The Astronomy Book (Big Ideas Simply Explained)

URANUS TO NEPTUNE 99

The evening of the third, Herschel qualified his suggestion The asteroid Vesta was visited by
my suspicion was converted by reserving for himself “the liberty the Dawn spacecraft from 2011–12.
into certainty, being assured of changing that name, if another, Its orbit lies within that of Ceres,
more expressive of their nature, and it is the brightest asteroid as
it was not a fixed star. should occur.” seen from Earth.
I waited till the evening of
the fourth, when I had the Nothing more expressive did that come particularly close
satisfaction to see it had occur, and after the Celestial Police to Earth—the Near Earth Asteroids
moved at the same rate as was disbanded in 1815, a steady (NEAs)—are monitored in the
trickle of asteroid discoveries hope of predicting and preventing
on the preceding days. continued. By 1868, their number devastating future impacts.
Giuseppe Piazzi stood at 100; by 1985, it was 3,000.
The advent of digital photography Trojans
The Celestial Police kept up the and image analysis has now There are also asteroids known
search and, in March 1802, Olbers boosted the number of recorded as trojans, which travel in the
discovered a second body like asteroids to more than 50,000, same orbits as planets, gathering
Ceres located at the same distance spread around the 28-Bode-unit gap. far from their host in gravitationally
from the sun, calling it Pallas. In Olbers and Herschel had discussed stable “libration points.” Most of
1804, Karl Harding found a third, the possibility that the asteroids these are in the Jupiter system,
named Juno, while it was Olbers were the remains of a planet that where they form two clusters: the
again who spotted the fourth, once orbited in the gap before “Trojan Camp” and “Greek Camp.”
Vesta, in 1807. All these bodies being smashed by an astronomical Mars and Neptune have trojans,
were later shown to be smaller cataclysm. Today, it is thought and the first Earth trojan was
than Ceres—Vesta and Pallas that the gravitational disruption discovered in 2011.
were slightly more than 300 miles of nearby Jupiter prevented the
(500 km) wide and Juno was asteroids from accreting into a In 2006, the International
half that size. planet in the first place, as similar Astronomical Union gave Ceres
disks had done elsewhere in the the status of dwarf planet, the only
Asteroid belt primordial solar system. one in the asteroid belt. At the
The Celestial Police called their same time, Pluto was reclassified
discoveries minor planets, but Under constant influence from as a dwarf planet. The orbits of
William Herschel chose another the cumulative gravity of other neither Neptune nor Pluto match
name—asteroid, which means asteroids, about 80 percent of the predictions of Bode’s law.
starlike. Herschel reasoned that, known asteroids have unstable Despite the fact that it was
unlike true planets, these small orbits. The 13,000 or so bodies instrumental in the discovery of
objects had no discernible features, Ceres, Bode’s law is now viewed
or at least none that could be made They resemble small stars as a mathematical coincidence,
out with the telescopes of the day, so much as hardly to be and not a key to unlocking the
so would be indistinguishable from distinguished from them. formation of the solar system. ■
starlight were it not for the fact that From this, their asteroidal
they moved. Perhaps still smarting appearance, if I take my name,
from his failure to name the planet and call them asteroids.
he had found 20 years earlier,
William Herschel

100

OWAFSHTUOHRLEVEEHSYEUAORVFFEATNCHSEE

THE SOUTHERN HEMISPHERE

IN CONTEXT B etween 1786 and 1802, sky, his son’s observations would
William Herschel published have to be made from somewhere
KEY ASTRONOMER catalogs listing more in the southern hemisphere.
John Herschel (1792–1871) than 1,000 new objects in the
night sky. Following his death in Herschel settled on South
BEFORE 1822, William’s son John continued Africa, then a part of the British
1784 Charles Messier his work, but expanded its scope Empire. He moved there in 1833,
publishes a list of 80 and ambition to carry out a taking with him his wife and
known nebulae. complete survey of the night sky. young family, an assistant, and
William’s observations had all been his father’s 20-ft (6-m) focal length
AFTER made from southern England, and telescope. This was the same
1887 The Cartes du Ciel, an so were limited to objects down instrument that had been used
ambitious project to survey to around 33° below the celestial to survey the northern skies,
the entire sky photographically, equator. To survey the rest of the and Herschel chose it to ensure
is initiated by the director that the new information gathered
of the Paris Observatory, from the southern hemisphere
Amédée Mouchez. was comparable to that already
produced. The family set
1918 The Henry Draper themselves up in a house near
Catalog, covering most the base of Table Mountain, far
of the sky, is published by enough away to avoid the clouds
Harvard College Observatory. that often gathered on its summit,
and Herschel spent the next four
1948–58 The Palomar years completing his survey.
Observatory in California
completes its major sky The southern skies
survey, which includes The Magellanic clouds are two
data from nearly 2,000 dwarf galaxies close to the Milky
photographic plates. Way, and are only visible from the
southern hemisphere. They can be
1989–93 The Hipparcos
satellite gathers data that The Milky Way’s core is clearest
allow more than 2.5 million from the southern hemisphere. The
stars to be cataloged. dark regions are where starlight is
blocked by interstellar dust.

URANUS TO NEPTUNE 101

See also: Messier objects 87 ■ The Milky Way 88–89 ■
Examining nebulae 104–05

From each hemisphere, part of the
celestial sphere is always hidden.

A survey taken from Britain misses everything
33° below the celestial equator.

Adding observations from South Africa John Herschel
would make a complete survey.
John Herschel left Cambridge
Combining observations from University in 1816, already
both hemispheres produces a survey a renowned mathematician.
of the whole surface of the heavens. He worked with his father,
William, and continued his
seen by the naked eye, but Herschel’s Due to the orientation of the work after William’s death
telescopic surveys provided the first solar system within the Milky in 1822. Herschel became
detailed observations available to Way, the brightest section of it, one of the founders of the
astronomers. He compiled a list of which is now known to be the core Royal Astronomical Society
more than 1,000 stars, star clusters, of the galaxy, is only visible low and served as president for
and nebulae within these galaxies. on the horizon from the northern three separate terms. He
hemisphere during the summer married in 1826 and fathered
Herschel also made careful when nights are short. From the 12 children. Herschel had
observations of the distributions southern hemisphere, the brighter numerous interests in addition
of stars within the Milky Way. core is visible higher in the sky to astronomy. While in South
and during the darker months of Africa, Herschel and his
The stars are the the year, allowing easier and more wife produced a portfolio
landmarks of the universe. detailed observations. of botanical illustrations.
He also made important
John Herschel The end result of Herschel’s contributions to photography,
labors, The General Catalog experimenting with color
of Nebulae and Clusters of Stars, reproduction, and published
listed more than 5,000 objects papers on meteorology,
in total. These included all the telescopy, and other subjects.
objects observed by John and his
father, and also many discovered Key works
by others such as Charles Messier,
since it was intended to be a 1831 A Preliminary
complete catalog of the stars. ■ Discourse on the Study
of Natural Philosophy
1847 Results of Astronomical
Observations Made at the
Cape of Good Hope
1864 General Catalog of
Nebulae and Clusters of Stars
1874 General Catalog
of 10,300 Multiple and
Double Stars

102

   MTAHNOEVAESPMPTAAERRNESTNOTF

STELLAR PARALLAX

IN CONTEXT P arallax is the apparent ab
movement of a nearby
KEY ASTRONOMER object against distant Earth’s Earth’s
Friedrich Bessel (1784–1846) objects due to the changing position position in
position of the observer. According in June December
BEFORE to this phenomenon, nearby stars
220 bce Aristarchus suggests should appear to change position Sun
that the stars are very far away against the background of more
since no parallax can be seen. distant stars as Earth moves Due to the effects of parallax, a nearby
around its orbit. The idea that it star’s apparent position against distant
1600 Tycho Brahe rejects might be possible to measure the background stars moves from b in June
the Copernican sun-centered distance to nearby stars using to a in December.
system partly because he parallax dates back to ancient
cannot detect stellar parallax. Greece. However, it was not 1838, Bessel measured parallax with
achieved until the 19th century, an angle of 0.314 arc seconds for the
AFTER due to the distances involved being star 61 Cygni, which indicated that
1912 Henrietta Swan Leavitt far greater than anyone supposed. it was 10.3 light-years away. The
discovers a link between the current estimate is 11.4 light-years,
period of a type of variable Much of German astronomer giving Bessel’s measurement an
star and its brightness, Friedrich Bessel’s career had error of just under 10 percent. ■
allowing these stars to be been dedicated to the accurate
used as “standard candles” determination of the positions of
for figuring out distances. stars and finding their proper motion
(changes in position due to the
1929 Edwin Hubble discovers motion of the star, rather than
the link between the redshift changes in apparent position due
of a galaxy’s light and its to the time of night or the season).
distance from Earth. By the 1830s, with improvements in
the power of telescopes, there was
1938 Friedrich Georg a race to carry out the first accurate
Wilhelm Struve measures the measurement of stellar parallax. In
parallax of Vega, and Thomas
Henderson measures the See also: The Tychonic model 44–47 ■ Measuring the universe 130–37 ■
parallax of Alpha Centauri. Beyond the Milky Way 172–77

URANUS TO NEPTUNE 103

SCAPYUCPNELSAEPSROTINS

  THE SURFACE OF THE SUN

IN CONTEXT S unspots are cooler areas the sun. He did not find Vulcan, but
on the sun’s surface caused he did discover that the number of
KEY ASTRONOMER by changes in its magnetic sunspots varied over 11-year cycles.
Samuel Heinrich Schwabe field. The first written observations
(1789–1875) of sunspots date from about 800 bce, Swiss astronomer Rudolf Wolf
in China, but it was not until 1801 studied Schwabe’s and other
BEFORE that the British astronomer William observations, including some
800 bce Chinese and Korean Herschel made the connection from as far back as Galileo, and
astrologers record sunspots between sunspots and changes numbered the cycles starting at
to help foretell events. in Earth’s climate. 1 for the 1755–66 cycle. Eventually,
he saw that there are long periods
1128 English chronicler John Samuel Schwabe, a German in each cycle when the number of
of Worcester draws sunspots. astronomer, started observing sunspots is low. Herschel had not
sunspots in 1826. He was looking noticed the pattern because he was
1801 William Herschel links for a new planet thought to orbit observing during what is now called
sunspot numbers and the price closer to the sun than Mercury, the Dalton Minimum, when overall
of wheat, due to the effect of provisionally named Vulcan. It numbers of sunspots were low. ■
sunspots on Earth’s climate. would have been very difficult to
observe such a planet directly, but Sunspots can last from a few
AFTER Schwabe thought he might see it days to several months. The largest
1845 French physicists as a dark spot moving in front of can be the size of Jupiter.
Hippolyte Fizeau and Léon
Foucault photograph sunspots. See also: Observing Uranus 84–85 ■ The properties of sunspots 129 ■
Carrington (Directory) 336
1852 Irish astronomer Edward
Sabine demonstrates that the
number of magnetic storms
on Earth correlates with the
number of sunspots.

