th-magnitude companion,
all visible through a small
HIGHEST IN SKY AT 10PM telescope. A more challenging

February–March

FULLY VISIBLE

90°N–28°S

Lynx is a fair-sized but faint THE LYNX ELUSIVE FELINE 2 THE NIGHT SKY
constellation in the northern sky. Lynx consists of nothing more than a few faint
It was introduced in the late 17th stars zigzagging between Ursa Major and Auriga.
century by Johannes Hevelius (see To spot it, keen eyesight or binoculars are required.
p.368), who wanted to fill the
gap between Ursa Major and
Auriga. Hevelius is reputed to
have named it Lynx because
only the lynx-eyed would
be able to see it –
Hevelius himself had
very sharp eyesight.
The animal he drew
on his star chart, however,
looked little like a real lynx.

the constellations

THE GREAT BEAR of the Plough points towards the binary relationship, orbiting every 60 THE OWL NEBULA 54
bright star Arcturus in the adjoining years, which is quick by the standards The dark, owl-like eyes of the faint planetary
Ursa Major constellation of Boötes. of visual binary stars. nebula M97 are visible only through large
telescopes or on photographs and CCD
SIZE RANKING 3 SPECIFIC FEATURES One of the easiest galaxies to images such as this.
The Plough is one of the most identify with binoculars is M81,
BRIGHTEST STARS famous patterns in the sky. Its shape which is in northern Ursa Major, and identify, however, is the Owl Nebula,
is formed by the stars Dubhe (α), is also known as Bode’s Galaxy (see or M97, which is located under the
Alpha (α) 1.8, Merak (β), Phad (γ), Delta (δ) Ursae p.304).This spiral galaxy is at an angle bowl of the Plough.This planetary
Epsilon (ε) 1.8. Majoris, Alioth (ε), Mizar (ζ) (see and can be seen on clear, dark nights nebula is one of the faintest objects
p.272), and Alkaid (η).With the as a slightly elongated patch of light. in Charles Messier’s catalogue, and a
GENITIVE Ursae exception of Dubhe and Alioth, these A telescope is needed to spot the telescope of around 75mm (3in)
Majoris stars travel through space in the same rather more elongated shape of the aperture is needed to make out its
direction, and they form what is smaller and fainter Cigar Galaxy (see grey-green disc, which is three times
ABBREVIATION UMa known as a moving cluster. p.304), or M82, which is found one larger than that of Jupiter. A telescope
diameter of the Moon away from with an even larger aperture reveals
HIGHEST IN SKY AT 10PM Mizar (ζ), the second star in the Bode’s Galaxy.This unusual-looking the two dark patches, like an owl’s
Plough’s handle, is next to Alcor object is now thought to be a spiral eyes, that give rise to its popular name.
February–May (see p.272), an eighth, fainter star in galaxy, seen edge-on, mottled with
the Plough, which can be seen with dust clouds and undergoing a burst
FULLY VISIBLE good eyesight. A small telescope of star formation following an
reveals that Mizar also has a closer encounter with M81.
90˚N–16˚S 4th-magnitude companion.
Another major spiral galaxy in this
Ursa Major is one of the best-known In southern Ursa Major lies a constellation is the Pinwheel Galaxy,
constellations and a prominent feature more difficult double star, Xi (ξ) Ursae M101 (p.306), which lies near the
of the northern sky. Seven of its stars Majoris, which needs a telescope with end of the Plough’s handle. Although
form the familiar shape of the Plough, an aperture of 75mm (3in) to divide larger than Bode’s Galaxy, it is fainter
also known as the Big Dipper. But it.This pair, with components of 4th and thus more difficult to see. An
as a whole, Ursa Major is much and 5th magnitudes, form a true even greater challenge to find and
larger than this; it is the third-largest
constellation in the sky.The two THE CIGAR GALAXY 54
stars in the Plough’s bowl farthest M82 is a peculiar-looking spiral galaxy
from the handle, Dubhe (α) edge-on to us, which is undergoing a
and Merak (β), point towards burst of star formation triggered by
the north Pole Star, a close encounter with the larger and
Polaris, while brighter spiral galaxy M81 about
the curved 300 million years ago.
handle

THE GREAT
BEAR

THE NIGHT SKY BODE’S GALAXY 54
This spiral galaxy was discovered by the
German astronomer Johann Elert Bode on
31 December 1774. Located approximately
11 million light-years away, M81 is
nevertheless one of the brightest and most
visible galaxies in the sky.

the constellations THE NIGHT SKY

THE HIDDEN DOUBLE 21
Although Mizar (ζ) and its neighbour Alcor
may appear to be a double star when seen
with the naked eye (see main picture), upon
further magnification, Mizar (on the left of
this image) is revealed to have an even
closer companion than Alcor (on the right).

