Sky & Telescope - July 2023 - 36

SHIM HARNO / ALAMY STOCK PHOTO
Stellar History
examine stellar spectra. But the one exception, Antonia
Maury, paved the way for Hertzsprung to arrive at a profound
discovery. Maury used lowercase letters to denote the widths
of a star's spectral lines. In particular, she labeled stars with
narrow spectral lines type c. Pickering didn't care for what
he saw as needless complexities and didn't adopt her system.
He suspected the line widths were instrumental defects that
indicated nothing about the stars themselves.
But Hertzsprung discovered otherwise. He noticed that
Maury's c-stars tended to have small proper motions. A star's
proper motion is its year-after-year apparent movement
against the stellar background, measured in fractions of
a degree per year or century. In general, the farther a star
is from us, the smaller its proper motion - just as distant
mountains appear nearly stationary as you drive down the
highway, but road signs in the foreground whiz by. From the
small proper motions of the c-stars, Hertzsprung correctly
deduced that they were distant. To look so bright from so
far away, they must emit profuse amounts of light - much
more than the Sun.
In 1906 Hertzsprung wrote Pickering a letter, reporting
that yellow, orange, and red stars come in two types: bright
and dim. Hertzsprung mentioned four pairs of stars with
similar spectral types but large luminosity differences. His
first pair was Capella and Alpha Centauri A. Both stars are
yellow and spectral type G, but Capella is dozens of times
more luminous. He also cited the orange K stars Arcturus and
70 Ophiuchi and noted that the former greatly outshines the
latter. The same pattern holds for the K stars Aldebaran and
61 Cygni. His final pair was brilliant Betelgeuse and Lalande
21258, a dim star in Ursa Major, both red stars of spectral type
M. Whereas Betelgeuse is bright but distant, which means
it's very luminous, Lalande 21258 is nearby yet too faint to be
seen with the naked eye, a clear sign its light is feeble.
Alas, Hertzsprung published his initial discoveries -
in 1905 and 1907 - in a German photography journal,
Zeitschrift für wissenschaftliche Photographie. Most astronomers
therefore never heard of Hertzsprung's work. " One of
the sins of your youth was to publish important papers in
inaccessible places, " British astronomer Arthur Eddington
later wrote.
Meanwhile, a young Princeton astronomer was discovering
the same startling pattern. Henry Norris Russell had gotten
into astronomy as a child: " I recall my parents showing me
the transit of Venus in 1882, when I was five years old. " Russell
determined the distances of numerous stars by painstakingly
measuring each one's parallax, the tiny apparent shift
the star shows because we view it from slightly different
vantage points as Earth circles the Sun. Once he knew a star's
distance, he could use its apparent brightness to calculate its
luminosity, that is, how much light it emits into space.
The coolest stars come in two types, he said. As Russell
put it during a 1913 talk in England, reported in the British
journal The Observatory, " . . . among the reddest stars (K
and M) there is a distinct separation into two groups. There
seem to be no stars of Class M which are closely comparable
with the Sun in brightness - they are either much brighter
or much fainter. "
Furthermore, both Hertzsprung and Russell deduced that
luminous red stars like Antares and Betelgeuse had to be
enormous. Because red stars are cool, their surfaces radiate
only a tiny amount of light per square inch. For example, a
star with half the Sun's temperature, as measured in Kelvin,
emits only 1
/16 as much light per square inch.
For a cool, red star to give off so much light, the star's
surface must have a gargantuan number of square inches. In
other words, the star is huge - so huge that German astronomer
Karl Schwarzschild, who thought highly of Hertzsprung's
work, named them " giants " ( " Giganten " ) during a 1908 lecture
and used the word in print the following year. Thus were
giant stars christened. They were much rarer than the more
compact stars that Russell started calling " dwarfs " in 1910.
DYNAMIC DUO German astronomer Karl Schwarzschild (left) coined
the term " giants " for the stars that Danish astronomer Ejnar Hertzsprung
(right) had deduced were large and luminous.
36 JULY 2023 * SKY & TELESCOPE
Red Giants as Stellar Youngsters
Although both Hertzsprung and Russell recognized the existence
of red giants, it was Russell who explored the implications
for the lives of stars. " He was really pre-occupied with
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