1908 US astronomer George
Ellery Hale discovers that
sunspots are caused by
magnetic fields.

104

OWAFSAAPSRIRDREAATLNEFGCOETRMEMDENT

EXAMINING NEBULAE

IN CONTEXT To the naked eye, nebulae are fuzzy patches
of light that could comprise gas or stars.
KEY ASTRONOMER
Lord Rosse (1800–1867) Larger telescopes Telescopes show
reveal a spiral form some nebulae to be
BEFORE clusters of stars.
1784 Charles Messier of arrangement.
publishes a catalog
of the visible nebulae. I n the 1840s, a British aristocrat Despite this difficulty, by 1845
named William Parsons, Lord Rosse had succeeded in casting
1785 William Herschel Rosse, decided to commit a mirror that was 72 in (1.8 m)
publishes catalogs of some of his considerable wealth in diameter. He mounted it in
nebulae and speculates that to building the world’s largest his telescope at Birr Castle, near
many are similar in shape reflecting telescope. Rosse was Parsonstown in Ireland, where it
and size to the Milky Way. curious to reexamine some of the became known as the Leviathan
nebulae listed by John Herschel in of Parsonstown. This telescope
1833 John Herschel expands the early 19th century, in particular remained the world’s largest
his father’s catalogs by those nebulae that did not appear reflecting type until the 100-in
surveying objects from the to be clusters of stars. (2.5-m) reflector was built at Mount
southern hemisphere. Wilson in California in 1917.
To re-observe these nebulae,
1864 William Huggins Rosse needed to build a larger and Central Ireland proved a far
discovers that some nebulae better telescope than that used by from ideal place to build a telescope,
are clouds of luminous gas, Herschel. He experimented for many as overcast or windy conditions
not aggregations of stars. years with methods for casting a often prevented viewing. The
36-inch (0.9-m) mirror. Mirrors at telescope itself had limited mobility,
AFTER the time were made from a metal meaning that only a small area
1917 Vesto Slipher concludes called speculum, an alloy of copper of the sky could be examined.
that spiral galaxies are “island and tin—a brittle material that was Nonetheless, when the weather
universes” and that the Milky prone to cracking as it cooled. was clear, Rosse was able to use
Way is one such galaxy seen
by us from within.

See also: Messier objects 87 ■ The Milky Way 88–89 ■ The southern URANUS TO NEPTUNE 105
hemisphere 100–01 ■ Properties of nebulae 114–15 ■ Spiral galaxies 156–61
Lord Rosse
the huge instrument to observe The light by which we
and record the spiral nature of recognize the nebulae now William Parsons was born
some nebulae—now called spiral must be merely that which in Yorkshire in 1800 and
galaxies—for the first time. The became Third Earl of Rosse
first of these spirals that Rosse left their surfaces a vast on the death of his father in
identified was M51, later known as number of years ago … 1841. He was educated at
the Whirlpool galaxy. Today, about phantoms of processes Trinity College, Dublin, and
three-quarters of all the galaxies completed long in the Past. at Oxford University, where
that have been observed are spiral he was awarded a first-class
galaxies. However, these are Edgar Allen Poe degree in mathematics. He
thought ultimately to transform married in 1836, but only four
into elliptical galaxies. Formed of of his 13 children survived
older stars, elliptical galaxies are to adulthood. Lord Rosse’s
dimmer and much harder to spot, estates were in Ireland,
but astronomers believe that they and this is where he built
are probably the most common his telescopes.
galaxy type in the universe.
In 1845, after he made
The nebular hypothesis stars in the Orion nebula, and public his findings on nebulae,
In the mid-19th century, so for a time, the idea of gaseous Rosse was criticized by John
astronomers debated whether nebulae was rejected. However, Herschel, who was convinced
nebulae consisted of gas or stars. although the stars were real, that nebulae were gaseous
In 1846, Rosse found numerous their presence did not mean that in nature. Both men accused
there was no gas. The gaseous each other of using flawed
The Leviathan Telescope at nature of some nebulae was not instruments. Ultimately,
Parsonstown held a mirror weighing demonstrated until spectroscopic however, neither succeeded
3.3 tons (3 metric tons), inside a 54 ft- analysis was used by William in demonstrating sufficient
(16.5 m-) long tube. The whole structure Huggins in 1864. ■ scientific evidence to resolve
weighed about 13 tons (12 metric tons). conclusively the question
of whether nebulae were
composed of gas or stars.

Key works

1844 On the construction of
large reflecting telescopes
1844 Observations on some
of the Nebulae
1850 Observations on
the Nebulae

106

YTWOHHUEOPHSLAEAVPNEOEPSTOITINIOTNED
OUT ACTUALLY EXISTS

THE DISCOVERY OF NEPTUNE

IN CONTEXT I n the months following William an undiscovered planet and using
Herschel’s discovery of Uranus Newton’s law of gravity to work out
KEY ASTRONOMER in 1781, astronomers found what its effect might be on Uranus.
Urbain Le Verrier (1811–1877) irregularities, or perturbations, This prediction was compared to
in its orbit. Most perturbations in observations of Uranus, and the
BEFORE orbits are caused by the gravitational position was revised according to
March 1781 William Herschel effects of other large bodies, but with the planet’s movements. After many
discovers Uranus. Uranus there were no known planets repetitions of this process, Le Verrier
that could cause the observed established the likely position of
August 1781 Finnish−Swedish motion. This led some astronomers an unknown planet. He presented
astronomer Anders Lexell finds to suggest that there must be a his ideas before the Académie des
irregularities in Uranus’s orbit planet orbiting beyond Uranus. Sciences in 1846, and he also sent
and suggests that they are due his predictions to Johann Galle
to other, undiscovered planets. Searching for the invisible (1812–1910) at the Berlin Observatory.
Frenchman Urbain Le Verrier tackled
1799–1825 Pierre-Simon the problem of the perturbations of Galle received Le Verrier’s letter
Laplace explains perturbations Uranus by assuming the location of on the morning of September 23,
mathematically. 1846, and obtained permission to

1821 French astronomer Alexis Unknown body Saturn
Bouvard publishes predictions
of future positions of Uranus. Calculations of Uranus’s Uranus Sun
Subsequent observations predicted orbit took into Jupiter
deviate from his predictions. account the gravitational Gravitational
effects of the sun, Jupiter, pull
AFTER and Saturn. However, the
1846 Briton William Lassell observed orbit deviated
discovers Triton, Neptune’s from the calculations in a
largest moon, only 17 days after way that suggested the pull
the discovery of the planet. of another massive body
farther out from the sun.
1915 Albert Einstein explains
perturbations in the orbit of
Mercury using relativity.

URANUS TO NEPTUNE 107

See also: The Milky Way 88–89 ■ Gravitational disturbances 92–93 ■
The theory of relativity 146–53

Perturbations in the orbits of planets can be explained by the
gravitational effects of other bodies in the solar system.

The orbit of Uranus has There may be another Urbain Le Verrier
perturbations that cannot planet beyond Uranus.
Urbain Le Verrier studied
be explained by any at the École Polytechnique,
known bodies. near Paris. After graduating,
his initial interests were in
Neptune is discovered Newton’s laws show chemistry, before he switched
very near the place where to look for to astronomy. His astronomical
predicted by the this planet. work was focused on celestial
mathematics. mechanics—the description
of the movements of the
look for the planet. Working with planet. There was some controversy bodies in the solar system
his assistant Heinrich D’Arrest, over who should have the credit for using mathematics. Le Verrier
he located an unknown object the discovery, but Adams always obtained a position at the
within 1° of the predicted position acknowledged that Le Verrier had Paris Observatory and spent
that same night. Observations on the better claim. most of his life there, acting as
subsequent nights showed that director from 1854. However,
the object was moving against the Galle was not the first person his management style was
background of stars and was, indeed, to observe Neptune. Once the orbit not popular and he was
a planet—one that would later be of Neptune had been worked out, replaced in 1870. He took
named Neptune at Le Verrier’s it was possible to go through old up the position again in 1873
suggestion. Galle later gave the records and find that others had after his successor drowned,
credit for the discovery to Le Verrier. already observed it without realizing and held it until his own
it was a planet, including both death in 1877.
Independent discovery Galileo and John Herschel. Later,
At the same time as Le Verrier Le Verrier used a similar technique Le Verrier spent his early
was calculating the position to analyze the orbit of Mercury career building on Pierre-
of the unknown planet, British and found that perturbations in Simon Laplace’s work on the
astronomer John Couch Adams its orbit could not be explained stability of the solar system.
(1819–92) was also looking at by Newtonian mechanics. He He later went on to study
the cause of the perturbations in suggested that this might be due periodic comets before turning
the orbit of Uranus. He arrived at to the influence of another planet his attention to the puzzle of
a similar conclusion to Le Verrier, even closer to the sun, provisionally Uranus’s orbit.
completely independently, but his named Vulcan. This speculation
results were not published until ended when Einstein explained Key work
after Galle had observed the new the perturbations using his general
theory of relativity. ■ 1846 Recherches sur les
Mouvements de la Planète
Herschel (Research on
the Movements of the
Planet Herschel)

TASHETRROISP

1850–1915

EHYOSFICS

110 INTRODUCTION

Germans Gustav Italian priest Angelo Pioneering American
Kirchhoff and Robert Secchi starts a project to astrophotographer Henry
classify stars according
Bunsen investigate Draper takes the first
the physics behind to their spectra. photograph of the
Orion nebula.
spectral lines.

1854 1863 1880

1862 1868 1888

Scottish physicist James British astronomer Using long-exposure
Clerk Maxwell produces Joseph Norman photography, British amateur
Lockyer discovers a astronomer Isaac Roberts
a set of equations that new element in the sun,
describe the wavelike which he calls helium. reveals the structure of the
Andromeda nebula.
behavior of light.

I n the early 19th century, from the longest wavelength (red) Such lines in the sun’s spectrum
astronomy was mainly to the shortest (violet). When a had been noted as early as 1802,
concerned with cataloging spectrum is examined in close but the first physicists to examine
the positions of stars and planets, detail, a multitude of fine variations the physics behind particular kinds
and understanding and predicting can emerge. A typical star of spectra were Gustav Kirchhoff
the movements of the planets. New spectrum appears crossed by and Robert Bunsen. Importantly, in
comets continued to be discovered, numerous dark lines, some fine about 1860, Kirchhoff showed that
and there was a growing awareness and faint, some broad and black. different patterns of dark lines are
of assorted distant phenomena, such the spectral fingerprints of different
as variable stars, binary stars, and Light is to us the sole chemical elements. Here was a
nebulous objects. However, there evidence of the existence way to investigate the composition
seemed little scope for learning of these distant worlds. of the sun and stars. It even led to
more about the nature of these James Clerk Maxwell the discovery of the previously
remote objects—their chemical unknown element, helium.
composition or temperature, for
example. The key that unlocked This new branch of astronomy
these mysteries was the analysis was enthusiastically taken up by
of light using spectroscopy. the British astronomer William
Huggins and his wife, Margaret,
Decoding starlight who also pioneered photography
A glowing object gives out light as a way of recording observations.
over a range of wavelengths, which They did not restrict themselves
we perceive as a rainbow of colors to stars, but studied the spectra
of nebulae as well.

THE RISE OF ASTROPHYSICS 111

The Harvard College While investigating X-rays, Harvard computer Henrietta
Observatory French physicist Henri Swan Leavitt shows how stars
Becquerel demonstrates called Cepheid variables can
produces the first be used to measure distances
Draper Catalog the effects of the radioactive
of Star Spectra. decay of uranium. in the universe.

1890 1896 1907
1895 1900 1912

In experiments with Max Planck lays the Austrian physicist
cathode ray tubes, foundation for quantum Victor Hess shows
German physicist mechanics by suggesting that powerful rays, now
Wilhelm Röntgen that energy can only exist called cosmic rays,
discovers X-rays. in distinct sizes of “quanta.”
come from space.

