Astronomy July_2020

By studying this strange ur solar system’s comets After planets form in a fledgling
star, astronomers hope to are believed to have deliv- system, the host star is surrounded
better understand what ered a wealth of material by a disk of leftover gas and dust
happened early in the life to early Earth. Among that didn’t get molded into new
of our own solar system. the icy visitors’ suspected gifts worlds. In our solar system, this
were rare gases, small amounts of surplus of material settled into two
BY NOLA TAYLOR REDD water, and organic material — all bands: the asteroid belt between
of which could have helped ter- Mars and Jupiter and the Kuiper
50 ASTRONOMY • JULY 2020 restrial life form and evolve. But Belt beyond Neptune. While mem-
while ancient Earth received only bers of the asteroid belt are rocky
a moderate influx of comets, any and relatively dry, comets in the
planets around young nearby star Kuiper Belt tend to be ice-rich and
Eta Corvi are likely raking in com- filled with gases. But both types of
etary currency. objects can help seed planets with

the ingredients necessary for the the James Clerk Maxwell Heavy Bombardment (LHB), Nearby star Eta
evolution of life as we know it. Telescope in Hawaii toward the which saw the inner planets pum- Corvi seems to be
star in 2005, he found a surprise: meled by asteroids some 4 billion surrounded by a
And then there’s Eta Corvi — The Eta Corvi system also boasts years ago. During this time, swarm of comets,
a 1.5-billion-year-old star that sits a ring of hot material much closer Neptune and Uranus performed as seen in this
just 60 light-years away. Decades to the star itself. “There is some- an intricate dance that banished artist’s concept.
ago, researchers used the Infrared thing unusual going on in the them to the outskirts of the solar By examining the
Astronomical Satellite (or IRAS) inner region of this system,” system. And as the ice giants infant system,
to find evidence that Eta Corvi Wyatt, now at the University migrated, they kicked up material which sports two
sports a massive outer ring much of Cambridge, says. from the Kuiper Belt and hurled it belts of debris,
larger and more distant than the back toward Earth and the other astronomers hope
solar system’s Kuiper Belt. But At first, astronomers thought rocky planets. Although the tim- to gain insight into
when Mark Wyatt, then an that Eta Corvi could just be going ing remains uncertain, some how young systems
astronomer with The Royal through a period much like the astronomers think the material form and evolve.
Observatory, Edinburgh, turned solar system’s suspected Late
NASA/JPL-CALTECH

WWW.ASTRONOMY.COM 51

e M104 e ¡ which shine hotter than our
b G-type Sun. Because A-type stars
Spica_ C R AT E R rotate quickly, astronomers can
more easily spot the light-blocking
VIRGO f cometary material that’s around
them. The faster rotation means
a Eta Corvi d b a a _ it’s easier to determine whether a
h star’s brightness is changing due to
HYDRA CORVUS dc passing dust, says Isabel Rebollido,
a Ph.D. student at Spain’s
Eta Corvi sits some c¡ ` Universidad Autónoma de Madrid.
60 light-years away `
in the constellation Eta Corvi, however, is the
Corvus the Crow. _ exception. As an F-type star, it’s
M68 still hotter than the Sun, but cooler
ASTRONOMY: ROEN KELLY than its fellow comet-bearing stars.
delivered during this cosmic bar- 5° Plus, the system boasts a massive
During the rage played a vital role in helping outer belt of material that stretches
Late Heavy our planet become habitable. Smashing comets, about 150 astronomical units (AU)
Bombardment, a smashing planets from its star, where 1 AU is equal
period when the But more recent studies seem to to the average distance between
inner planets were rule out an LHB equivalent for the Comets are common, both within Earth and the Sun. The Kuiper
rocked by asteroids, Eta Corvi system, instead favoring the solar system and beyond. And Belt, for comparison, reaches about
Earth would have a steady stream of comets rather although they are incredibly chal- 55 AU from the Sun.
been transformed than a flood. Whether that’s good lenging to spot around other stars,
into a hellish news for the evolution of life these icy chunks are expected to be In addition to its massive exo-
landscape. NASA GSFC remains an open question. But widespread. Thanks to improved Kuiper Belt, Eta Corvi also has an
even with its stunning differences, technology and methods, astrono- interior belt of warm material
CONCEPTUAL IMAGE LAB Eta Corvi and its strange set of mers have identified more than 25 within about 3 AU of its star. The
belts could help reveal what our systems that are thought to host inner band is rich in carbon mon-
own solar system was like billions exocomets, as well as minor bodies oxide, which gets destroyed by
of years ago. like dwarf planets. stellar radiation in just over a cen-
tury. “The fact that we’re seeing
Most of the exocomets seen this carbon monoxide now, it
today are around A-type stars, either means that we’re very lucky
to be observing it, [or] it’s a con-
tinuous process,” says Sebastian
Marino, an exoplanet researcher

52 ASTRONOMY • JULY 2020

THE SOLAR SYSTEM ETA CORVI SYSTEM

Sun Eta Corvi

Asteroid be lt

DUCHÊNE, G., ET AL. (APJ, 784, 2, 19 [2014])Jupiter

2 AU Inner disk is 2–4 AU 2 AU Inner disk starts at 3 AU
20 AU
Uranus Similar to the
Neptune solar system, the
Eta Corvi system
Saturn displays two
distinct belts of
20 AU Kuiper Belt Outer disk Outer disk starts dust and debris.
30–55 AU at 100–150 AU But unlike the
asteroid belt, Eta
Corvi’s inner belt
is surprisingly
warm and rich in
carbon monoxide.

ASTRONOMY: ROEN KELLY

at the Max Planck Institute for settling,” Rebollido says. After a ETA CORVI’S ODDITIES
Astronomy in Germany. The com- billion and a half years, Eta Corvi
position of the interior disk also should have calmed, most of its 5" 5" 5"
suggests it originally formed far- debris swept up by successful
ther from the star before later planets. “It doesn’t quite agree PACS 70 PACS 100 SCUBA2 850
migrating inward. with our idea of how planetary
systems form and when they Previous studies suggest that, Eta Corvi’s two-
So where is the inner collection settle down,” Rebollido says. during the solar system’s LHB, component debris
of carbon monoxide coming from? the Kuiper Belt would have been disk was imaged by
One possibility is that astronomers Planetary broad, and features such as spiral the Herschel Space
caught a glimpse of debris ejected bucket brigade arms could have formed within it. Observatory at 70, 100,
after a large object collided with and 850 micrometers,
an exoplanet. Astronomers haven’t When Neptune and Uranus Using the Atacama Large as seen in the top row.
directly detected any planets underwent what scientists call Millimeter/submillimeter Array The middle row shows
around Eta Corvi yet, but if the dynamical instability, trading (ALMA), Marino’s team probed Eta synthetic images that
material was expelled during a col- places while moving outward Corvi’s outer disk for signs of a were created based
lision, it could indicate the pres- through the solar system, they giant planet stirring things up. on best-fit models.
ence of a world inside the star’s cleared debris from the inner Their observations showed no hints Finally, the bottom
habitable zone — the region where edge of a once-wider Kuiper Belt. of spiral arms or other structures. row shows residual
liquid water can exist on the plan- To get an idea of how Eta Corvi’s “It seems unlikely that a dynamical features that were
et’s surface. Such a collision would belts formed, Marino and his col- instability in the system similar to left over after the
be remarkable timing for astrono- leagues decided to model possible the LHB is responsible,” the authors synthetic images
mers, though — a stroke of luck configurations of worlds and how concluded in their paper. were subtracted
that it occurred in modern times. they might pass material inward, from the real views,
as well as carve out the observed highlighting features
Another possibility is that Eta exo-Kuiper Belt. Marino wanted that the models did
Corvi is still suffering a violent to figure out what kind of planets not fully replicate.
childhood, with excess material it would take to feed comets from
smashing together well after any the system’s outermost edges to its
planets should have cleared out innermost regions.
their orbits. “While smashing,”
Marino says, “they release some of First, he and his colleagues
the gases and ices that are inside.” looked at whether Eta Corvi could
However, that stands at odds with be undergoing its own version of
astronomers’ idea for an aging LHB. When Neptune and Uranus
toddler system like Eta Corvi. migrated, they not only hurled
cometary material inward, they
“We expected that the younger also foundationally changed the
the system, the higher the dynami- structure of the Kuiper Belt.
cal interactions because it’s still