A FAMILIAR SIGHT 2
The saucepan shape of the Plough stars
is one of the most easily recognized
sights in the night sky, but it makes up
only part of the whole constellation
pattern of Ursa Major.

MYTHS AND STORIES

THE TALE OF THE GREAT BEAR

The Plough is one of the oldest, most recognized
patterns in the sky. In Greek mythology, it
represents the rump and long tail of the
Great Bear.Two different characters are
identified with it: Callisto, who was
one of Zeus’s lovers (see p.185); and
Adrastea, a nymph who nursed the
infant Zeus and was later placed
in the sky as the Great Bear.

RECURRING PATTERN
The shape of the Plough can be seen clearly (below,
centre) on this northern polar chart from Dunhuang,
China, dating from AD 940 or earlier.

the constellations

THE HUNTING DOGS

Canes Venatici

SIZE RANKING 38

BRIGHTEST STAR

Cor Caroli (α) 2.9
GENITIVE Canum
Venaticorum
ABBREVIATION CVn

HIGHEST IN SKY AT 10PM

April–May

FULLY VISIBLE

90°N–37°S

Canes Venatici lies in the northern THE WHIRLPOOL GALAXY 54 THE HUNTING
sky between Boötes and Ursa Major. The core of this beautiful spiral galaxy (also DOGS
This constellation represents two dogs known as M51) appears as a point of light in
held on a leash by the herdsman a small telescope, as does its companion
Boötes. It was formed by Johannes galaxy NGC 5195 (top) at the end of one arm.
Hevelius (see p.368) at the end of the
17th century from stars that had Canes Venatici also contains some fine
previously been part of Ursa Major. galaxies, such as the Whirlpool Galaxy
(see p.305), or M51, which is found
SPECIFIC FEATURES seven diameters of the full Moon
The constellation’s brightest star, from the star at the end of the handle
Alpha (α) Canum Venaticorum, is of the Plough (in Ursa Major).The
known as Cor Caroli, meaning Whirlpool Galaxy was the first galaxy
Charles’s Heart, in commemoration in which spiral form was detected –
of King Charles I of England.This the observation being made in 1845
wide double star, with components by William Parsons (see p.305) in
of magnitudes 2.9 and 5.6, is easily Ireland.The galaxy appears as a
separated with a small telescope.The round patch of light through
brighter star is slightly variable, by binoculars, but a moderate-sized
about one-tenth of a magnitude, telescope is needed to make out
which is too small to be noticeable the spiral arms. At the end of
to the naked eye. Larger variation one of the arms lies a smaller
is found in Gamma (γ) Canum galaxy, NGC 5195, which is
Venaticorum, a deep red supergiant passing close to M51.
popularly known as La Superba. It
fluctuates between magnitudes 5.0 Two spiral galaxies worth
and 6.5 every 160 days or so. looking for through a small
telescope are the Sunflower
Galaxy (M63) and M94.

THE SUNFLOWER GALAXY 54
M63 is a spiral galaxy, with patchy outer
arms, that is seen at an angle from Earth.
The arms give rise to comparisons with the
appearance of a sunflower. The star to its
right in this photograph is of 9th magnitude.

THE NIGHT SKY GLOBULAR CLUSTER M3 15 TWO BRIGHT STARS 2
This cluster is one of the biggest and Canes Venatici represents a pair of
brightest globular clusters in the northern hounds, but the unaided eye can see
sky. A telescope with 100mm (4in) aperture little more than the constellation’s
is needed to resolve its individual stars. brightest stars, Cor Caroli and Beta
Canum Venaticorum.