By the end of the 19th century, spectrum. This information quickly would strongly influence the
it seemed that, in order to fully paid dividends as astronomers future direction of astronomy.
understand the nature of stars, it analyzed the new data. Cannon’s Significant developments in basic
was necessary to systematically colleague at Harvard, Antonia physics impacted on astronomy.
record their spectra and classify Maury, realized that the simple For example, Briton James Clerk
them into different types. temperature sequence did not take Maxwell published his theory
account of subtle variations within of electromagnetism in 1873,
Star classification each star type. Ejnar Hertzsprung describing electromagnetic radiation
This immense task was undertaken and Henry Norris Russell such as light in terms of its wavelike
at Harvard College Observatory, independently followed this up, properties. X-rays were discovered
where the director Edward leading to the discovery that stars in 1895 and radioactivity in 1896. In
Pickering employed a large team of the same color could be giants or 1900, German physicist Max Planck
of women to carry out the exacting dwarfs, and the identification of the prepared the ground for quantum
work. Here, Annie Jump Cannon first known white dwarf star. physics with a leap of inspiration,
devised the stellar classification postulating that electromagnetic
system still used today, based The physics of the stars energy comes in “packets” of a
on a temperature sequence. In an interval of some 50 years, particular size, called “quanta.”
Cannon personally classified cutting-edge astronomy had These discoveries would lead
some 500,000 stellar spectra. changed its focus. By the early to new ways of looking at the
The star catalog included not only 20th century, physics—the study skies, and shed new light on the
their position but also precise of matter, forces, and energy, and processes taking place within stars.
information about their magnitude how they are related—could be Physics and astronomy would be
(apparent brightness) and applied to the sun and stars, and inseparable from this point on. ■

112

   BSS  EOODLFAIOURUMNATIDSMITNOOSTPHHEERE

THE SUN’S SPECTRUM

IN CONTEXT I n 1814, a German maker of hot, dense gas, such as the sun,
optical instruments named will emit light at all wavelengths
KEY ASTRONOMER Joseph von Fraunhofer and thus produce a continuous
Gustav Kirchhoff invented the spectroscope (see spectrum. However, if the light
(1824–1887) diagram on p.113). This allowed the passes through a cooler, lower-
spectrum of the sun, or any other density gas, such as the sun’s
BEFORE star, to be displayed and measured atmosphere, some of that light
1802 After creating an image with high precision. Fraunhofer might be absorbed by an element
of the sun’s spectrum by noticed that there were more than (sodium, for example), at the same
passing sunlight through 500 dark lines crossing the sun’s wavelengths at which the element
a narrow slit and prism, spectrum, each located at a precise emits light when heated. The
English chemist William wavelength (color). These came absorption of the light causes
Hyde Wollaston notices seven to be known as Fraunhofer lines. gaps in the spectrum, which are
dark lines in the spectrum. now known as absorption lines. ■
By the 1850s, German scientists
1814 Joseph von Fraunhofer, Gustav Kirchhoff and Robert Bunsen The path is opened for
the German inventor of the had discovered that, if different the determination of the
spectroscope, discovers 574 chemical elements are heated chemical composition of
of the same dark lines in the in a flame, they emit light at one the sun and the fixed stars.
sun’s spectrum. He maps or more wavelengths that are
these in detail. characteristic for that element, Robert Bunsen
acting like a fingerprint to indicate
AFTER its presence. Kirchhoff noticed
1912 Danish physicist that the wavelengths of light given
Niels Bohr introduces a off by some elements corresponded
model of the atom in which to the wavelengths of some
movements of electrons Fraunhofer lines. In particular,
switching between different sodium’s emissions at wavelengths
energy levels cause radiation of 589.0 and 589.6 nanometers
to be emitted or absorbed exactly matched two Fraunhofer
at particular wavelengths. lines. Kirchhoff suggested that a

See also: Analyzing starlight 113 ■ The characteristics of stars 122–27 ■
Refining star classification 138–39 ■ Stellar composition 162–63

THE RISE OF ASTROPHYSICS 113

STGHTRAEORIURSPSECPDAENBCYTBREA

 ANALYZING STARLIGHT

IN CONTEXT A ngelo Secchi was Star
one of the pioneers of
KEY ASTRONOMER astrophysics, an arm Light
Angelo Secchi (1818–1878) of science that focuses on the
properties of a star, rather than Prism
BEFORE merely its position in the sky.
1802 William Hyde Wollaston He was the first to group stars Spectroscopy uses Spectrum
notices that there are dark according to their spectra, or the a prism to refract
gaps in the sun’s spectrum. particular colors of light they emit. the light from a star,
splitting the light to
1814 German lensmaker A Jesuit priest as well as a allow its constituent
Joseph von Fraunhofer noted physicist, Secchi founded wavelengths to be
measures the wavelengths a new observatory at the order’s measured with a high
of these dark lines. Collegio Romano in Rome. There he degree of accuracy.
became a pioneer of the technique
1860 Gustav Kirchhoff and of spectroscopy, a new way to were orange, with a complex array
Robert Bunsen use a gas measure and analyze starlight. of elements present. In 1868, Secchi
burner to make systematic added Class IV for redder stars with
recordings of the wavelengths Gustav Kirchhoff had shown carbon present, and finally in 1877
produced by burning elements. that gaps in a stellar spectrum came Class V for stars that showed
were caused by the presence of emission lines (not absorption lines,
AFTER specific elements (see opposite). as in the other four).
1868 English scientist Armed with this knowledge, Secchi
Norman Lockyer identifies began to class stars according to Secchi’s stellar classes were
a new element, helium, their spectra. At first he used three later amended by other scientists,
from emission lines in the classes: Class I were white or blue and in 1880 became the foundation
sun’s light. stars that showed large amounts of of the Harvard System, which is
hydrogen in their spectra; Class II used to classify stars to this day. ■
1901 The Harvard System were yellow stars, with metallic
for the classification of spectral lines (for astronomers,
stellar spectra, devised “metallic” refers to any element
by Williamina Fleming heavier than helium); and Class III
and Annie Jump Cannon,
supersedes Secchi’s system. See also: The sun’s spectrum 112 ■ The sun’s emissions 116 ■
The star catalog 120–21 ■ The characteristics of stars 122–27

114

ELMNUAOMSRISNMEOSOUUOSSFGAS

PROPERTIES OF NEBULAE

IN CONTEXT I n the 1860s, a pioneering wavelengths. Huggins, encouraged
British astronomer named by his astronomer wife, Margaret,
KEY ASTRONOMER William Huggins made turned his attention deeper into
William Huggins (1824–1910) key discoveries by studying the space, toward nebulae, the fuzzy
composition of stars and nebulae patches of light that had long
BEFORE using a spectroscope. This mystified astronomers. He used
1786 William Herschel instrument, a glass prism attached spectroscopy to divide these
publishes a list of nebulae. to a telescope, splits white light into patches into two distinct types.
its constituent light wavelengths,
1850s Gustav Kirchhoff and producing a spectrum of color. The spectra of nebulae
Robert Bunsen realize that hot Gustav Kirchhoff and Robert Bunsen Huggins observed that nebulae,
gases produce bright emission had already noted the chemical such as the Andromeda nebula, had
lines in their spectra of light, composition of the sun by studying a spectrum of light similar to that
whereas cool gases absorb the the dark absorption lines that occur of the sun and other stars—a broad
same wavelength, producing in its spectrum. These lines are band of color with dark absorption
dark lines in spectra. caused by the atoms of different lines. The reason for this (not
chemical elements absorbing discovered until the 1920s, after
AFTER radiation at certain precise Huggins’s death) was that such
1892 Margaret Huggins is
made an honorary Fellow of the Spectroscopes allow Some nebulae are found
Royal Astronomical Society. astronomers to measure to have spectra similar
a nebula’s spectrum
1913 Dane Niels Bohr depicts to those of stars.
atoms as containing a central of light.
nucleus surrounded by
electrons. Spectral lines are These nebulae are Others have spectra that
produced when the electron enormous masses emit energy at a single
moves between energy levels. of luminous gas.
wavelength.
1927 American Ira Bowen
realizes that the two green
lines caused by “nebulium”
are produced by oxygen atoms
that have lost two electrons.

THE RISE OF ASTROPHYSICS 115

See also: Observing Uranus 84–85 ■ Messier objects 87 ■
The sun’s spectrum 112

nebulae are indeed composed of Huggins was the first to analyze William Huggins
stars, and are galaxies in their own the spectrum of a planetary nebula (the
right. The second type of nebula he Cat’s Eye nebula), confirming that it After selling the family
observed was entirely different. Its was gaseous and not composed of stars. drapery business when he
light spectrum was made of single- was 30, William Huggins
wavelength emission lines—energy energy in two strong green lines, ran a private observatory in
was being emitted as one color; which did not correspond to any Tulse Hill in South London.
there were no absorption lines. known chemical element. Some He used his new wealth
astronomers suggested that they to buy a powerful 8-in
Huggins realized that these were produced by a new element, (20-cm) refracting telescope.
second kind of nebula were huge dubbed nebulium.
clouds of hot, low-density gas. In 1875, age 51, Huggins
Some of this gas could be in the Huggins concluded from his married 27-year-old Irish
process of forming new stars; spectroscopic observations that astronomy enthusiast
other gas clouds, like the planetary all of the heavenly bodies he had Margaret Lindsey, who
nebulae, could have been ejected studied were made of exactly the encouraged him to adopt
from evolving stars. same elements as Earth. However, photography to record his
the mystery of nebulium was spectra and was an active
Huggins’ 1864 observations not solved until after his death. partner in his later research,
of the Cat’s Eye planetary nebula in In 1927, it was found to be simply co-authoring many papers.
the constellation Draco revealed a doubly ionized oxygen—oxygen Huggins was a pioneer in the
spectrum with a single absorption atoms that have lost electrons and use of photography to record
line, produced by hot hydrogen gas. have a double positive charge. ■ astronomical objects. He
However, the nebula also emitted also developed a technique
to study the radial velocity
of stars using the Doppler
shift of their spectral lines.

As a pioneer astronomical
spectroscopist, Huggins was
elected to serve as president
of the Royal Society from 1900
to 1905. He died at his home
in Tulse Hill in 1910, age 86.