WWW.ASTRONOMY.COM 53

ETA CORVI’S OUTER RING of it winds up getting diverted else- other comets or a planet. In order
where, sometimes being ejected for the inner disk to have formed
200 from the solar system entirely. from mutual cometary collisions,
“Only one in a million comets that though, the range in size of the
100 got scattered inwards made it to cometary dust grains would have
collide with Earth,” Wyatt says. to be dramatically different from
AU 0 what’s expected, and the grains
If Eta Corvi has its own chain themselves would have to shine
-100 of worlds, the planetary conga line significantly brighter than they
could allow material to leapfrog do. On the other hand, the excess
-200 inward. But the existence of a giant warm dust could form if a comet
inner dust cloud suggests that collided with a rocky world
-200 -100 0 100 200 there is no Jupiter-like world between four and 10 times as
AU guarding any rocky planets near massive as Earth, making this
Eta Corvi. According to Wyatt, the the more likely scenario.
A model of Eta However, if a relatively low- rate at which comets are hurled
Corvi’s outer debris mass planet is either moving toward Eta Corvi is roughly 100 While most of Earth’s water
disk suggests it’s slowly outward through the belt times less than the solar system came from asteroids, comets pro-
most heavily or has settled in its midst, the experienced in its prime. However, vided some of the ingredients nec-
concentrated about researchers think it could send without a Jupiter-like sentinel, essary for life to evolve. Because
100 to 130 AU from material inward. The planet would more of that material actually suc- of that, Eta Corvi “can have some
the star. J. LEBRETON, have to be small enough that it ceeds in making it to the inner quite important implications for
hasn’t made much of a dent in part of the Eta Corvi system. what’s going on on a planet in
C. BEICHMAN, G. BRYDEN, clearing out the debris around it, terms of the potential for the
however, since the outer belt shows Using simulations, Marino and development of life,” Wyatt says.
ET AL. (APJ, 817, 2, 165 [2016]) no sign of a gap. A planet between his colleagues found that a chain of
3 and 30 Earth masses, orbiting fewer than 10 worlds of roughly the Some researchers have sug-
between 75 and 100 AU, would be same size — each weighing gested Earth got part of its atmo-
sufficient to fling comets inward. between 3 and 30 Earth masses sphere from comets, while others
— could easily hand off cometary think that impacts could have
When Neptune and Uranus material from one to the next. This aided in the development of life. So
plowed into the Kuiper Belt, they is one possible way in which the by studying Eta Corvi, astronomers
sent roughly 30 Earth masses’ system could continue to maintain may get a better understanding of
worth of cometary debris to the its inner, warm disk of material what happened early in the life of
inner solar system over a period of that we see today. the solar system. “Whenever we
15 million years. But not all of the look at a system younger than our
comets made it. Jupiter, the largest The ingredients for life solar system, we’re kind of looking
of the planets, stands as sentinel in the past,” Rebollido says.
for the terrestrial planets. While Eta Corvi may provide more than
the gas giant lets some of the just an intriguing glimpse of giant Of course, the best way to
Kuiper Belt material fly by, most exoplanets working as a bucket understand how Eta Corvi’s hypo-
brigade, passing cometary ice and thetical worlds evolve is to actually
ALMA observations gas to the inner system. Something observe them. “It would be great
of Eta Corvi’s disk else unusual is happening only if we could detect the planets,”
indicate debris rich 3 AU from the star, roughly the Wyatt says. But that won’t happen
in carbon monoxide same distance our asteroid belt is in the near future.
can be ushered from the Sun. And both scenarios
inward by a chain of have the potential to blow out Eta Corvi’s outer planets are
so-far undetected enormous amounts of hot dust. likely Neptune-sized worlds.
planets. MARINO, S. ET AL. Direct imaging, which is akin to
When Wyatt and his colleagues photographing a planet, is the best
(MNRAS, 465, 3, 2595 [2017]) reexamined the system, they found method for spotting worlds that lie
Eta Corvi’s inner dust cloud was so far out, but exo-Neptunes are
not spread evenly around the star. too small to be spotted with cur-
Instead, a massive clump of mate- rent or even upcoming instru-
rial appeared close to the star. ments. Directly imaging an
Once again, the clump could have exoplanet often relies on capturing
been produced by either comet- the residual heat seeping from a
comet or comet-planet collisions. recently formed world, and Eta
Corvi’s planets are likely too old
Marino’s team investigated how to have enough heat left from their
likely it might be for the icy mate- birth to be seen from afar.
rial cast inward to collide with

54 ASTRONOMY • JULY 2020

As for the inner planets, the Wyatt says. “That will tell us more proof that cometlike bodies are Comet 67P/
dust that hints at their existence about what’s going on.” being formed [in other systems],” Churyumov-
ironically might keep us from she says. “I’m curious about the Gerasimenko, seen
spotting them. Eta Corvi’s giant At the same time, the recent composition — and, specifically, in this image taken
outer cloud is a much more effec- arrival of an exocomet in our own whether it’s similar to our solar by the Rosetta
tive veil than the dust surrounding solar system could help decode the system comets or not.” spacecraft on
our inner solar system, where mysteries of Eta Corvi. At the time January 31, 2015,
material dropped by comets dis- of this writing, Comet 2I/Borisov, Researchers might also better originated from
plays itself on Earth as zodiacal discovered in August 2019, is cur- understand our solar system’s own the solar system’s
light. This has led researchers to rently breaking up. By studying its past by continuing to study the Eta distant Kuiper Belt.
conclude that the Eta Corvi system spilled guts, astronomers hope to Corvi system. But with its unique Until last June,
is not a good target for missions probe the object’s composition to pair of disks, Eta Corvi clearly has NASA was strongly
hoping to image an exo-Earth. learn how comets in other star something special going on. considering a
systems compare to our own. sample-return
Either way, Wyatt and his col- Regardless, this single young mission to Comet
leagues want to re-observe the sys- “This kind of comet visiting us system rich in cometary material 67P, but instead
tem. If the clump they spotted in could have formed in a system like may reveal a lot about how our opted to proceed
the inner disk is orbiting the star, Eta Corvi,” Marino says. But solar system and others formed with a mission that
they should see it move with time. instead of colliding with another and evolved over time. According will send a large
The researchers also hope to con- comet or planet, Borisov was to Marino, “Eta Corvi is kind of rotorcraft to Saturn’s
firm the system’s cometary finger- ejected. This, he adds, “will tell us the tip of the iceberg.” moon Titan.
prints with further observations. about comets that are farther out.”
“We should be able to map the Nola Taylor Redd is a freelance ESA/ROSETTA/NAVCAM
carbon monoxide distribution,” Rebollido is also excited to see science writer and frequent
the interstellar interloper pass contributor to Astronomy.
through our solar system. “It’s the

WWW.ASTRONOMY.COM 55

RR Lyrae variables

allow precise distance

measurements, reveal

the history of the

regions they populate,

and trace how galaxies

are structured.

BY ATA SARAJEDINI

arth is fortunate to be orbiting a
star that is fairly constant in its
light production. The amount
of light energy reaching Earth
from the Sun has varied by much
less than 1 percent over the past few
hundred years.
Our star is stable because the balance
between the inward force of gravity and
the outward force exerted by the hot gas
in its interior allows it to maintain a con-
stant radius and thus a constant luminos-
ity. But this will not always be the case.
The Sun is a typical main sequence star
fusing hydrogen into helium in its core. In
about 5 billion years, when it runs out of
hydrogen in its core, the Sun will evolve
into a red giant, and soon after, it will
enter the “horizontal branch” chapter
of its life. In this phase, it will shine by
combining three helium nuclei to produce
one carbon nucleus at temperatures of
100 million kelvins in its central regions.
During its life in the horizontal branch
phase, the Sun may experience an imbal-
ance between the outward flow of energy
and the force of gravity trying to hold it
together. When this happens, our star will
become unstable and begin to pulsate, its
radius increasing and decreasing by as
much as 20 percent over a period between
12 and 24 hours. As it experiences this

This infrared view of the Milky Way’s crowded
center was taken as part of the European Southern
Observatory’s VISTA Variables in the Via Lactea
Survey. Images such as this have revealed numerous
RR Lyrae stars in our galaxy’s bulge, hinting that it is
old and may have been built up as primordial star
clusters merged together over time. ESO/VVV SURVEY/D. MINNITI