THE HERDSMAN binoculars. In billions of years’ time, MYTHS AND STORIES the constellations
our Sun will swell into a red giant
Boötes similar to this star. THE BEAR KEEPER Boötes represents a man herding
a bear (Ursa Major). Myths differ
SIZE RANKING 13 Boötes is noted for its double stars, DOUBLE STAR IZAR 5 as to whether he is a hunter or
the most celebrated of which is Izar Epsilon (ε) Boötis, which is also a herdsman, as the constellation’s
BRIGHTEST STAR (see p.273), or Epsilon (ε) Boötis, at known as Izar or Pulcherrima, is a brightest star, Arcturus, means
the heart of the constellation.To the challenging double star consisting
Arcturus (α) -0.1 naked eye, it appears of magnitude of a bright orange star with a “bear guard” or “bear keeper”
GENITIVE Boötis 2.4, but high magnification on a fainter blue-green companion star. in Greek.The man’s two dogs
telescope of at least 75mm (3in) are represented by adjoining
ABBREVIATION Boo aperture reveals a close, 5th-magnitude Canes Venatici. In Greek
companion that is blue-green in myth, Boötes was
HIGHEST IN SKY AT 10PM colour, providing one of the most identified with Arcas,
beautiful contrasts of all double stars. son of Zeus and Callisto.
May–June
Much easier to divide with any ADJACENT STARS
FULLY VISIBLE small telescope are Kappa (κ) and Boötes is depicted here
Xi (ξ) Boötis. Kappa’s stars, with leading the two hunting dogs,
90°N–35°S components of 5th and 7th on an 18th-century star chart
magnitudes, are unrelated but Xi, by Sir James Thornhill.
The Greek constellation Boötes with stars also of 5th and 7th
contains the brightest star north of magnitudes, is a true binary with
the celestial equator, Arcturus – Alpha an orbital period of 150 years and
(a) Boötis – which is also the fourth- has warm yellow-orange hues.
brightest star in the entire sky.This
large and conspicuous constellation Easiest of all are the doubles Mu
extends from Draco and the handle of (μ) Boötis, with components of 4th
the Plough (in Ursa Major) to Virgo. and 6th magnitudes, and Nu (ν)
Faint stars in the northern part of Boötis, with two 5th-magnitude
Boötes once formed the now-defunct components – both are widely spaced
constellation of Quadrans Muralis, enough to divide with binoculars.
which gave its name to the
Quadrantid meteor shower that THE HERDSMAN
radiates from this area every January.

SPECIFIC FEATURES
Arcturus is classified as a red giant,
but as with most supposedly “red”
stars, it actually looks orange to the
unaided eye. Its colouring becomes
stronger when viewed through

KITE-SHAPED CONSTELLATION 2 THE NIGHT SKY
Boötes, containing the bright star
Arcturus, stands aloft in spring skies in
the northern hemisphere. The crown of
Corona Borealis can be seen to its left.

The constellations

HERCULES The most distinctive feature of this
constellation is a quadrilateral
Hercules of stars called the Keystone, which is
composed of Epsilon (ε), Zeta (ζ),
SIZE RANKING 5 Eta (η), and Pi (π) Herculis.

BRIGHTEST STAR SPECIFIC FEATURES
Alpha (α) Herculis, (see p.281), or
Kornephoros (β) 2.8 Rasalgethi, is actually the second-
GENITIVE Herculis brightest star in Hercules. It fluctuates
between 3rd and 4th magnitudes.
ABBREVIATION Her As with most such erratic variables,
Rasalgethi is a bloated red giant that
HIGHEST IN SKY AT 10PM pulsates in size, causing the brightness
changes. A small telescope brings a
June–July 5th-magnitude blue-green companion
star into view.
FULLY VISIBLE
On one side of the Keystone lies
90°N–38°S M13, which is regarded as the finest
globular cluster of northern skies.
This large but not particularly Under ideal conditions, M13 can be
prominent constellation of the glimpsed with the naked eye, and
northern sky represents Hercules, through binoculars it appears like a
the strong man of Greek myth. In hazy star half the width of the full
the sky, Hercules is depicted clothed Moon. Slightly farther away from the
in a lion’s pelt, brandishing a club Keystone is a second globular cluster
and the severed head of the watchdog – M92. This often overlooked cluster
Cerberus, and kneeling with one foot is smaller and fainter than M13, and
on the head of the celestial dragon, when seen through binoculars it can
Draco – the tools and conquests of easily be mistaken for an ordinary star.
some of his 12 labours.
Several readily seen double stars
are to be found in Hercules, including
Kappa (κ) Herculis, with components
of 5th and 6th magnitudes, and
100 Herculis, with its two 6th-
magnitude stars. Postioned closer
together, and hence requiring higher
magnification, are 95 Herculis, with
two 5th-magnitude components, and
Rho (ρ) Herculis, with components
of 5th and 6th magnitudes.

HERCULES UPSIDE DOWN 2
In the sky, Hercules is
positioned with his feet
towards the pole (top left in
this picture) and his head
pointing south.

GLOBULAR CLUSTER M13 15
Through binoculars, this cluster appears
as a rounded patch of light. It breaks up
into countless starry points when
viewed through a small telescope.

THE NIGHT SKY THE HERCULES GALAXY CLUSTER 3
Every fuzzy object in this picture is a faint
galaxy in the cluster Abell 2151, some 500
million light-years away.