Key works

1870 Spectrum analysis
in its application to the
heavenly bodies
1909 Scientific Papers

116

  DTPT  HREIFROEFRMSEERIUNSSNET’NFRSRCIYAOEELMLFLALONAWYME

THE SUN’S EMISSIONS

IN CONTEXT I n August 1868, the French A total eclipse of the sun reveals
astronomer Pierre Jules César the chromosphere. This image of an
KEY ASTRONOMERS Janssen traveled to India to eclipse was captured in 1919 by British
Jules Janssen (1824–1907) observe a solar eclipse. The eclipse astronomer Arthur Eddington.
Joseph Norman Lockyer covered the sun’s bright disk,
(1836–1920) leaving only a narrow ring of light. he changed his mind—the light was
This was the chromosphere, the not from sodium but from a hitherto
BEFORE middle of three layers in the sun’s unknown element, which he named
1863 Gustav Kirchhoff develops atmosphere, which was normally helium, after helios, the Greek word
spectroscopy, showing how hidden by the glare. Janssen for the sun. For some years, it was
light can be used to identify found that the spectrum of the thought that helium only existed
hot substances. chromosphere’s light contained on the sun, but in 1895, Scottish
numerous bright emission lines. chemist William Ramsay succeeded
1864 William and Margaret Using discoveries made by Gustav in isolating a sample from a
Huggins find that the spectra Kirchhoff, Janssen was able to radioactive uranium mineral. ■
of nebulae contain different confirm that the chromosphere
emission lines, showing that was a layer of gas. He also noticed a
they are largely clouds of gas. previously unseen yellow emission
line in the sun’s spectrum. He
AFTER assumed this unknown light was
1920 Arthur Eddington states produced by sodium, helping to
that stars are fueled by the give the sun its yellow hue.
fusion of hydrogen into helium.
In October that year, English
1925 Cecilia Payne-Gaposchin astronomer Joseph Norman Lockyer
shows that stars are largely developed a spectroscope for
made from the elements observing the chromosphere directly.
hydrogen and helium. He also detected its curious light
and also assumed it was produced
1946 US cosmologist Ralph by sodium, but after consulting
Alpher calculates that most the chemist Edward Frankland,
of the universe’s helium was
formed in the first few minutes See also: The sun’s spectrum 112 ■ Nuclear fusion within stars 166–67 ■
after the Big Bang. The primeval atom 196–97

THE RISE OF ASTROPHYSICS 117

BMOYFARACSHDAEISNNNSTEERLANSVEETRWSOEDRK

  MAPPING MARS’S SURFACE

IN CONTEXT B y the mid-19th century, Schiaparelli described various dark
scientists were increasingly areas as “seas” and lighter areas
KEY ASTRONOMER speculating about the as “continents.” He also portrayed
Giovanni Schiaparelli possibility of life on Mars, which what seemed to him to be a
(1835–1910) had been found to have certain network of long, dark, straight lines,
similarities to Earth, including ice or streaks, crisscrossing Mars’s
BEFORE caps, a similar length of day, and an equatorial regions. In his book Life
1858 Angelo Secchi first uses axial tilt that meant it experienced on Mars, Schiaparelli suggested
the word canali (channels) in seasons. However, it had also been that, in the absence of rain, these
connection with Mars. found that it did not rain on Mars. channels might be the mechanism
by which water was transported
AFTER Between 1877 and 1890, Italian across the dry surface of the
1897 Italian astronomer astronomer Giovanni Schiaparelli planet to allow life to exist there.
Vincenzo Cerulli theorizes that carried out a series of detailed
the Martian canals are just an observations of Mars to produce In the following years, many
optical illusion. a map of the planet’s surface. eminent scientists, including
American astronomer Percival
1906 A book by American Lowell, speculated that these
astronomer Percival Lowell, dark lines were irrigation canals
Mars and Its Canals, promotes constructed by intelligent beings
the idea that there may be on Mars. However, others could
artificial canals on Mars, not see the channels at all when
built by intelligent beings. they looked for them—and by 1909,
observations with telescopes of
1909 Photographs of Mars higher resolution had confirmed that
taken at the new Baillaud the Martian canals did not exist. ■
dome at the Pic du Midi
observatory in France discredit Schiaparelli’s 1888 atlas of Mars
the Martian canals theory. shows land, seas, and a network of
straight channels. Here, the south
1960s NASA’s Mariner flyby pole is shown at the top.
missions fail to capture any See also: Observing Saturn’s rings 65 ■ Analyzing starlight 113 ■
images of the canals or find Life on other planets 228–35
any evidence of them.

118

PTHHOETSOTGARRASPHING

ASTROPHOTOGRAPHY

IN CONTEXT Photographs of the Photographing
stars can be used to the stars requires
KEY ASTRONOMER make very accurate long exposures.
David Gill (1843–1914)
star maps.
BEFORE
1840 The first clear Accurate maps However, Earth’s
photograph of the moon reveal that stars are rotation makes images
is taken by American moving at different
John Draper, using a blurry. A precise
20-minute exposure. speeds and in tracking mechanism
different directions.
1880 John Draper’s son is needed to move
Henry takes a 51-minute the camera.
exposure of the Orion
nebula. He also takes the I saac Newton’s theory Gill was a pioneer in the field of
first wide-angle photograph of gravitation, like many astrophotography. In the mid-1860s,
of the tail of a comet. advances of the Scientific then still an amateur astronomer
Revolution (pp.42–43), was based working in his father’s backyard,
AFTER on a belief that the universe he built a tracking mount for his
1930 US astronomer worked like clockwork. In the 12-in (30-cm) reflecting telescope,
Clyde Tombaugh discovers 1880s, David Gill, a master and used it to photograph the
Pluto by spotting a moving clockmaker from Aberdeen, moon with a clarity that had never
object on photographic plates. Scotland, applied his precision been seen before. The photographs
clockmaking machinery to earned Gill a fellowship in the
1970s Charge-coupled astronomical telescopes—and, Royal Astronomical Society,
devices replace photographic ironically enough, offered a way and, by 1872, his first job as a
plates and film with digital of showing that the stars were professional astronomer at the
photographs. not all moving in clocklike unison. Dunecht Observatory in Aberdeen.

1998 The Sloan Digital
Sky Survey begins to make
a 3-D map of galaxies.

THE RISE OF ASTROPHYSICS 119

See also: The Tychonic model 44–47 ■ Mapping southern stars 79 ■ Messier objects 87 ■ Space telescopes 188–95 ■
A digital view of the skies 296 ■ Roberts (Directory) 336 ■ Kapteyn (Directory) 337 ■ Barnard (Directory) 337

Gill applied clockwork tracking Frank McClean, an astronomer motion of nearby stars relative to
mechanisms to telescope mounts friend of David Gill, donated the more distant ones. This information
so that the telescope could move McClean Telescope to the Cape was invaluable for measuring
in near-perfect harmony with the Observatory in 1897. David Gill stellar distances on a large
rotation of Earth. This allowed the used it extensively. scale, and it began to reveal to
instrument to remain fixed and astronomers the true scale of the
focused on a single patch of sky. photographic record of the southern galaxy and the universe beyond. ■
Gill was not the first to attempt sky. The result was the Cape
to photograph the heavens with Photographic Durchmusterung
telescopes, but imaging faint (catalog), showing the position
celestial light required exposures and magnitude of nearly half a
of several minutes at least, and million stars. Gill also became
poor tracking meant that early a key figure in the Carte du Ciel
star photographs were mostly (“Map of the Sky”) project, a global
incomprehensible blurs. collaboration of observatories
begun in 1887 with the goal of
Southern sky making a definitive photographic
In 1879, Gill became the chief map of the stars. This ambitious,
astronomer at the Cape Observatory expensive, and decades-long
in South Africa. By now, he was project involved teams of human
using the latest dry-plate system computers who would measure
(a photographic plate pre-coated the plates by hand. It was, however,
in light-sensitive chemicals), which superseded by new methods and
he employedto capture the “Great technologies before it was finished.
Comet” that appeared over the
southern hemisphere in 1882. The accurate maps produced
by Gill’s photographic techniques
Working in partnership with may seem fairly unremarkable
the Dutch astronomer Jacobus today, but at the turn of the 20th
Kapteyn, Gill spent the best part century they were the first reliable
of the next two decades creating a means for showing the proper

David Gill The eldest son of a successful measuring stellar parallax
clockmaker, David Gill was (p.102). His measurements,
destined to take over the family used in conjunction with his star
business. However, while at the maps, did much to reveal the
University of Aberdeen, he distances between stars. By the
became a student of the great time he left the Cape Observatory
physicist James Clerk Maxwell, in 1906, Gill was a renowned
whose lectures gave Gill a passion astronomer. In one of his final
for astronomy. When offered a job jobs, the government consulted
as a professional astronomer in him on the implementation of
1872, Gill sold the family business daylight saving hours.
and began work at the Dunecht
Observatory, Aberdeen. Key work

In addition to his pioneering 1896–1900 Cape Photographic
work in astrophotography, Durchmusterung (with
Gill developed the use of Jacobus Kapteyn)
the heliometer, a device for

120

MOAFEPARTSHEUCERISSETEMAERNST

THE STAR CATALOG

IN CONTEXT E dward C. Pickering, in his help of the Harvard “computers”—
role as director of the Harvard a team of mathematically
KEY ASTRONOMER College Observatory from minded women upon whom
Edward C. Pickering 1877 to 1906, laid the foundations Pickering relied to process the
(1846–1919) for precise stellar astronomy. His huge amounts of data required
team carried out star surveys that to create the catalog.
BEFORE broke new ground in understanding
1863 Angelo Secchi develops the scale of the universe. Pickering More than 80 computers,
spectral classification for stars. combined the latest techniques in known in those less enlightened
astrophotography with spectroscopy days as “Pickering’s Harem,”
1872 American amateur (splitting light into its constituent worked at the Harvard Observatory.
astronomer Henry Draper wavelengths) and photometry The first of them was Williamina
photographs the spectral (measuring the brightness of stars) Fleming, who had been Pickering’s
lines of Vega. to create a catalog that listed maid. Upon taking over the
a star’s location, magnitude, and observatory, Pickering fired his
1882 David Gill begins to spectral type. He did this with the male assistant, deeming him
survey the southern sky “inefficient,” and hired Fleming
using photography. in his place. Other notable names
among the computers included
AFTER Antonia Maury, Henrietta Swan
1901 Annie Jump Cannon, Leavitt, and Annie Jump Cannon.
along with Pickering, creates
the Harvard Classification A woman had no chance Color and brightness
Scheme, which forms the at anything in astronomy Pickering’s individual contributions
basis of stellar classification. except at Harvard in the to the star catalog were twofold. In
1880s and 1890s. And even 1882, he developed a method of
1912 Henrietta Swan Leavitt there, things were rough. photographing multiple star spectra
links the period of Cepheid William Wilson Morgan simultaneously by transmitting the
variables to their distance. stars’ light through a large prism
US astronomer and onto photographic plates.
1929 Edwin Hubble In 1886, he designed a wedge
measures the distance photometer, a device for measuring
to nearby galaxies using the apparent magnitude of a star.
Cepheid variables. Magnitudes had previously been
recorded psychometrically—using

THE RISE OF ASTROPHYSICS 121

See also: The sun’s spectrum 112 ■ The characteristics of stars 122–27 ■
Classifying star spectra 128 ■ Measuring the universe 130–37

the naked eye as a means of I do not know if God Edward C. Pickering
comparing one star’s brightness is a mathematician,
with that of another. The wedge but mathematics is the Edward C. Pickering was the
photometer was much more loom on which God dominant figure of American
objective; the observer viewed weaves the universe. astronomy at the turn of the
a target star alongside one of Edward C. Pickering 20th century. Many of the
several stars with an accepted first steps in the development
brightness and then edged a Peru, to survey the southern of today’s astrophysics and
wedge of calcite in front of the sky and produce the first all-sky cosmology were made by
known source, diminishing its photographic map. people he employed at the
magnitude in increments until Harvard College Observatory.
the two sources looked to have In combination with the Known as a progressive for
the same brightness. work of the Harvard computers, his attitudes to the education
Pickering’s data was the basis of women and their role in
In 1886, Mary Draper, the for the Henry Draper Catalogue research, Pickering nonetheless
widow of spectral photography published in 1918, which contained asserted a rigid authority
pioneer Henry Draper, agreed spectral classifications for 225,300 over his team. On more than
to fund Pickering’s work in her stars across the entire sky. ■ one occasion, he forced out
husband’s name. In 1890, the first researchers with whom he
Draper Catalogue of Stellar Spectra did not agree, only for them
was published. Pickering then later to be proved right; one
opened an observatory in Arequipa, example of this is Antonia
Maury, whose work on stellar
Many of the Harvard computers spectra Pickering dismissed.
were trained in astronomy, but as
women, they were excluded from Pickering spent his whole
academic positions. Their wages were career in academia, but was
similar to those of unskilled workers. also an avid outdoorsman, and
was a founding member of the
Appalachian Mountain Club.
The club became a leading
voice in the movement to
preserve wilderness areas.