56 ASTRONOMY • JULY 2020

WWW.ASTRONOMY.COM 57

M33 is the second-closest spiral galaxy to our own. Observations of RR Lyrae stars in this galaxy What RR Lyraes reveal
have allowed astronomers to decipher clues about its structure, including where the majority of its
dust resides. ESO The most basic property of a stellar pop-
ulation, such as a star cluster or a galaxy,
change, the Sun’s light will vary by RR Lyrae stars are grouped into two is its distance. RR Lyrae variables are
as much as four times between its faint- classes based on the shapes of their standard candles — objects for which
est and brightest points. It will become light curves. A light curve shows the their intrinsic brightness is known, so
a type of variable known as an RR change in the star’s brightness over that a comparison with their observed
Lyrae star. time and measures the period, ampli- apparent brightness allows astronomers
tude, and shape of the variation. The to calculate their distance. Thus, these
RR Lyrae stars period is how long it takes the RR stars can be used to map out the regions
Lyrae to undergo one complete cycle, of our galaxy in which they reside.
Named after the prototype of the and the amplitude is the amount of
group — RR Lyrae, a variable star in brightening that the star exhibits. Another important property of stel-
the constellation Lyra the Harp — these RRab variables are the most common lar populations is age, or how long ago
pulsating variables are common in the and exhibit sawtooth-shaped light the stars in a galaxy or star cluster
night sky and are favorite targets of curves, while RRc variables have formed. RR Lyrae variables “shine” here
amateur astronomers such as those in shorter periods and roughly sinusoidal as well. In order to become an RR Lyrae
the American Association of Variable light curves. The RRab-type variables variable, a star must evolve beyond the
Star Observers. RR Lyraes are impor- can help astronomers decipher many main sequence and onto the horizontal
tant to professional astronomers as well, of the properties of galaxies and, as branch. Thus, these stars are advanced
because they serve as the lynchpin for such, researchers such as myself refer in age. As it turns out, only stars older
galactic structure studies, revealing to them as the Swiss Army knives than about 10 billion years can become
important clues about the regions in of astronomy. RR Lyrae variables. If we find such a
which they are found. star in a particular galaxy or globular
cluster, then that object must be at least
10 billion years old. Since the universe
is about 13.8 billion years old, this
implies that the globular clusters and
galaxies that contain RR Lyrae variables
were formed relatively soon after the
Big Bang that created the universe itself.

Along with their distances and ages,
globular clusters and galaxies are also
characterized by the metal content of
their stars. (In astronomy, “metals” are
all elements heavier than helium.) The
Big Bang produced mostly hydrogen
and helium, along with a negligible
fraction of metals. The vast majority of
metals in the universe today — and,
thus, all of the elements that make up
the everyday objects we encounter on
Earth — were synthesized in stars.

RR LYRAE LIGHT CURVES There are two major classes of RR Lyrae stars, based on the shape of their light curve, which
measures a star’s brightness over time: RRab- (left, middle) and RRc-type stars. ASTRONOMY: ROEN KELLY
14.8
14.8 14.8
RRa
RRb RRc
15.0
15.0 15.0

Magnitude 15.2
Magnitude
Magnitude
15.2 15.2

15.4 15.4 15.4

15.6 15.6 15.6

15.8 1.0 Phase 1.5 15.8 1.0 Phase 1.5 15.8 1.0 Phase 1.5
0.5 0.5 0.5

58 ASTRONOMY • JULY 2020

WHAT IS THE Effective temperature (kelvins)
HORIZONTAL BRANCH?
30,000 10,000 7,000 6,000 4,000
-10

SUPERGIANTS
-8

Astronomers use a tool call the Because RR Lyraes are moving Absolute magnitude-6
Hertzsprung-Russell (HR) diagram across the horizontal branch, they Instability strip
to easily characterize stars. The HR can be used as standard candles. -4
diagram plots a star’s temperature This is because their luminosity RR Lyrae
and luminosity, or brightness. (brightness) remains constant in
Where a star falls on the HR dia- this phase of their lifetime, even as -2 variables
gram can tell researchers how far their temperature changes. 0 Horizontal branch
along the star is in its life cycle, as
the temperature and brightness of Furthermore, as stars move GIANTS Helium
a star change as it evolves. through the horizontal branch, they Brighterburning
may cross a region astronomers call 2 WHITEMAIN SEQUENCE ignites
Stars located on the main the instability strip. While in this 4
sequence part of the diagram, like region, which is associated with
our Sun, are fusing hydrogen into specific properties, stars can 6 Sun
helium. After a star exhausts the become unstable and pulsate as
hydrogen in its core and becomes a variables, such as RR Lyrae stars. 8
red giant, it begins fusing hydrogen
in a spherical shell just outside the Although their brightness cycles 10
core. It also moves onto the giant during each pulsation, RR Lyraes 12 DWARFS
branch of the HR diagram. Next, exhibit a period-luminosity relation- 14
helium fusion ignites in the core, as ship, where the time it takes to com-
the star fuses helium into carbon. plete one full cycle of dimming and O BA F G K M
This process causes the star’s tem- brightening is related to the star’s
perature to increase, while its radius intrinsic brightness. By measuring Hotter
decreases; as a result, its luminosity, the period of an RR Lyrae’s bright-
or brightness, remains the same. ness changes, astronomers can When helium fusion ignites in a star’s core, its temperature increases
This causes the star to move hori- infer how bright it is, allowing them but its brightness remains constant and it moves horizontally in the
zontally on the diagram, creating to calculate the distance to that star HR diagram, creating the horizontal branch. The black line shows the
the horizontal branch. based on how bright it appears. path a star with slightly less mass than the Sun will take as it evolves,
— Alison Klesman moving across the horizontal branch and into the instability strip,
becoming an RR Lyrae star for a time. ASTRONOMY: ROEN KELLY

Each generation of stars produces metals; M3 is an 8-billion-year-old globular cluster in our galaxy. The cluster, arguably one of the Milky Way’s most
upon reaching the end of its life, each beautiful, contains more than 270 variable stars, of which at least 170 are RR Lyraes. ESA/HUBBLE & NASA, G. PIOTTO ET AL.
star distributes those metals through the
interstellar medium, where the enriched
gas is used as raw material to make more
stars. As a result, each successive genera-
tion of stars has more metals than the
previous generation. This is known as
chemical enrichment.

The Sun, which was born roughly
5 billion years ago, has a metal content of
2 percent. That is to say, the mass of the
Sun is composed of 70 percent hydrogen,
28 percent helium, and 2 percent all of
the other elements. So, elements that are
major constituents of Earth, such as car-
bon, nitrogen, oxygen, iron, and calcium,
make up only 2 percent of the Sun’s com-
position by mass. Within the range of
measured metal abundances of most
stars, the Sun is relatively metal rich.

The chemical enrichment history of
a stellar population is related to its age,
but not always in an expected and pre-
dictable manner. For example, one of
the most perplexing aspects of open star
clusters in the Milky Way is that there is
no apparent correlation between their

WWW.ASTRONOMY.COM 59

Here, too, RR Lyrae variables show
their promise by providing a means by
which to measure the amount of extinc-
tion and reddening along the line of
sight. Observations of nearby ab-type
RR Lyrae variables — the ones with the
sawtooth-shaped light curves — that
present zero or negligible reddening
show they all have the same color at the
faintest part of their light curve, also
known as minimum light. Therefore,
the intrinsic minimum-light color of
these variable stars is constant regard-
less of their other properties. This
means that we can compare the
observed minimum-light color of an
ab-type variable to what we know it
should be in the absence of dust and
calculate the amount of reddening
along the line of sight to the RR Lyrae.
From that knowledge, we therefore get
the amount reddening to the star clus-
ter or galaxy that hosts the star.