Key works

1886 An Investigation in
Stellar Photography
1890 Draper Catalogue of
Stellar Spectra
1918 Henry Draper Catalogue

CTLHAESSSITFAYIRNSG

ACCORDING TO THEIR

SAPGECETRAARNEVDEALSS TIHZEEIR

THE CHARACTERISTICS OF STARS



124 THE CHARACTERISTICS OF STARS

IN CONTEXT A merican astronomer Each substance sends out its
Annie Jump Cannon was own vibrations of particular
KEY ASTRONOMER the early 20th-century’s wavelengths, which may be
Annie Jump Cannon leading authority on the spectra likened to singing its own song.
(1863–1941) of stars. When she died in 1941,
Cannon was described as “the Annie Jump Cannon
BEFORE world’s most notable woman
1860 Gustav Kirchhoff shows astronomer.” Her great contribution stars according to their spectra.
that spectroscopy can be used was to create the basis of the Pickering’s team modified this
to identify elements in starlight. system for classifying the spectra system. By 1924, the catalog
of stars that is still in use today. contained 225,000 stars.
1863 Angelo Secchi classifies
stars using their spectra. Cannon worked at the Harvard Early approaches
College Observatory, as part of the Williamina Fleming, the first of
1868 Jules Janssen and Joseph team of “Harvard Computers,” a Pickering’s female computers,
Norman Lockyer discover group of women employed by the made the earliest attempt at a more
helium in the solar spectrum. director Edward C. Pickering to help detailed classification system, by
compile a new stellar catalog. The subdividing Secchi’s classes into
1886 Edward Pickering begins college’s catalog, begun in the 13 groups, which she labeled with
compiling the Henry Draper 1880s with funding from the widow the letters A to N (excluding I), then
Catalogue using a photometer. of astrophotographer Henry Draper, adding O, P, and Q. In the next
used new techniques to collect phase of the work, fellow computer
AFTER data on every star in the sky Antonia Maury, working with better
1910 The Hertzsprung−Russell brighter than a certain magnitude, data received from observatories
diagram reveals the different including obtaining the spectra of
sizes of stars. as many stars as possible. In the
1860s, Angelo Secchi had set out
1914 US astronomer Walter a provisional system for classifying
Adams records a white dwarf.

1925 Cecilia Payne- The seven main classes of star,
Gaposchkin finds that stars categorized according to spectra and
are composed almost entirely temperature, are, from left to right:
of hydrogen and helium. O, B, A, F, G, K, and M, with O the
hottest and M the coolest.

THE RISE OF ASTROPHYSICS 125

See also: The sun’s spectrum 112 ■ Analyzing starlight 113 ■ The sun’s emissions 116 ■ The star catalog 120–21 ■
Analyzing absorption lines 128 ■ Refining star classification 138–39 ■ Stellar composition 162–63

around the the world, noticed more The spectra of stars cover a wide
variety in the detail. She devised a range of star types.
more complex system of 22 groups
designated by Roman numerals, A star’s spectrum can Classifying the
each divided into three subgroups. reveal its temperature, stars according
Pickering was concerned that to their spectra
applying such a detailed system luminosity, and
would delay the task of compiling composition. reveals their
the catalog. However, Maury’s age and size.
approach to stellar classification
proved a crucial step toward the appearance of its spectrum and Harvard system
creation of the Hertzsprung–Russell made her classes a temperature Cannon’s 1901 system laid the
diagram in 1910, and consequent sequence from hotter to cooler. foundations for the Harvard
discoveries about stellar evolution. On this, Cannon followed Maury’s Spectral Classification system.
lead. Some of Fleming’s letters By 1912, she had extended it to
Cannon joined the Harvard were dropped because they were introduce a range of more precise
College Observatory staff in 1896 unnecessary, so the final sequence subclasses, adding 0 to 9 after
and began working on the next became O, B, A, F, G, K, M, based the letter, with 0 the hottest in the
part of the catalog, which was on the presence and strength of class and 9 the coolest. A few new
published in 1901. With Pickering’s certain spectral lines, especially classes have been added since.
approval, to make classification those due to hydrogen and helium.
clearer and easier, she reverted Students of astronomy still learn it The Harvard system essentially
to Fleming’s spectral classes by remembering the mnemonic, classifies stars by temperature and
designated by letters, but she “Oh Be A Fine Girl, Kiss Me,” takes no account of the luminosity
changed the order. attributed to Henry Norris Russell. or size of the star. In 1943, however,
luminosity was added as an ❯❯
Maury had realized that stars
of similar colors have the same
characteristic absorption lines
in the spectra. She had also
deduced that a star’s temperature
is the main factor affecting the

126 THE CHARACTERISTICS OF STARS

additional dimension, creating The strengths of the absorption lines of different elements
the Yerkes classification system, vary according to the surface temperature of the star. Lines of
otherwise called the MKK system heavier elements are more prominent in the spectra of cooler stars.
after William Morgan, Philip
Keenan, and Edith Kellman, the Neutral Ionized Hydrogen Ionized Neutral Molecules
astronomers based at the Yerkes helium helium metals metals
Observatory in Wisconsin who
formulated it. This system denotes RELATIVE STRENGTH
luminosity with Roman numerals,
although a few letters are also used. OBA FGKM
DECREASING TEMPERATURE
The advantage of the MKK
system is that it gives a star a size as are this hot. O-type stars burn their twice as large as the sun, main
well as a temperature, so that stars fuel very quickly and release huge sequence A-type stars have a
can be described in colloquial terms amounts of energy. As a result, they surface temperature of between
such as white dwarf, red giant, or have a short life expectancy, which 7,500 and 10,000 K. They have
blue supergiant. The main sequence is measured in tens of millions strong hydrogen lines in their
stars, including the sun, are all small of years, compared to billions for spectra and emit a wide spectrum
enough to be called dwarfs. The sun cooler stars. Members of this class of visible light, which makes them
is a G2V star, which indicates that it have weak lines of hydrogen in look white (with a blueish tinge).
is a yellow dwarf with a surface their spectra, and strong evidence As a result, they are some of the
temperature of about 5,800 K. of ionized helium, which is present most easily seen stars in the night
because of the high temperature. sky, and include Vega (in Lyra),
Classes and characteristics Gamma Ursae Majoris (in the Big
The hottest class of star, O-types With a surface temperature of Dipper), and Deneb (in Cygnus).
have a surface temperature in excess between 10,000 and 30,000 K, B-type However, only 0.625 percent of main
of 30,000 K. Most of the radiation stars are brighter in visible light than sequence stars are A-type stars.
these stars emit is in the ultraviolet O-types, despite being cooler. This
part of the spectrum and appear is because more of the radiation is Cooling stars
blue when viewed in visible light. emitted as visible light, making them As dwarf stars cool, the hydrogen
O stars are mainly giants, typically “blue-white.” Again, B-type dwarfs in their spectra becomes less
20 times as massive as the sun and are rare, making up less than 0.1 intense. They also exhibit more
10 times as wide. Only 0.00003 percent of main sequence stars. absorption lines due to metals. (To
percent of main sequence stars When they do occur, they are an astronomer, everything heavier
perhaps 15 times more massive than helium is a metal.) This is
The prism has revealed to us than the sun. B-type stars have not because their composition is
something of the nature of non-ionized helium in their spectra different from that of hotter stars
and more evidence of hydrogen. but because the gas near the surface
the heavenly bodies, and the Because they live for only a short is cooler. In hotter stars, the atoms
photographic plate has made time, B-type stars are found in are too ionized to create absorption
molecular clouds or star-forming lines. F-type stars have a surface
a permanent record of the regions, since they have not had temperature of between 6,000 and
condition of the sky. time to move far from the location 7,500 K. Called yellow-white dwarfs,
in which they formed. About
Williamina Fleming

THE RISE OF ASTROPHYSICS 127

they make up 3 percent of the main main sequence stars, making up Annie Jump Cannon
sequence and are a little larger than 76 percent of the total, although
the sun. The spectra of these stars no red dwarf is visible to the naked Born in Delaware, Annie Jump
contain medium-intensity hydrogen eye. They are just 2,400–3,700 K Cannon was the daughter
lines and strengthening lines for on the surface and their spectra of a state senator, and was
iron and calcium. contain absorption bands for oxide introduced to astronomy
compounds. The majority of the by her mother. She studied
The sun’s class yellow, orange, and red dwarfs are physics and astronomy at
Type-G yellow dwarfs, of which believed to have planetary systems. Wellesley College, an all-
the sun is one, make up 8 percent women’s college. Graduating
of the main sequence. They are Extended classification in 1884, Cannon returned to
between 5,200 and 6,000 K on the Stellar spectral classes now cover her family home for the next
surface and have weak hydrogen even more types of stars. Class W 10 years. On the death of her
lines in their spectra, with more are thought to be dying supergiant mother, in 1894, she began to
prominent metal lines. Type-K stars. Class C, or carbon stars, are teach at Wellesley and joined
dwarfs are orange and make up declining red giants. Classes L, Y, Edward C. Pickering’s Harvard
12 percent of the main sequence. and T are a diminishing scale of Computers two years later.
They are between 3,700 and colder objects, from the coolest red
5,200 K on the surface and have dwarfs to the brown dwarfs, which Cannon suffered from
very weak hydrogen absorption are not quite large or hot enough to deafness, and the ensuing
lines but strong metallic ones, be classed as stars. Finally, white difficulties in socializing led
including manganese, iron, and dwarfs are class D. These are the hot her to immerse herself in
silicon. Type-M are red dwarfs. cores of red giant stars that no longer scientific work. She remained
These are by far the most common burn with fusion and are gradually at Harvard for her entire
cooling. Eventually they should fade career, and is said to have
A white dwarf sits at the heart of the to black dwarfs, but it is estimated classified 350,000 stars over
Helix planetary nebula. When its fuel it will take a quadrillion years for 44 years. Subject to many
ends, the sun will become a white dwarf. that to happen. ■ restrictions over her career
due to her gender, she was
finally appointed a member
of the Harvard faculty in 1938.
In 1925, she became the
first woman to be awarded
an honorary degree by
Oxford University.

Key work

1918–24 The Henry Draper
Catalogue

128

  OTTHWFEROREEKDIANSRDTESAR

ANALYZING ABSORPTION LINES

IN CONTEXT I n the late 19th and early 20th Red giant
centuries, Edward Pickering
KEY ASTRONOMER and his assistants carried Sun
Ejnar Hertzsprung out extensive work classifying
(1873–1967) star spectra. They cataloged the Red dwarf
range of light wavelengths coming
BEFORE from a star, which, among other A typical red giant has a diameter
1866 Angelo Secchi creates information, contains dark absorption about 50 times that of the sun, and
the first classification of stars lines. These lines indicate the 150 times that of a typical red dwarf.
according to their spectral presence of particular elements However, a red giant has only about
characteristics. in the star’s atmosphere that are 8–10 times the mass of a red dwarf.
absorbing those wavelengths.
1880s At the Harvard College category, or red-colored stars,
Observatory, Edward Pickering One of Pickering’s assistants, he noticed that the “c-types”
and Williamina Fleming Antonia Maury, developed her own were highly luminous, high-mass,
establish a more detailed classification system, taking into comparatively rare stars—today,
classification system. account differences in the width of these are called red giants or red
absorption lines in star spectra. She supergiants, depending on their
1890s Antonia Maury develops noticed that some spectra, which she size. The remaining majority of
her own system for classifying denoted as “c,” had sharp, narrow non-“c-type” M-class stars were
star spectra, taking into lines. Using Maury’s system, Danish low-mass, faint stars that are now
account differences in width astronomer Ejnar Hertzsprung saw known as red dwarfs. A similar
and sharpness of spectral lines. that stars with “c-type” spectra were distinction of two main kinds also
far more luminous than other stars. applied to K-class (orange) stars. ■
AFTER
1913 Henry Norris Russell Bright and dim red stars
creates a diagram, similar Hertzsprung figured out that what
to one made by Hertzsprung, Maury had identified as “c-type”
that plots the absolute stars were radically different from
magnitude (intrinsic brightness) other types in the same category.
of stars against spectral class. For example, within the M-class
This later becomes known as a
Hertzsprung−Russell diagram. See also: The sun’s spectrum 112 ■ Analyzing starlight 113 ■ The star catalog
120–21 ■ The characteristics of stars 122–27 ■ Refining star classification 138–39

THE RISE OF ASTROPHYSICS 129

SARUENSMPAOGTNSETIC

  THE PROPERTIES OF SUNSPOTS

IN CONTEXT A merican George Hale was including the 60-inch (150-cm)
just 14 when his wealthy Hale Telescope, built at California’s
KEY ASTRONOMER father bought him his first Mount Wilson Observatory in 1908,
George Ellery Hale telescope, and 20 when his father paid for by a bequest from his
(1868–1938) built him an observatory on the father. Working at Mount Wilson
family property. Two years later, that same year, Hale was able
BEFORE while at MIT, Hale developed a new to take clear images of sunspots
800 bce The appearance of dark design for a spectroheliograph— in a deep red wavelength emitted
spots on the sun is recorded in a device for viewing the surface by hydrogen. The speckled images
the Chinese Book of Changes. of the sun one wavelength of light reminded Hale of the way iron
at a time. He used this device to filings mapped the force field
1600 English physicist William study the spectral lines of sunspots. around a magnet. This led him to
Gilbert discovers that Earth look for signs of the Zeeman effect
has a magnetic field. Some years later, Hale organized in light coming from the sunspots.
the building of some of the largest
1613 Galileo demonstrates telescopes in the world at the time, The Zeeman effect is a split
that sunspots are features in spectral lines caused by the
on the surface of the sun. presence of a magnetic field, first
observed by the Dutch physicist
1838 Samuel Heinrich Pieter Zeeman in 1896. The
Schwabe notes a cycle in spectral lines in light coming from
the numbers of sunspots sunspots had indeed been split,
seen each year. which suggested to Hale that
sunspots were swirling magnetic
1904 British astronomers storms on the surface of the sun. ■
Edward and Annie Maunder
publish evidence of an 11-year The variations in the strength
sunspot cycle. of the sun’s magnetic field are shown
in this magnetogram, produced
AFTER using the Zeeman effect. The marks
1960 US physicist Robert correspond to the locations of sunspots.
Leighton introduces the field See also: Galileo’s telescope 56–63 ■ The surface of the sun 103 ■
of helioseismology, a study of The sun’s vibrations 213 ■ Maunder (Directory) 337
the motion of the solar surface.

THE KEY

SCALETO A DISTANCE

OF THE UNIVERSE

MEASURING THE UNIVERSE



132 MEASURING THE UNIVERSE

IN CONTEXT S ome of the most important, A remarkable relation
but often most challenging, between the brightness
KEY ASTRONOMER measurements for astronomers of these (Cepheid) variables
Henrietta Swan Leavitt to make have been the distances to and the length of their
(1868–1921) extremely remote objects—which periods will be noticed.
includes most celestial objects Henrietta Swan Leavitt
BEFORE aside from the moon, sun, and other
1609 German pastor David planets of the inner solar system. by the year 1900, the distances
Fabricius discovers the Nothing in the light coming from to only about 60 stars had been
periodically variable star Mira. distant stars and galaxies gives measured. Furthermore, the parallax
any direct indication of how far method could be applied only to
1638 Dutch astronomer that light has traveled through nearby stars. The difference in
Johannes Holwarda observes space to reach Earth. perspective for more distant stars
Mira’s variation in brightness over the course of a year was too
over a regular 11-month cycle. For several hundred years, small to be accurately determined.
scientists realized that it should be New methods were therefore needed
1784 John Goodricke discovers possible to measure the distances to measure large distances in space.
a periodic variation in the star to relatively nearby stars by a
Delta Cephei: the prototypic method called parallax. This is Measuring brightness
example of a Cepheid variable. based on comparing the position In the 1890s and early 1900s, the
of a nearby star against the Harvard College Observatory in
1838 Friedrich Bessel measures background of more distant stars Massachusetts was one of the
the distance to the star 61 Cygni from two perspectives—usually world’s leading astronomical research
using the parallax method. Earth’s different positions in space
six months apart in its orbit around
AFTER the sun. Although many others
1916 Arthur Eddington had tried (and failed) before him,
studies why Cepheids pulsate. the first astronomer to measure a
star’s distance accurately using
1924 Edwin Hubble uses this method was Friedrich Bessel,
observations of a Cepheid in 1838. However, even with
in the Andromeda nebula increasingly powerful telescopes,
to calculate its distance. measuring star distances by
parallax proved difficult and,

Henrietta Swan Leavitt Henrietta Swan Leavitt developed Due to the prejudices of the
an interest in astronomy while day, Leavitt did not have the
studying at Radcliffe College, chance to use her intellect
Cambridge, Massachusetts. After to the fullest, but she was
graduation, she suffered a serious described by a colleague as
illness that caused her to become “possessing the best mind
increasingly deaf for the rest of at the Observatory.” She was
her life. From 1894 to 1896 and remembered as hardworking
then again from 1902, she worked and serious-minded, “little given
at Harvard College Observatory. to frivolous pursuits.” Leavitt
Leavitt discovered more than worked at the Observatory until
2,400 variable stars and four her death from cancer in 1921.
novae. In addition to her work
on Cepheid variables, Leavitt Key work
also developed a standard of
photographic measurements, 1908 1777 Variables in the
now called the Harvard Standard. Magellanic Clouds

THE RISE OF ASTROPHYSICS 133

See also: A new kind of star 48–49 ■ Stellar parallax 102 ■ The star catalog 120–21 ■ Analyzing absorption lines 128 ■
Nuclear fusion within stars 166–67 ■ Beyond the Milky Way 172–77 ■ Space telescopes 188–95

Milky Way At the time, the SMC and LMC
were thought to be very large star
Earth clusters within the Milky Way,
which itself was assumed to
Large The Cepheid variable stars that comprise the entire universe.
Magellanic Leavitt studied are in the Magellanic Today, they are known to be
Cloud Clouds, known today to be galaxies relatively small, separate galaxies
outside the Milky Way. The Large that lie outside the Milky Way.
Magellanic Cloud is about 160,000 The Magellanic Clouds are visible
light-years away; the Small Magellanic to the naked eye in the night sky
Cloud is about 200,000 light-years away. of the southern hemisphere, but are
Both are part of the Local Group galaxy never visible from Massachusetts,
cluster that includes the Milky Way. where Leavitt lived and worked.
Small Magellanic Cloud Therefore, although she examined
numerous photographic plates
institutions. Under the supervision eventually became the head of of the LMC and SMC obtained
of its director, Edward C. Pickering, the photographic photometry by astronomers at an observatory
the Observatory employed many department. This mainly involved in Peru, it is highly unlikely that
men to build equipment and take measuring the brightness of stars, she ever physically observed
photographs of the night sky, but a specific aspect of Leavitt’s them in the sky.
and several women to examine work was to identify stars that
photographic plates taken from fluctuate in brightness—known After several years’ work,
telescopes throughout the world, as variable stars. To do this, Leavitt had found 1,777 variables in
measure their brightness, and she would do a comparison of the SMC and LMC. One particular
perform computations based on photographic plates of the same part kind that caught Leavitt’s attention,
their assessment of the plates. of the sky, made on different dates. representing a small fraction of all
These women had little chance Occasionally she would find a star the variables she had found (47
to do theoretical work at the that was brighter on different dates, out of 1,777), was of a type called
Observatory, but several of them, indicating that it was a variable. a Cepheid variable. Leavitt called
including Williamina Fleming, them “cluster variables”—the term
Henrietta Swan Leavitt, Antonia Cluster variables Cepheid variable was introduced ❯❯
Maury, and Annie Jump Cannon, A specific task that Leavitt took
nevertheless left a lasting legacy. on was to examine some of the One of the most striking
photographic plates of stars in the accomplishments of Miss
Henrietta Swan Leavitt, who had Small Magellanic Cloud (SMC) and Leavitt was the discovery
originally joined the Observatory the Large Magellanic Cloud (LMC). of 1,777 variable stars in
as an unpaid volunteer in 1894,
the Magellanic Clouds.
Solon I. Bailey

Colleague of Leavitt

134 MEASURING THE UNIVERSE

The period of the fluctuation in brightness of a galaxies. In examining her records
Cepheid variable is closely related to its intrinsic brightness. of Cepheid variables in either the
LMC or SMC, Leavitt noticed
Measuring its period Comparing its intrinsic something that seemed significant.
gives a value for its brightness to its Cepheids with longer periods
seemed to be brighter on average
intrinsic brightness. apparent brightness than those with shorter periods.
from Earth gives a value In other words, there was a
for its distance from Earth. relationship between the rate at
which Cepheids “blinked” and their
Cepheid variables can be used as “standard candles” brightness. Furthermore, Leavitt
to measure distances in the universe. correctly inferred that, since the
Cepheids she was comparing were
later. These are stars that regularly in brightness followed by a slower all in the same distant nebula
vary in brightness with a period tailing off. Today, they are known to (either the LMC or the SMC), they
(cycle length) that could be anything be giant yellow stars that “pulsate”— were all at much the same distance
from one to more than 120 days. varying in diameter as well as from Earth. It followed that any
Cepheid variables are reasonably brightness over their cycles—and difference in their brightness as
easy to recognize because they are are very rare. As a class of stars, viewed from Earth (their apparent
among the brightest variable stars, they also have an exceptionally magnitude) was directly related
and they have a characteristic light high average brightness, which to differences in their true or
curve, showing fairly rapid increases means they stand out even in other intrinsic brightness (their absolute
magnitude). This meant there was
a definite relationship between the
periods of Cepheid variables and
their average intrinsic brightness
or their optical luminosity (the rate
at which they emit light energy).

Leavitt published her initial
findings in a paper that first
appeared in the Annals of the

Hottest state Coolest state

A straight line can readily LUMINOSITY Period of one pulsation Light
be drawn among each of curve
the two series of points
corresponding to maxima and TIME
minima, thus showing that A Cepheid variable belongs to a class of star called a pulsating
there is a simple relation variable. These stars expand and contract over a regular cycle, at the
between the brightness of the same time regularly varying in brightness. They are hottest and brightest
variables and their periods. shortly after reaching their most contracted phase. The graph of the
Henrietta Swan Leavitt star’s luminosity (light output) against time is called its light curve.

THE RISE OF ASTROPHYSICS 135

Brightness and magnitudes of stars information about the distance to
the SMC, nor indeed any accurate
data about the intrinsic brightness
of any Cepheid variable.