This jam-packed Hubble Space Telescope image shows a portion of the Andromeda Galaxy’s stellar Beyond the Milky Way
stream — a structure made of stars that astronomers believe came from a past companion galaxy that
has since been torn apart by the larger Andromeda’s gravity. Observations of this stream, which harbors Not only are RR Lyrae variables the
RR Lyrae variables, have helped researchers characterize the ages and origins of the stars that make up Swiss Army knives of astronomy in
Andromeda today. NASA, ESA AND T.M. BROWN (STSCI) galactic studies, they are also appli-
cable to extragalactic environments.
ages and metal abundances. This Lyrae variables — as metal-rich as the With the high-resolution imaging
means younger open clusters don’t Sun — are not uncommon in the Milky capabilities of the Hubble Space
necessarily have more metals than Way, presenting an especially puzzling Telescope (HST), it is routinely pos-
older ones. The reason for this is not scenario to researchers working to sible to identify and characterize RR
immediately clear. Nevertheless, the understand their origins. Lyrae stars in galaxies well beyond
metal abundance of a star cluster or the Milky Way. A good example that
galaxy is an important clue that helps Lifting the veil illustrates the broad applicability of RR
us to piece together its formation and Lyrae variables is my own work using
evolutionary chronology. I want to highlight one more way in HST observations of these stars in the
which these stars help astronomers bet- nearby late-type spiral galaxy M33,
RR Lyrae variables are also a power- ter understand the universe. To appreci- also known as the great spiral galaxy
ful tool for measuring the metal content ate this aspect of RR Lyrae stars, it is in Triangulum. This system is located
of stellar populations. As it turns out, important to realize that the dust in at a distance of 2.8 million light-years
the periods and amplitudes of these interstellar space acts like an absorbing and we think it is a satellite of the
stars can be used to determine their curtain that limits our ability to study Andromeda Galaxy (M31), which is
metal abundances, and thus infer the certain parts of the galaxy and in some 2.5 million light-years distant.
metal content of their host stellar sys- directions of the sky. This dust makes
tems. In general, the shorter the period background starlight fainter and redder. M33 is seen nearly face-on in the
of an RR Lyrae variable, the more We use the term extinction to refer to sky. Prior to HST, ground-based obser-
metal-rich it is. Given that a stellar pop- the former and reddening for the latter. vations using the Palomar 5.1-meter
ulation must be ancient to produce an Researchers also call reddening “color telescope and early-generation CCD
RR Lyrae star, it stands to reason that excess”; it is calculated by comparing detectors in 1985 were able to show
most, if not all, RR Lyraes are relatively the observed color of an object (through that M33 contains RR Lyrae stars, but
deficient in metals. That is to say, their dust) to its intrinsic color in the absence had a difficult time measuring their
progenitors were formed early in the of dust. Dust extinction and reddening properties such as their periods and
history of the universe when the overall are often the most challenging obstacles amplitudes of variation. HST’s superior
metal content of galaxies was compara- to our ability to study objects in the gal- high-resolution imaging capabilities
tively low. However, metal-rich RR axy and located outside of it. have allowed us to not only identify
these stars in M33, but also to precisely
measure their properties.

60 ASTRONOMY • JULY 2020

The discovery of RR Lyrae stars in The Draco dwarf spheroidal galaxy is a small companion of the Milky Way. Astronomers have used variable
M33 yielded the first evidence that stars, including RR Lyraes, to accurately measure its distance and decipher its star formation history. GIUSEPPE
this galaxy contains a population as
old as the ones in the Milky Way and DONATIELLO USING SDSS/PANSTARRS DATA
Andromeda. As described above, the fact
that RR Lyrae variables are in the hori- abundance, are similar. This is the first construction in the Chilean Andes, the
zontal branch phase of evolution means time we have isolated stellar populations 8.4-meter Simonyi Survey Telescope
that they are older than about 10 billion in distinct line-of-sight components in a at the Vera C. Rubin Observatory (for-
years. Therefore, soon after the Big Bang, galaxy that is so far away. merly called the Large Synoptic Survey
all three of the spiral galaxies in our local Telescope) will map the entire southern
galactic neighborhood began forming The future of RR Lyraes sky every few nights. As it does, the
stars at about the same time, within telescope will observe close to 1 million
1 billion years of one another. RR Lyrae stars have taught us much RR Lyrae stars in the Milky Way.
about our galaxy and even those nearby,
The observations of M33 also show but their contribution to astronomy These RR Lyrae stars and the infor-
that its RR Lyrae stars fall into two dis- is far from complete. Currently under mation on age, metal abundance, and
tinct reddening regimes. One set exhibits reddening that they yield will revolution-
low reddening consistent with being ize the field of galactic archeology. This
affected solely by dust in the Milky Way field endeavors to understand the struc-
Galaxy. The other set is fainter and red- ture, formation, and evolution of the
der, apparently affected by an additional Milky Way — our galactic home. And
source of reddening and extinction. At the distances inferred for every single
first, the presence of an additional layer one of these stars will provide a struc-
of dust in the direction of these RR Lyrae tural map of the Milky Way, allowing us
stars presents an intriguing puzzle. But a to visualize it in three dimensions.
glance at M33 itself reveals the possibility
that we may be viewing these stars With the power of the Rubin
through the spiral arms of M33 and on Observatory, particularly over time, the
the other side of the galaxy. The stars reach of new RR Lyrae research will also
with low reddening are located on the extend out into the Local Group. The
near side of M33, while those with high growing pool of RR Lyrae stars will
reddening are on the far side. Yet, besides allow astronomers to bring to bear this
their reddening, all of their other proper- versatile tool on our efforts to better
ties, such as period, amplitude, and metal understand the formation and evolution
of our larger galactic neighborhood.
Old-population tracers Age THE SWISS
ARMY Ata Sarajedini is dean of the Charles
RR 10 KNIVES OF E. Schmidt College of Science at Florida
byiellaiorsn ASTRONOMY Atlantic University in Boca Raton, Florida.

Metallicity In addition to his research on stellar
old RR Lyrae stars can populations in Local Group galaxies, he
indicators be considered the is an avid model railroading enthusiast.
Chemistry Swiss Army knives of
astronomy. These stars
Distance provide astronomers
with invaluable details
Dust about the places in
which they are found,

including distance,
Standard candles chemistry, history, and
even the amount of

obscuring material
between Earth and
the target region.

ASTRONOMY: ROEN KELLY

Reddening estimates

WWW.ASTRONOMY.COM 61

SECRET SKY

Is T CrB rising? Birmingham was on a road near his hometown in
Tuam, Galway County, when he noticed a nova in
In the next few years, we may witness a stellar outburst. Corona Borealis (T CrB) “at least equal to Alphecca
(Alpha CrB) and decidedly more brilliant.”
Corona Borealis The extraordinary dimming of Betelgeuse
the Northern Crown (Alpha [α] Orionis) that began in October Birmingham reported his observation to W.T. Lynn
(left) is a small 2019 captured the attention of astronomers of the Royal Observatory Greenwich. Julius Schmidt,
constellation in the worldwide. The star’s dip was a golden moment for director of the National Observatory of Athens, inde-
spring northern sky. Edward Guinan and colleagues at Villanova University, pendently discovered T CrB’s outburst the following
To find it, look about who have been monitoring the star for the past 25 years. night, writing to Lynn that he had surveyed Corona
20° northeast of They were the first to report its recent dimming. Borealis four hours prior to Birmingham’s discovery
brilliant Arcturus Betelgeuse’s brightness varies on three known cycles: and did not notice a change to the constellation. Lynn
(Alpha Boötis). The one between 100 to 180 days, one about every 425 days, concluded, therefore, that the star must have risen from
photo illustration to and one about every 5.9 years. “The current faintness beyond naked-eye visibility to 2nd magnitude, “either
the right shows how of Betelgeuse appears to arise from the coincidence of suddenly or with great rapidity,” in about four hours.
T CrB may appear the star being near the minimum light of the 5.9-[year]
in outburst. STEPHEN light-cycle as well as near the deeper than usual mini- Birmingham added that the star’s color was “nearly
mum of the 425-[day] period,” Guinan announced in white with a bluish tinge,” though others reported see-
JAMES O’MEARA an International Astronomical Union Circular on ing warmer hues. For instance, on May 18, 1866, F. Bird
December 23, 2019. of Birmingham, England, said that through apertures
BY STEPHEN We live in a time when predicting events like the up to 12 inches and powers up to 400x, the star appeared
JAMES O’MEARA eruption of recurrent novae — ordinary novae that have “decidedly yellow, inclining toward red.”
Stephen is a globe- multiple eruptions — are becoming ever-more familiar.
trotting observer who Take, for example, how Louisiana State University T CrB dropped to 10th magnitude in only a few days.
is always looking astronomer Bradley Schaefer predicted the 2010 erup- This behavior was repeated during its next outburst
for the next great tion of the recurrent nova U Scorpii and the 2014 erup- when it achieved roughly magnitude 3 on February 9,
celestial event. tion of the recurrent nova V745 Sco. 1946. So, the next few years may prove important for
T CrB, which is again showing signs of unrest.
A bit of history
An upcoming event?
Now that Orion has left the evening sky, it’s a perfect
time to start keeping an eye on yet another stellar won- T CrB is a binary star system comprising a red giant
der: recurrent nova T Coronae Borealis (T CrB). with a close white dwarf companion. Gas from the giant
Popularly known as the Blaze Star, it may erupt in star spills onto the white dwarf until a runaway ther-
brightness any time in the near future. monuclear explosion occurs on the dwarf star’s surface,
which we see as a novalike outburst.
Originally recorded as a magnitude 9.5 star in the
Bonner Durchmusterung (1859–62), T CrB made history The American Association of Variable Star Observers
on May 12, 1866. That night, Irish astronomer John (AAVSO) data show that T CrB’s minimum brightness
plateaued between April 2011 and February 2015. It
then entered an active state, including fluctuations in
its minimum brightness of up to a magnitude.

In July 2019, Schaefer related by private communica-
tion that T CrB was then “in the middle of its decade-
long pre-eruption plateau, with this being a weird and
very high energy event.” Specifically, he says, the star’s
“continuum has gone very blue and very high excitation
lines are popping in and changing fast. Any such long-
term pre-eruption plateau is ‘impossible.’  The identical
pre-eruption rise happened back in 1936–1946. So I am
predicting that T CrB will have its third known erup-
tion in the year 2023.6 +/-1.0.”

So, keep your eyes on this star. Familiarize yourself
with this constellation and watch for an aberration in
its shape. Or head to the AAVSO’s website for a chart to
monitor the star telescopically. As always, let me know
what you see or don’t see at [email protected].

BROWSE THE “SECRET SKY” ARCHIVE AT
www.Astronomy.com/OMeara

62 ASTRONOMY • JULY 2020

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FOR YOUR CONSIDERATION

The coolest cat Bohr’s interpretation is powerful in a shut-up-and-
calculate way, but it is also a shell game. Bohr knew in

on the web his gut what reality was supposed to look like, and
funny mixed-state wave functions weren’t it. To force
the theory into the mold of his preconceptions, Bohr

posited ill-defined “observers” that somehow stand

Quantum entanglement: Coming soon to a cellphone near you. apart from physical systems, then used them to arbi-

trarily collapse wave functions into the kind of classical

The other day, a friend reality his gut demanded.

showed me what has to be the Bohr’s sleight of hand did not go completely unno-

coolest thing on the internet. ticed. Eventually, a Princeton University graduate

It’s an app that turns you into a human student named Hugh Everett III had the temerity to call

version of Schrödinger’s cat (minus the foul. Everett threw out Bohr’s assumptions about what

whole dead-in-a-box thing, of course). it means to be “real,” stripped observers of their special

You read Astronomy magazine, so status, added them back into the wave function with

you know the basics. A cat is sealed in a everything else, then let the machinery of quantum

box, along with a Geiger counter and a mechanics do its thing.

radioactive source with a half-life of Amazingly, the wave functions naturally split into

one hour. The Geiger counter is rigged multiple isolated, noninteracting components, each

so that if it registers a single radioactive corresponding to a different classical outcome. The

The Universe Splitter decay, it will break a vial of deadly entire wave function remained, but you wouldn’t know
app lets you explore hydrocyanic acid. Bye-bye, kitty. it from the inside. Applied to Schrödinger’s cat, the
different quantum live-cat-happy-observer component of the wave func-
states. UNIVERSE SPLITTER Schrödinger probably needed therapy. tion and the dead-cat-sad-observer component of the
Anyway, in quantum mechanics (which, having

never made an incorrect prediction, is the crème de la wave function cannot interact after they split.

crème of theories), physical systems are described by Everett’s 1956 Ph.D. thesis, “The Theory of the

wave functions. Those wave functions con- Universal Wave Function,” kicked off what

tain all of the information there is about the How does it is now called the Many Worlds Interpretation
system, and evolve according to feel to be the of quantum mechanics. At first, Everett’s
Schrödinger’s Equation. In the case of heresy wasn’t taken very seriously, but some

Schrödinger’s cat, after an hour, the wave real-life star ideas have to await their time. As experi-
function of the stuff in the box is an equal of one of the ments have slammed the door on more and
mix of two quantum states. One state cor- most famous more of its alternatives, Many Worlds has
responds to no radioactive decay and a very and mind- moved squarely into the mainstream.
lucky cat, while in the other the cat doesn’t
Which brings me back to the coolest

have to worry about it anymore. bending thing on the internet. Check out the smart-
Living/dead quantum cats aren’t the thought phone app called Universe Splitter. The app
experiments asks you to let it choose between two
worst of it. “What about the observer?” you courses of action you might take. Hit “go”
might ask. “She is also part of the physical ever? and Universe Splitter contacts a machine
system. Shouldn’t the wave function

describing her also be a mixture of live-cat that automatically generates a single photon

and dead-cat states?” in a mixture of two quantum states; “mea-

Bingo! Here’s where Schrödinger throws up his sures” the photon, finding it in one state or the other;

hands and declares, “I don’t like it and I’m sorry I ever then tells you what to do. Of course, while you are doing

had anything to do with it!” one thing, the version of you in the other branch of the

In 1920, one of the early pioneers of quantum newly split universal wave function is doing the other.

mechanics, Niels Bohr, confronted the same basic prob- Like Schrödinger’s cat, your macroscopic reality is

lem. In Bohr’s view, a quantum system in a mixed state now and forever entangled with the mixed state of a

isn’t quite real until it is measured by an observer, at single quantum particle. How does it feel to be the real-

BY JEFF HESTER which point the wave function collapses unpredictably life star of one of the most famous and mind-bending
Jeff is a keynote into a single one of the allowed classical states. It’s thought experiments ever?
speaker, coach, meaningless to even talk about the state of the cat until
and astrophysicist. the observer opens the box. This is the “Copenhagen Meow.
Follow his thoughts Interpretation” that became physics orthodoxy for most
at jeff-hester.com of the 20th century. BROWSE THE “FOR YOUR CONSIDERATION”
ARCHIVE AT www.Astronomy.com/Hester

64 ASTRONOMY • JULY 2020

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7,& 81,9(56(¶ RQ ZHEVLWH www.newtonugeam.com

OBSERVING BASICS

Summertime At the other end of the telescope, your own body mois-
ture can fog your lens as you peer into the eyepiece.

observing The good news is that a hazy evening is often accom-
panied by steady seeing conditions. Jupiter and Saturn
both reach opposition this month, so look for planetary

Combat shorter nights, hazy skies, and irritating insects. details you might not be able to capture on a turbulent,
cool-weather night. And though faint galaxies and

nebulae may be off the menu, double stars and bright

One of the all-time star clusters are a worthy alternative.

classic summer-themed When it comes to the fogging of optical surfaces,

hits is the 1902 song “In reflecting telescopes are the least susceptible. Nestled

the Good Old Summertime,” inside the tube assembly, the primary and secondary

written by George Evans (music) mirrors are reasonably immune to condensation. The

and Ren Shields (lyrics). I don’t front-end objectives of refractors and catadioptrics

know much about either song- can be protected with the use of a dew shield. If your

writer, but I can make the follow- scope doesn’t have one, or if the existing one isn’t

ing statement with the utmost doing the job, make your own from a sheet of flexible

confidence: Neither was an black foam board purchased from a local craft shop.

amateur astronomer. Not only does a dew shield limit lens fogging, it also

Summertime astronomy for reduces the amount of stray light entering the tube. If,

Northern Hemisphere observers, despite your best efforts, your eyepiece still fogs up,

especially those who live in fan it vigorously with your hand to evaporate the dew.

higher latitudes, is nothing less Ditto with the telescope’s finder. If an electric outlet

than a nightmare. We suffer from is handy, you can even use a hairdryer dialed to its

three seasonal ills: god-awfully lowest heat setting.

late sunsets, god-awfully hazy Then there are the bugs. Not only are they a nuisance,

and humid weather, and god- but we also must be vigilant against mosquito-borne

awfully annoying mosquitoes. illnesses like West Nile virus, Zika virus, and eastern