Apparent magnitude Absolute visual Optical luminosity is Calibrating the variables
is the brightness magnitude is the the rate at which a star To turn Leavitt’s finding into
of a star as viewed brightness of a star emits light energy from a system that could be used to
from Earth. as viewed from a set its surface and is closely determine absolute distances, not
distance and indicates related to absolute just relative distances, it needed
the true or intrinsic visual magnitude. calibrating in some way. In order
brightness of a star. to do this, it would be necessary to
measure accurately the distances
Astronomical Observatory of One of the first people to appreciate to and intrinsic brightness of a few
Harvard College in 1908. Then, the significance of Leavitt’s Cepheid variables. Hertzsprung
in 1912, after further study, which discovery was Danish astronomer therefore set about determining the
included plotting graphs of the Ejnar Hertzsprung. Due to the distances to a handful of Cepheids
periods of Cepheid variables period−luminosity relationship in the Milky Way galaxy, using an
in the SMC against values for discovered by Leavitt, Hertzsprung alternative complex method called
their minimum and maximum realized that by measuring the statistical parallax, which involves
brightness, she confirmed her period of any Cepheid variable calculating the average movement
discovery in more detail. It became it should be possible to determine of a set of stars assumed to be at
known as the “period−luminosity” its luminosity and intrinsic a similar distance from the sun.
relationship. Formally, it stated brightness. Then by comparing
that the logarithm of the period its intrinsic brightness to its Having obtained the stars’
of a Cepheid variable is linearly apparent magnitude (measured distances, it was a straightforward
(i.e., directly) related to the star’s average brightness from Earth), it step to figure out the intrinsic
average measured brightness. should be possible to calculate the brightness of each of the nearby
distance to the Cepheid variable. Cepheids. Hertzsprung used
Building on Leavitt’s work In this way, it should also be these values to calibrate a scale,
Although it is possible that possible to determine the distance which allowed him to calculate
Leavitt did not realize the full to any object that contained one the distance to the SMC and the
implications right away, she had or more Cepheid variable star. intrinsic brightness of each of
discovered an extremely valuable Leavitt’s Cepheids in the SMC. ❯❯
tool for measuring distances in the However, there was still a
universe, far beyond the limitations problem to be solved: although I should be willing to pay
of parallax measurements. Cepheid Leavitt had established the thirty cents an hour in view
variables were to become the first important period−luminosity of the quality of your work,
“standard candles”—a class of relationship, initially all this
celestial objects that have a known promised was a system for although our usual price,
luminosity, allowing them to be measuring the distance to remote in such cases, is twenty
used as tools to measure vast objects relative to the distance
distances in space. to the SMC. The reason for this five cents an hour.
is that Leavitt had no accurate Edward C. Pickering

136 MEASURING THE UNIVERSE

Leavitt left behind a legacy of
a great astronomical discovery.

Solon I. Bailey

Following these calibrations,
Hertzsprung was able to establish
a system for determining the
distance to any Cepheid variable
from just two items of data—its
period and its apparent magnitude.

Further applications to the first realistic estimate of The star RS Puppis is one of
It was not long before Leavitt’s the true size of the Milky Way, the brightest Cepheid variables
findings, tuned by the work of was an important milestone in in the Milky Way. It is about 6,500
Hertzsprung, led to further galactic astronomy. light-years from Earth and has a cycle
important results in terms of of variability lasting 41.4 days.
helping to understand the scale Right up to the 1920s, many
of the universe. From 1914 to 1918, scientists (including Harlow Andromeda nebula, allowing its
the American astronomer Harlow Shapley) maintained that the distance to be measured. This
Shapley (who was also the first Milky Way galaxy was the whole led directly to the confirmation
person to show that Cepheid universe. Although there were that the Andromeda nebula is a
variables are pulsating stars) those that believed otherwise, separate large galaxy (and is now
was one of the first to use the neither side could conclusively called the Andromeda galaxy)
newly developed concept that the prove their argument one way outside the Milky Way. Later,
distances of variable stars could be or another. In 1923, however, the Cepheids were similarly used to
found from knowing their periods American astronomer Edwin show that the Milky Way is just one
and apparent brightness. Shapley Hubble, using the latest in of a vast number of galaxies in the
found that objects called globular telescopic technology, found universe. The study of Cepheids
star clusters—all part of the Milky a Cepheid variable in the
Way—were distributed roughly in
a sphere whose center lay in the
direction of the constellation of
Sagittarius. He was able to
conclude from this that the center
of the Milky Way galaxy is at a
considerable distance (tens of
thousands of light-years) in the
direction of Sagittarius and that
the sun is not, as had previously
been supposed, at the center of the
galaxy. Shapley’s work, which led

THE RISE OF ASTROPHYSICS 137

was also employed by Hubble in classical and Type II Cepheids, The measurement of distances
his discovery of the relationship and started to be used for different to Cepheid variables for more
between the distance and purposes in distance measuring. accurate calibration of period–
recessional velocity of galaxies, luminosity relationships is still
leading to confirmation that the Today, classical Cepheids are considered extremely important,
universe is expanding. used to measure the distance of and it was one of the primary
galaxies out to about 100 million missions of the Hubble Space
Revising the scale light-years—well beyond the Telescope project when it was
In the 1940s, the German local group of galaxies. Classical launched in 1990. A better
astronomer Walter Baade was Cepheids have also been used to calibration is crucial, among
working at the Mount Wilson clarify many characteristics of other things, to calculate the age
Observatory in California. Baade the Milky Way galaxy, such as its of the universe. Leavitt’s findings
made observations of the stars local spiral structure and the sun’s from over a century ago are still
at the center of the Andromeda distance from the plane of the having significant repercussions
galaxy during the enhanced galaxy. Type II Cepheids have been in terms of truly understanding
viewing conditions afforded used to measure distances to the the scale of the cosmos. ■
by the wartime blackout. He galactic center and globular clusters.
distinguished two separate
populations, or groups, of Cepheid A simplified version Gravity Pressure
variables that have different of the mechanisms that forces
period–luminosity relationships. cause Cepheid variables A
This led to a dramatic revision in to fluctuate in size is B
the extragalactic distance scale— shown here. The pressure Pressure forces exceed
for example, the Andromeda forces inside a star include gravity. The star begins Pressure and gravity are now
galaxy was found to be double gas pressure, maintained to expand. in balance but inertia causes
the distance from the Milky by heat output from the the star to expand further.
Way that Hubble had calculated. star’s core, and radiation
Baade announced his findings pressure. Another
at the International Astronomical mechanism that may
Union in 1952. The two groups be involved is a cyclical
of Cepheids became known as change in the opacity
(resistance to the
transmission of radiation)
in gas within the star’s
outer layers.

Hubble’s underwhelming C D E
acknowledgment of Leavitt
is an example of the ongoing With continued expansion, Pressure and gravity As the star contracts, the
denial and lack of professional the pressure forces decrease, are in balance again pressure forces increase until
and public recognition that as does the gravity, though but inertia causes the they exceed the inward-
she suffers from, despite her to a lesser extent. Eventually, star to shrink further. pulling gravity. The star
gravity exceeds the pressure stops shrinking and begins
landmark discovery. from the pressure forces, and to expand again, starting
Pangratios Papacosta the star stops expanding a new pulsation cycle.
and begins to shrink.
Science historian

138

DSGWITAAANRRTSFSSAORRE

REFINING STAR CLASSIFICATION

IN CONTEXT Among most stars, A round 1912, American Henry
blue stars are brighter Russell began comparing
KEY ASTRONOMER than yellow stars, which are stars’ absolute magnitude
Henry Norris Russell brighter than orange/red (or true brightness) and their color,
(1877–1957) or spectral class. Before the early
stars. These are 20th century, no one had figured out
BEFORE dwarf stars. how different star types might be
1901 Annie Jump Cannon, However, a few related in some overall scheme, but
working at the Harvard College exceptionally bright stars it had long been recognized that
Observatory, introduces the do not follow this rule. they differ in certain properties,
star spectral classes O, B, A, F, These are giant stars. such as color. While some stars
G, K, and M (based on surface Stars fall into two shine with a pure white light, others
temperature of stars). distinct groups when have distinct colors: many have
plotted on a diagram reddish or bluish hues, while the
1905 Based on analyses of star showing luminosity sun is yellow. In 1900, German
spectra, Ejnar Hertzsprung and temperature. physicist Max Planck worked out
states that there are two the precise mathematics to describe
fundamentally different kinds Stars are either how the mix of wavelengths of light
of star within some spectral giants or dwarfs. given off by hot objects, and hence
classes, one of which is far their color, varies according to their
more luminous. temperature. Thus, star colors are
related to surface temperature—red
AFTER stars have the coolest surfaces, and
1914 Walter Adams discovers blue stars the hottest. By around
white dwarf stars—white-hot 1910, stars were considered to fit
but relatively faint. into spectral classes related to their
colors and surface temperatures.
1933 Danish astronomer
Bengt Strömgren introduces The other obvious way in which
the term “Hertzsprung–Russell stars differ is in their brightness.
diagram” to denote a plot Since ancient times, stars have
of the absolute magnitudes been classified into brightness
of stars against spectral class. classes. This developed into the
apparent magnitude scale, which
rated stars according to how bright

THE RISE OF ASTROPHYSICS 139

See also: Analyzing starlight 113 ■ The characteristics of stars 122–27 ■ Analyzing absorption lines 128 ■
Measuring the universe 130–37 ■ Discovering white dwarfs 141 ■ Stellar composition 162–63

they look from Earth. However, it ABSOLUTE MAGNITUDE -10 Supergiants Giants The Hertzsprung−
was realized that, in order to know Russell diagram
a star’s absolute brightness, it -5 shows the distribution
would be necessary to correct for of stars by absolute
its distance from Earth: the farther 0 magnitude and
away a star is, the dimmer it will spectral class. The
appear. From the mid-19th century, +5 Main sequence diagram formed the
reasonably precise distances to (Dwarfs) basis for developing
some stars began to be calculated, theories about how
and the absolute brightness of +10 stars evolve. (In the
these stars could be established. +15 White dwarfs absolute magnitude
scale, the lower the
Russell’s discovery number, the higher
Among the majority of stars, the magnitude.)
Russell found a definite
relationship—hot blue-white stars 20,000 10,000 5,000 2,500
(spectral classes B and A) tend to
have higher absolute magnitudes TEMPERATURE (°C)
than cooler white and yellow stars
(classes F and G), while white and 1913. However, unknown to him, Russell called these ordinary stars
yellow stars have higher absolute Danish chemist and astronomer “dwarfs”; Hertzsprung referred to
magnitudes than orange and red Ejnar Hertzsprung had performed them as “main sequence.” The
stars (classes K and M). However, a similar exercise a couple of years newly discovered hot but faint
some exceptionally bright red, orange, earlier, and the diagram is now white dwarfs were later added to
and yellow stars departed from this known as the Hertzsprung–Russell the diagram, forming a third group.
rule. These were the “giant” stars. diagram. The diagram shows stars Today, it is known that most stars
divided into a group of bright giant spend most of their lives on the
Russell plotted the absolute stars and a much larger group of main sequence, some later evolving
magnitudes of stars against ordinary stars running diagonally. into giants or supergiants. ■
their spectral classes on a scatter
diagram, which he published in

Henry Norris Russell Henry Norris Russell was born appointed as an instructor
in Oyster Bay, Long Island, in in astronomy at Princeton
1877. At age 5, his parents University, and in 1911, he
encouraged him to observe became professor of astronomy
a transit of Venus across the there. He was also director
sun’s disc, which inspired an of Princeton University
interest in astronomy. He was Observatory from 1912 to 1947.
awarded a doctoral degree
by the astronomy department Key works
of Princeton University for an
analysis of the way that Mars 1927 Astronomy: A Revision of
perturbs the orbit of the asteroid Young’s Manual of Astronomy;
Eros. From 1903 to 1905, he Volume 1: The Solar System;
worked at the Cambridge Volume 2: Astrophysics and
Observatory, England, on star Stellar Astronomy
photography, binary stars, and 1929 On the Composition of the
stellar parallax. In 1905, he was sun’s Atmosphere