The author’s summer What can we do to combat equine encephalitis (or EEE). The assault isn’t just from
observing setup relies
on a dew shield and these summertime maladies? We’re pretty much at the the air, either. We also must deal with ground attacks
a hairdryer to deal
with the fogging of mercy of late summer sunsets. In mid-northern lati- from ticks, which can lead to diseases like Lyme disease
optical surfaces, as
well as bug spray to tudes, July sunsets don’t happen until after and Rocky Mountain spotted fever.
combat mosquitoes
and other biting 8 p.m. local time. And the onset of true dark- What can There are several ways to handle the
insects. GLENN CHAPLE ness — the end of astronomical twilight — insect problem. Avoid observing from areas

occurs after 10 p.m. local time. If you need we do to frequented by nighttime biting insects. And

to get up early for work the next morning, combat before going outside, spray insect repellent
observing this late is impractical. And even these (preferably containing DEET) on all exposed
if you don’t have to work at first light, late- summertime skin and thin clothing. Unless the heat is
night lethargy still usually trumps any maladies? unbearable, also wear a long-sleeved shirt
enthusiasm you might have had for a back- and full-length slacks or jeans. Then, once

yard astronomy session. back indoors, undress and thoroughly check

If a must-see happening like an occulta- yourself for ticks.

tion is in the offing, you can always head to bed early The good news is that the days begin to shorten dur-

and set your alarm to wake you when it’s time to go ing July. Cool, dry air from the polar regions will begin

outside. Or, for casual observing, limit your summer to push away the summer murk and drive mosquitoes

evening outings to an hour or so after it gets dark. That and other biting insects into hibernation. Before you

way, you can still be in bed by midnight. know it, you’ll be trudging with your telescope through

But this isn’t a perfect solution. Even if you’re able to knee-deep snow while enduring frostbite-inducing cold

BY GLENN CHAPLE muster the energy to go outside, you’ll come face to face — and wishing it were summer.
Glenn has been an with summertime haze and humidity. A hot summer Questions, comments, or suggestions? Email me at
avid observer since day leads to a lot of evaporation, and the cooling night
a friend showed air causes that water vapor to condense. That means [email protected]. In the next two columns:
him Saturn through hazy skies. Worse yet, humid air at ground level loves Women in astronomy clubs. Clear skies!
a small backyard to condense on your telescope — particularly on the
scope in 1963. objective lenses of refractors and catadioptric scopes. BROWSE THE “OBSERVING BASICS” ARCHIVE
AT www.Astronomy.com/Chaple

66 ASTRONOMY • JULY 2020

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BINOCULAR UNIVERSE but rather its distance. M4 is the closest globular to our
solar system, about 5,600 light-years away. M13 is about

22,000 light-years away. M4 would appear even brighter

were it not for all of the intervening clouds of interstellar

Riches of the dust between us and it, which muffle the view.
M4 is arguably the easiest globular in the sky to

locate, owing to its proximity to such a bright star. But

Scorpion that can also make it a challenge to see through lower-
power binoculars for the same reason. Antares can be
distracting. If you have a problem seeing M4, move

The region of Scorpius contains numerous breathtaking Antares just off the eastern (likely the left) edge of the
field and try again. Once you spot it, you’ll wonder how

binocular targets for amateur observers. you ever missed it in the first place.

Through my 16x70s, M4 reveals a feature that is

This month, we dive head- unique among globulars: a bright central “bar.” In real-

first into the deep end of ity, this bar is formed by a coincidental queue of

the summer Milky Way by brighter-than-average stars. Some of those stars are

visiting everyone’s favorite bad guy, resolvable in 70mm and larger binoculars if you look

Scorpius the Scorpion. In one Greek carefully using averted vision.

myth, the goddess of the hunt Those same giant binoculars just might show a sec-

Artemis and her mother, Leto, dis- ond, far fainter globular cluster that appears even closer

patch Scorpius to kill Orion after the to Antares. NGC 6144 often goes unnoticed because of

hunter brags that he could slay any its proximity to both the star and M4. Messier missed it

animal on Earth. Another version altogether, but it was finally noticed by William Herschel

puts the blame on Artemis’ twin in May 1784. Lying some 30,000 light-years away, NGC

brother, the god Apollo. Either way, 6144 is also subdued by intervening clouds of dust. With

the scorpion prevails. To memorial- careful scrutiny, I’ve seen it through my 16x70s by first

The globular cluster ize Orion and Scorpius, Zeus places moving the distracting glare of Antares out of the field.
M4, located near the them in the sky directly opposite each another, so they Be aware that any interference from sky haziness, or dust
bright star Antares, are never visible at the same time. or dew on a lens, will render it invisible.
makes an appealing
binocular target. Like Orion, Scorpius is also famous as the home to If NGC 6144 proves difficult, try a more accessible

GERALD RHEMANN one of the largest stars visible to the naked eye. Antares Scorpion globular. M80 is fainter than M4, but far easier

(Alpha [α] Scorpii) is a monster of a star, a spectral-type than NGC 6144. To find it, scan 4.5° northwestward

M1.5 red supergiant. Compared to the Sun, its through your binoculars from Antares to a

outer edge would encompass the orbit of Mars. position just east of the halfway point

Through binoculars, Antares looks like a Scorpius is between Sigma (σ) and Nu (ν) Scorpii in the
glimmering ruby surrounded by countless home to one Scorpion’s head. There, we find M80 nestled
grains of diamond dust. Depending on Earth’s among faint field stars and framed by Sigma
atmospheric turbulence, Antares may look of the and Nu, still in the field of most 10x and
more like a flashing kaleidoscope of colors. At largest stars lower-power binoculars. Discovered by

their most turbulent, those arrhythmic pulses visible to the Messier on a winter morning in January 1781,
can have an almost hypnotic effect. naked eye: M80 shines at 7th magnitude and appears
about one-third as large as M4. In reality,
After snapping out of the Antarian trance, Antares. however, M80 is both larger and more con-
shift your attention just a degree to the star’s

west. There, you will find a hidden surprise: centrated. It’s just farther away, at more than

a small puff of celestial cotton afloat amongst 32,000 light-years from us. Binoculars reveal

the stars. That’s one of my favorite globular clusters: a perfect sphere focusing to a brighter central core.

M4, a colossus of some 100,000 stars. Buried within M80’s several hundred thousand stars

M4 was discovered in 1746 by Swiss astronomer are many so-called “blue stragglers.” These blue giant

Jean-Philippe Loys de Chéseaux. But, in a sort of “pub- stars appear much younger than other cluster members,

lish or perish,” no one else knew about it until French seemingly defying the cluster’s overall age. Studies con-

astronomer Nicolas-Louis de Lacaille independently clude that these stars may have lost their cooler outer

found it and published an account six years later. shells in close encounters with other stars, thereby expos-

Charles Messier added it to his fledgling catalog in ing hotter inner shells.

BY PHIL 1764, noting it as a “cluster of very small stars.” Indeed, Suggestions for future columns? Contact me through
HARRINGTON M4 is the only globular among the 29 in his catalog that my website, philharrington.net. Until next time,
Phil is a longtime he saw as anything more than a nebulous smudge. remember that two eyes are better than one.
contributor to
Astronomy and the At magnitude 5.8, M4 appears as bright as M13 in BROWSE THE “BINOCULAR UNIVERSE” ARCHIVE AT
author of many books. Hercules, even though it is not as large or as concen- www.Astronomy.com/Harrington
trated. It appears so bright not because of its size or girth,

68 ASTRONOMY • JULY 2020

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WWW.ASTRONOMY.COM 69

A S K A S T R O Astronomy’s experts from around the globe answer your cosmic questions.

The Sun appears here The Sun’s at longer wavelengths, in the
in ultraviolet light, light infrared, microwave, and radio.
which has a wave- Our Sun emits light at progres-
length slightly shorter Q IIN WHAT PART OF THE sively shorter wavelengths,
than that of visible ELECTROMAGNETIC SPECTRUM too: the ultraviolet, X-ray, and
light. Looking at the DOES THE SUN EMIT ENERGY? even gamma-ray parts of the
Sun in this portion of spectrum.
the electromagnetic Jason Perry
spectrum highlights Phoenix, Arizona But most of the Sun’s light is
its delicate — and in the infrared, visible, and
extremely hot — outer A I The Sun emits light in virtually every part of ultraviolet parts of the electro-
atmosphere, the the electromagnetic spectrum, albeit some magnetic spectrum. Its output
corona. SOHO (ESA & NASA) more than others. The sunlight that we see — aptly at the longest (radio) wave-
named visible light — falls into only a very narrow lengths is much less than its
range of the spectrum, from about 400 to 750 nano- output at visible wavelengths;
meters (a nanometer is one-billionth of a meter, or conversely, the Sun’s short-
about 400 millionths of an inch). The Sun also emits wavelength X-rays come only
from the hottest and most active
parts of its outer atmosphere,
the corona. The gamma rays our
star generates through fusion
processes in its core never make
it out of the Sun before they are
converted into lower-energy
light. So, the only gamma rays
from the Sun we receive here on
Earth are from extreme solar
events, such as the most power-
ful solar flares.

Alison Klesman
Senior Associate Editor

Q ION PAGE 51 OF THE FEBRUARY
ISSUE, THERE IS A PICTURE
CLAIMING TO SHOW VENUS IN
RETROGRADE. I HAVE BELIEVED
AND TAUGHT FOR 40 YEARS THAT
RETROGRADE MOTION ONLY OCCURS
WITH THE PLANETS BEYOND EARTH’S
ORBIT. SO PLEASE EXPLAIN HOW VENUS
COULD BE IN RETROGRADE, OR WAS
THIS A MISTAKE?

John Shonle
Amherst, New Hampshire

A I The apparent retrograde motion of planets (and
other objects) on the sky is an illusion caused
by the fact that objects in our solar system orbit the
Sun at different distances and speeds. This is certainly
easiest to picture for superior planets — those outside
of Earth’s orbit — such as Mars. Earth circles the Sun
every 365 days; Mars takes 687 Earth days to do the

70 ASTRONOMY • JULY 2020

same. Our planet has a shorter path to travel, so at some VENUS’ RETROGRADE MOTION j
point, we “catch up” to Mars and then pass it. Just as PERSEUS
passing a slower-moving car on the highway makes that N
car artificially appear to move backward from your point Pleiades
of view, Mars appears to move backward, or retrograde, AURIGA 20 Path of Venus April 1
relative to the background stars for a period of time. Once June 1 10
your car (or our planet) has pulled far enough ahead, the a May 1
retrograde motion disappears.
10
But as your question brings up, can this happen with
the inferior planets Venus and Mercury? The answer is 20
still yes, these planets do exhibit retrograde motion. Their
retrograde motion occurs because they circle the Sun E
much faster than Earth and sometimes overtake our
planet as they swing around our star. That same effect 10 TAU R U S
causes them to first pause, then move “backward” (or ¡ 20
westward) relative to the background stars, before paus- 10
ing and resuming their eastward motion. July 1
Aldebaran a
Alison Klesman
Senior Associate Editor 5°

Q I IF BETELGEUSE WERE TO brightness recently, astronomers still aren’t sure when When an inferior
EXPLODE, WOULD IT DAMAGE planet, such as
OUR EYES TO STARE AT IT? WHAT ABOUT exactly in the next 100,000 years or so it will explode as Venus, passes Earth,
LOOKING THROUGH A TELESCOPE? differences in the
a supernova. But they do know it will explode and can planets’ orbits make
John Hanson the planet appear
Huntsville, Alabama estimate how bright it will become. to stop and then
move backward, or
A IBetelgeuse is a red supergiant star some Currently, Betelgeuse is about the 10th-brightest star retrograde, in the sky
650 light-years away. Although it underwent for a brief time. This
some interesting — and noticeable — changes in in the sky, shining at magnitude 0.58. When it goes chart shows Venus’
path between April 1
This artist’s concept shows how the constellation Orion might supernova, it will experience a few different phases, and July 10, 2020, as
appear when Betelgeuse, the Hunter’s shoulder, goes supernova it appears to back-
and briefly becomes the brightest object in the night sky. first brightening for a short period of time, then fading track against the
Although the star will be visible during the day and cast a background stars
shadow at night, you still would be able to look at it safely, slightly before again brightening more slowly, and for about 40 days.
with or without a telescope. HENRYKUS/WIKIMEDIA COMMONS
finally fading for good. In the first phase, Betelgeuse ASTRONOMY: ROEN KELLY

would flare up quickly to appear brighter than Venus, SEND US YOUR
QUESTIONS
but only for a few moments. It would then fade to a few
Send your
times its current brightness, before slowly brightening astronomy questions
via email to askastro@
again over the course of about a week. Estimates vary, astronomy.com, or
write to Ask Astro,
but ultimately, Betelgeuse could shine roughly as bright P.O. Box 1612,
Waukesha, WI 53187.
as the Full Moon, although it might appear even Be sure to tell us
your full name and
brighter because the Moon’s light is spread out over where you live.
Unfortunately, we
about 0.5° on the sky, while Betelgeuse would appear cannot answer all
questions submitted.
only as a point source. It will be visible during the day

and, at night, likely cast shadows similar to those from

the Full Moon. The star will certainly appear as the

brightest point in the sky until it finally begins to fade

a few weeks later.

Many observers know that looking at the bright

Moon through a telescope without a filter, especially

with dark-adapted eyes, can be a bit uncomfortable and

certainly ruin your night vision. Even at its brightest,

though, Betelgeuse would not damage your eyes, with

or without a telescope. Alison Klesman

Senior Associate Editor

WWW.ASTRONOMY.COM 71

READER GALLERY

Cosmic portraits

1 2

1. A DEEP VIEW
Globular cluster M15 lies in the
constellation Pegasus the Winged
Horse. The imager’s goal here was
to capture the stars deep in the core
without overexposing them.
• Rodney Pommier

2. RING OF FIRE
These exposures, taken through
thickening clouds, capture the
climactic moments of the annular
eclipse that occurred December 26,
2019, at Rempang Island, Batam,
Indonesia. • Muhammad Rayhan

72 ASTRONOMY • JULY 2020

3 3. COMET AND CLUSTERS
4 Comet PanSTARRS (C/2017
T2) passed the Double Cluster
5 in Perseus (NGC 869 and
NGC 884) on January 24,
2020. This image combines
116 minutes of exposures
taken through a 12-inch f/3.6
astrograph. • Gerald
Rhemann

4. A LIT PAIR
This pairing of the crescent
Moon and Venus occurred
December 28, 2019.
Earthshine (sunlight reflected
from Earth onto the lunar
surface) illuminated the dark
part of the Moon. The
photographer captured a
1/20-second exposure at ISO
2500 from Pittsburgh.
• Matt Dieterich

5. GLOWING GAS
Sharpless 2–207 (left) and
Sh 2–208 are two emission
nebulae in the constellation
Camelopardalis the Giraffe.
Both objects have begun
the process of star formation,
but are so faint that this
image required 20.4 hours
of exposures to record them.
• Douglas J. Struble

6. EXTRAGALACTIC
The small group of galaxies to
the left includes two spirals
(NGC 5350 and NGC 5371)
and two lenticulars (NGC
5354 and NGC 5353). The
luminary of the group is NGC
5371 at magnitude 10.0, just
0.1 magnitude brighter than
NGC 5350. All of these
objects lie in the constellation
Canes Venatici the Hunting
Dogs. • Vasilis Misirlis

NGC 5350 SEND YOUR IMAGES TO:
NGC 5354 Astronomy Reader Gallery,
NGC 5353 P.O. Box 1612, Waukesha,
WI 53187. Please include
NGC 5371 the date and location of the
image and complete photo
data: telescope, camera,
filters, and exposures.

6 Submit images by email to
readergallery@
astronomy.com.

WWW.ASTRONOMY.COM 73

BREAKTHROUGH

VIRGO’S STRIKING SUPERNOVA FACTORY

The loosely wrapped spiral arms of NGC 5468 sparkle with brilliant star clusters glowing blue from the high-energy
radiation pouring from the hot young stars within. A more careful look also reveals a smattering of pinkish nebulae well on
their way to creating a new generation of dazzling clusters. This face-on galaxy lies about 140 million light-years from Earth
in eastern Virgo. With a diameter of 110,000 light-years, NGC 5468 closely matches the Milky Way’s size. But the stars in this
distant galaxy explode at a far greater rate than those in our home system. Astronomers have observed five supernovae in
NGC 5468 in the past 21 years, but haven’t seen one in the Milky Way in more than 400 years. ESA/HUBBLE AND NASA/W. LI ET AL.