140

    IPRFSREANOCDEMOIATMTSRIIPANOATGNCINEG

COSMIC RAYS

IN CONTEXT A ustrian-born physicist by substances in the ground,
Victor Hess made a series meaning that air ionization should
KEY ASTRONOMER of dangerous high-altitude decrease with altitude. However,
Victor Hess (1883–1964) ascents over eastern Germany in a measurements made at the top
hydrogen balloon in the years 1911 of the Eiffel Tower in Paris in
BEFORE and 1912. His goal was to measure air 1909 indicated a higher level
1896 French physicist Henri ionization at a height of 3 miles (5 km). of ionization than expected.
Becquerel detects radioactivity.
Ionization is the process by Hess’s results showed that
1909 German scientist which electrons are stripped from ionization decreased up to an
Theodor Wulf measures air atoms. In the early years of the altitude of about half a mile
ionization near the top of 20th century, scientists were (1 km), and then increased above
the Eiffel Tower. Levels are puzzled by the levels of ionization that point. He concluded that
higher than expected. in Earth’s atmosphere. After the powerful radiation from space
discovery of radioactivity in 1896, was penetrating and ionizing the
AFTER it was suggested that ionization atmosphere. This radiation later
1920s American physicist was caused by radiation emitted became known as cosmic rays.
Robert Millikan coins the
term “cosmic ray.” In 1950, scientists found that
cosmic rays consisted of charged
1932 American physicist particles, some possessing very
Carl Anderson discovers the high energies. They smash into
positron (antiparticle of the atoms in the atmosphere, creating
electron) in cosmic rays. new subatomic particles that may
themselves create collisions, which
1934 Walter Baade and Fritz in turn cause a cascade of collisions
Zwicky propose the idea that called a cosmic ray shower. ■
cosmic rays come from
supernova explosions. In 1951, the Crab nebula was found
to be a major source of cosmic rays.
2013 Data from the Fermi Since then, supernovae and quasars
Space Telescope suggest that have also been identified as sources.
some cosmic rays come from
supernova explosions. See also: Supernovae 180–81

THE RISE OF ASTROPHYSICS 141

SAISTWATRHOIOTTHEFAAHTIONTT

DISCOVERING WHITE DWARFS

IN CONTEXT I n the first decade of the 20th The answer could only be that,
century, American astronomer although it was small (about the
KEY ASTRONOMER Walter Adams developed a size of Earth), its density must be
Walter Adams (1876–1956) method for calculating the absolute immense—about 25,000 times
magnitude of stars from the relative that of the sun. 40 Eridani B was
BEFORE intensities of particular wavelengths the first white dwarf star to be
1783 William Herschel in their spectra. One of the original discovered. White dwarfs were
discovers 40 Eridani B and C. team members at the Mount Wilson later shown to be the hot stellar
Observatory in California, Adams cores left behind when main
1910 Williamina Fleming used his method to investigate the sequence stars run out of fuel
answers an inquiry from triple-star system 40 Eridani, which for nuclear fusion. ■
Henry Norris Russell about contained a mysterious star that
the spectrum of 40 Eridani B, seemed very dim but also very hot.
confirming that it is a Type
A star. White dwarf Composed of material 3,000
The brightest of the three stars, times denser than anything
AFTER 40 Eridani A, was being orbited you have ever come across, a
1926 British astronomer by a much dimmer binary pair, ton of [this] material would be
Ralph Fowler applies new 40 Eridani B and C. Stars as a little nugget that you could
ideas in quantum physics faint as 40 Eridani B and C were
to explain the nature of the expected to be of spectral class M, put in a matchbox.
extremely dense material meaning that their starlight is red, Arthur Eddington
in white dwarfs. indicating a relative coolness. 40
Eridani C fitted this profile, but 40 describing white dwarfs
1931 Subrahmanyan Eridani B was one of the whitest
Chandrasekhar calculates and hottest types of star. When
that white dwarfs cannot be Adams published the data in 1914,
more massive than 1.4 times astronomers were presented with
the mass of the sun. a puzzle: a star that hot had to be
getting its energy from somewhere.
1934 Walter Baade and Fritz
Zwicky suggest that stars See also: Observing Uranus 84–85 ■ Refining star classification 138–39 ■
too massive to become white The life cycles of stars 178 ■ Energy generation 182–83
dwarfs form neutron stars.

AATNODMGSAL

1915–1950

LSATXAIRESS

144 INTRODUCTION

Albert Einstein Observing a solar eclipse, Edwin Hubble finds a
publishes his general Arthur Eddington shows relation between the redshift
theory of relativity,
which explains gravity that light from stars is and distance of nebulae,
bent by the sun’s gravity, showing that spiral
as a warping of just as relativity predicts.
spacetime. nebulae are galaxies.

1916 1919 1924

1917 1920 1926

Vesto Slipher shows that At the Smithsonian Austrian physicist Erwin
many nebulae show large museum, a “Great Schrödinger formalizes
redshift, meaning that Debate” takes place over the equation describing
whether or not spiral quantum mechanics, which
they are moving away nebulae are galaxies. describes strange behavior
from us rapidly.
at the quantum level.

D espite the vast difference understanding how the hierarchy In 1919, the New Zealand physicist
in scale, atoms, stars, of matter in the universe is Ernest Rutherford was able to
and galaxies share a organized. Underpinning these transmute atoms of nitrogen
property in common: each in its developments was Einstein’s into oxygen by firing particles at
own size domain is a fundamental general theory of relativity, in them from a radioactive element.
construction unit of the universe. which the concepts of mass There was now ample evidence
Galaxies define the distribution and energy are inseparable in that nuclear processes could
of matter in the universe on the a unified fabric of space and time. produce new elements and
grandest scale; stars are a defining release unimaginable quantities
constituent of those galaxies Looking inside a star of energy. For any remaining
(although galaxies may harbor Between 1916 and 1925, Briton doubters, Eddington reflected
quantities of gas, dust, and Arthur Eddington worked on the on the experiments conducted at
mysterious dark matter as well); physical nature of ordinary stars Cambridge University by pointing
atoms are the units of matter that such as the sun. He pieced together out that “what is possible in the
make up the hot gas of stars (with a detailed physical description Cavendish Laboratory may not
some simple molecules in cooler of a sphere of hot gas, in which be too difficult in the sun.”
stars). If we think of galaxies as energy makes its way from a central
cities, stars are like individual source to the surface, from where it When British astronomer Cecilia
buildings within the city, and then radiates into space. Eddington Payne-Gaposchkin, working in
atoms are the bricks. also did much to convince the US, concluded in 1925 that
astronomers that stars are fueled stars are overwhelmingly made
In a mere 30-year period in by subatomic processes—what of hydrogen atoms, astronomers
the first half of the 20th century, we would now call nuclear energy. at last had a real grasp on the
astronomy took huge leaps in true nature of “ordinary” stars.

ATOMS, STARS, AND GALAXIES 145

At the Lowell Observatory in Georges Lemaître American astrophysicist
Arizona, Clyde Tombaugh publishes a paper in Lyman Spitzer Jr.
which he proposes that proposes putting
discovers Pluto, which is the universe began telescopes in space.
initially classified as from a tiny “atom.”
the ninth planet.

1930 1931 1946
1930 1933 1946

Subrahmanyan Using an antenna he had British astronomer
Chandrasekhar calculates built himself, American Fred Hoyle shows
the conditions under which radio engineer Karl Jansky how elements are
discovers radio waves
a star can collapse into a made in stars.
neutron star or black hole. coming from space.

However, not all stars are quite holes was born, although many tiny “primeval atom” like a firework.
so ordinary. White dwarfs, for astronomers found it hard to believe In just a handful of years, astronomers
example, are clearly extraordinarily they could really exist. In any event, had learned that the universe was
dense. In the 1930s, the tools of the it was four decades before the first far larger and more complex than
new quantum physics were used to neutron stars and candidate black they had ever imagined. ■
explain how a star could become holes were identified.
so compacted and predicted even We used to think that if
more exotic types of collapsed The universe of galaxies we knew one, we knew two,
star. It turned out that 1.46 solar Meanwhile, the whole concept of the
masses would be the upper limit nature of the universe was changing because one and one are
to make a white dwarf, but there rapidly. In 1917, American Vesto two. We are finding that
was nothing to stop more massive Slipher recognized that many we must learn a great deal
stars from collapsing into a much so-called “nebulae” were galaxies,
denser neutron star or a black hole. akin to our own Milky Way, and in more about “and.”
rapid motion. Some 10 years later, Arthur Eddington
Black holes may be real Belgian priest Georges Lemaître
Walter Baade and Fritz Zwicky realized that an expanding universe
speculated that the central remnant was consistent with Einstein’s theory
of a supernova explosion would of relativity. American Edwin Hubble
be a neutron star, and with the discovered that the more distant a
work of Indian Subrahmanyan galaxy, the faster it is receding from
Chandrasekhar and others, the us, and Lemaître suggested that the
theoretical concept of black universe began by exploding from a

GTIMREAAVNDITSPAATCEIOANND

EHAXVEISNOTSEEPNARCATEE

FROM MATTER

THE THEORY OF RELATIVITY



148 THE THEORY OF RELATIVITY

IN CONTEXT The speed of light A person undergoing
is always constant acceleration cannot tell
KEY ASTRONOMER even when observers if this is due to gravity or
Albert Einstein (1879–1955) another force. Their body
are moving.
BEFORE This must mean that could be thought of as
1676 Ole Rømer shows that moving through space moving, or the universe
light speed is not infinite. around it could be thought
makes the flow of
1687 Isaac Newton publishes time slower. of as changing.
his laws of motion and Mass exists not
universal law of gravitation. just in space but in
spacetime. Mass itself
1865 James Clerk Maxwell distorts spacetime.
shows that light is a
wave moving though an The slowing of time Gravity is best described
electromagnetic field at makes an object’s as the result of spacetime
a constant speed. mass increase. being warped by mass.

AFTER Time and space and gravitation have
1916 Karl Schwarzschild uses no separate existence from matter.
Einstein’s equations to show
how much matter warps space.

1919 Arthur Eddington
provides evidence for the
warping of spacetime.

1927 Georges Lemaître shows
that a relativistic universe can
be dynamic and constantly
changing, and proposes the
Big Bang theory.

A lbert Einstein’s general The theory of relativity arose from Measuring the speed of light is
theory of relativity has a contradiction between the laws of not an easy thing to do. Danish
been called the greatest motion described by Isaac Newton astronomer Ole Rømer tried in 1676
act of thought about nature ever and the laws of electromagnetism by measuring the time delay in the
to take place in a person’s head. defined by Scottish physicist light arriving from Jupiter’s moons.
It explains gravity, motion, matter, James Clerk Maxwell. Newton His answer was 25 percent too
energy, space and time, the described nature in terms of matter slow, but he did show that light’s
formation of black holes, the Big in motion governed by forces that speed was finite. By the 1850s,
Bang, and possibly dark energy. act between objects. Maxwell’s more accurate measurements
Einstein developed the theory over theories concerned the behavior had been made. However, in a
more than a decade at the start of electric and magnetic fields. Newtonian universe, there must
of the 20th century. It went on to Light, he said, was an oscillation also be changes in the speed
inspire Georges Lemaître, Stephen through these fields, and he of light to account for the relative
Hawking, and the LIGO team, predicted that the speed of light motion of its source and observer.
which searched for the gravitational was always constant, regardless Try as researchers might, no such
waves predicted by the theory. of how fast the source was moving. differences could be measured.