74 ASTRONOMY • JULY 2020

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SOUTHERN SKY BY MARTIN GEORGE

September 2020

Jupiter and Saturn excel

With spring arriving of their structure. Look more brilliance. On September 1, its on its lesser-known gems: the
this month and the closely and you should see disk measures 18.9" across; by open star cluster M23.
nights growing warmer, Saturn’s shadow falling on the the 30th, it spans 22.4". As it
September offers nearly perfect rings behind and just east of climbs higher in the sky, take You can locate M23 in
conditions for observing the the planet’s limb. a few moments to explore the the northwestern corner of
Sun’s two largest planets. Both planet through a telescope. Sagittarius, some 6° north-
Jupiter and Saturn stand high Normally elusive Mercury Several dusky surface features northwest of the Lagoon
in the northeast during evening begins its best evening appear- as well as the south polar cap Nebula (M8). I find it to be
twilight and pass almost over- ance of 2020 in September. It should pop into view during a beautiful object through a
head within a couple of hours. becomes easy to see low in the moments of good seeing. telescope-eyepiece combination
Jupiter appears far brighter west after sunset by the month’s that yields a field of view of 0.5°
than its companion. The mag- second week, when it shines at As dazzling as Jupiter and or a bit more. The cluster con-
nitude –2.5 planet actually out- magnitude –0.2. The planet Mars appear, neither comes tains many stars of seemingly
shines every other point of light passes 0.3° north (to the lower close to matching magnitude almost equal brightness scat-
in the evening sky. The giant right) of Spica, Virgo’s most –4.2 Venus. The inner planet tered across its 30' diameter.
world remains nearly stationary prominent star, on the 22nd. rises before the first hint of
this month against the back- At magnitude –0.1, Mercury dawn and stands out in the The famous 18th-century
drop of eastern Sagittarius. shines a magnitude brighter northeast as twilight brightens. French astronomer Charles
Jupiter’s high altitude makes than Spica. The pair lies 11° Venus begins September among Messier discovered M23 on
it a showpiece object through high in the west an hour after the background stars of eastern June 20, 1764. He clearly
any telescope. Even the smallest sunset. Mercury climbs another Gemini, 9° due south of 1st- noticed a 6th-magnitude star
instrument shows two dark degree higher by month’s end. magnitude Pollux. It then nearby, and mentioned in his
cloud belts straddling a brighter moves eastward through notes that it appeared very
zone that coincides with the When viewed through a tele- Cancer and into Leo, ending close to the star English astron-
planet’s 43"-diameter equator. scope, the planet starts to look the month 4° west of 1st- omer John Flamsteed had des-
You can also track the planet’s attractive late in the month. On magnitude Regulus, the Lion’s ignated 65 Ophiuchi.
four bright moons as they circle the 30th, it sports a 7"-diameter brightest star.
the planet from night to night. disk that appears 63 percent lit. The problem is that astrono-
Beautiful Saturn follows Mercury’s appearance improves Unfortunately, Venus no mers can’t identify 65 Ophiuchi.
less than 10° — about 30 min- in October as its size swells and longer shows the dramatic They assume it was an errone-
utes — behind Jupiter as they its phase wanes to a crescent. changes in its telescopic ous position for 6 Sagittarii.
cross the sky from east to west. appearance we experienced Indeed, no star brighter than
The ringed planet also resides Shortly after Earth’s rotation this past autumn and winter. 8th magnitude lies within a
in eastern Sagittarius, just a few carries Jupiter and Saturn past In mid-September, the planet degree or so of the position
degrees from that constella- the zenith, Mars rises in the shows a disk that spans 17" and Flamsteed gave.
tion’s border with Capricornus. east. The Red Planet appears appears two-thirds lit.
Many observers consider unmistakable against the back- Sixth-magnitude 6 Sgr
Saturn their favorite telescopic drop of Pisces by late evening. The starry sky stands 2° northeast of M23, so
object, and a glance through a Mars nearly doubles in bright- it’s tempting to consider it to be
any instrument shows why. The ness during September, increas- The Milky Way arches across the star Messier was referring
planet’s spectacular ring sys- ing from magnitude –1.8 to the early evening sky during to. But I suspect Messier’s star
tem, which spans 40" in mid- –2.5. And by month’s end, the September, with Sagittarius is more likely magnitude 6.5
September, surrounds the Red Planet shines a bit brighter — and the two bright planets SAO 160909, which stands only
world’s 18"-diameter globe. The than Jupiter. Earth’s neighbor it currently hosts — near the 20' northwest of M23’s center,
rings tilt 23° to our line of sight, will continue to improve as it zenith. The constellation holds just beyond the cluster’s edge.
affording a mesmerizing view heads toward opposition in a treasure-trove of deep-sky
mid-October. objects visible through binocu- Unfortunately, we may never
lars and small telescopes. This know which star Flamsteed
Mars’ apparent diameter month I want to highlight one designated 65 Ophiuchi, nor
increases in tandem with its why Messier thought it was the
star tucked up against M23.

STAR DOME S

VOLANS

X U R b CHAMAELEON
CARINA
C
_a
HOW TO USE THIS MAP MENSA
NGC 5128 4755 NGC C
This map portrays the sky as seen `
near 30° south latitude. Located SW NGC 2070
inside the border are the cardinal
directions and their intermediate TAURUS N E C _ TAR IU SATNRGAULLEU M O CTANS NGC 104
points. To find stars, hold the map NGC 5139 ` SCP SMC
overhead and orient it so one of HYDRUS
the labels matches the direction CIRCINUS
you’re facing. The stars above
the map’s horizon now match T E LNEGSCC6O3P9I7UM PAV O TUCANA
what’s in the sky. ARA
LU P U S Antares NORMA INDUS
The all-sky map shows
how the sky looks at: NGC GRUS

10 P.M. September 1 LIBRA SCO ACUOS TRROANLAI S
9 P.M. September 15 6231
8 P.M. September 30
M4 R
Planets are shown
at midmonth P MICROSCOPIUM

I

M6 M7 SAGIT TARIUS
US

MAP SYMBOLS M22 S C U T UM S
M8 M17
Open cluster W M20 M16 U
Globular cluster
Diffuse nebula N
Planetary nebula
Galaxy Saturn IS
RI

C
T

S
S

M5 Jupiter PI
AU

OPHIUCHUS CAPRICORNUS

STAR SCEAR PP M11 AQUILA EQUULEUS
MAGNITUDES SERPENS Altair Enif

Sirius EN CAUDA
0.0 3.0 UT
1.0 4.0
2.0 5.0 S

SAGIT TA M15
VULPECULA
DELPHINUS

STAR COLORS NW HERCULES LYRA
Vega
A star’s color depends LACER
on its surface temperature.
CYGNUS
The hottest stars shine blue Deneb

•• Slightly cooler stars appear white
• Intermediate stars (like the Sun) glow yellow
• Lower-temperature stars appear orange
• The coolest stars glow red
• Fainter stars can’t excite our eyes’ color

receptors, so they appear white unless you
use optical aid to gather more light

N

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT
www.Astronomy.com/starchart.

SEPTEMBER 2020 SAT.

SUN. MON. TUES. WED. THURS. FRI.

Achernar ERIDANUS 1 2345 ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY
HOROLOGIUM
F OR NAX SE 6 7 8 9 10 11 12
RETICULUM L
13 14 15 16 17 18 19
PHOENIX
20 21 22 23 24 25 26
Fomalhaut NGC 253 CETUS Mira
SCULPTOR SGP 27 28 29 30
E
Note: Moon phases in the calendar vary in size due to the distance
AQUARIUS Path of the Sun (ecliptic) PISCES from Earth and are shown at 0h Universal Time.
Mars
CALENDAR OF EVENTS
PEGASUS
1 Venus passes 9° south of Pollux, 17h UT
RTA ANDROMEDA NE 2 Full Moon occurs at 5h22m UT

Asteroid Pallas is stationary, 13h UT
The Moon passes 4° south of Neptune, 21h UT
6 The Moon passes 0.03° north of Mars, 5h UT
The Moon is at apogee (405,607 kilometers from Earth), 6h29m UT
7 The Moon passes 3° south of Uranus, 4h UT
9 Mars is stationary, 18h UT
10 Last Quarter Moon occurs at 9h26m UT
11 Asteroid Fortuna is at opposition, 7h UT
Neptune is at opposition, 20h UT
13 Jupiter is stationary, 0h UT
14 The Moon passes 4° north of Venus, 5h UT
17 New Moon occurs at 11h00m UT
18 The Moon is at perigee (359,082 kilometers from Earth), 13h48m UT
The Moon passes 6° north of Mercury, 22h UT
22 Mercury passes 0.3° north of Spica, 9h UT
September equinox occurs at 13h31m UT
24 First Quarter Moon occurs at 1h55m UT
25 The Moon passes 1.6° south of Jupiter, 7h UT
The Moon passes 2° south of Saturn, 21h UT
29 Saturn is stationary, 3 UT
30 The Moon passes 4° south of Neptune, 2h UT
Asteroid Leto is at opposition, 3h UT