The Astronomy Book

139

The Hertzsprung−

Russell diagram

shows the distribution

of stars by absolute

magnitude and

spectral class. The

diagram formed the

basis for developing

theories about how

stars evolve. (In the

absolute magnitude

scale, the lower the

number, the higher

the magnitude.)

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

THE RISE OF ASTROPHYSICS

they look from Earth. However, it
was realized that, in order to know
a star’s absolute brightness, it
would be necessary to correct for
its distance from Earth: the farther
away a star is, the dimmer it will
appear. From the mid-19th century,
reasonably precise distances to
some stars began to be calculated,
and the absolute brightness of
these stars could be established.

Russell’s discovery
Among the majority of stars,
Russell found a definite
relationship—hot blue-white stars
(spectral classes B and A) tend to
have higher absolute magnitudes
than cooler white and yellow stars
(classes F and G), while white and
yellow stars have higher absolute
magnitudes than orange and red
stars (classes K and M). However,
some exceptionally bright red, orange,
and yellow stars departed from this
rule. These were the “giant” stars.
Russell plotted the absolute
magnitudes of stars against
their spectral classes on a scatter
diagram, which he published in

1913. However, unknown to him,
      Danish chemist and astronomer
      Ejnar Hertzsprung had performed
      a similar exercise a couple of years
      earlier, and the diagram is now
      known as the Hertzsprung–Russell
      diagram. The diagram shows stars
      divided into a group of bright giant
      stars and a much larger group of
      ordinary stars running diagonally.

Russell called these ordinary stars

“dwarfs”; Hertzsprung referred to

them as “main sequence.” The

newly discovered hot but faint

white dwarfs were later added to

the diagram, forming a third group.

Today, it is known that most stars

spend most of their lives on the

main sequence, some later evolving

into giants or supergiants. ■

Henry Norris Russell Henry Norris Russell was born^

in Oyster Bay, Long Island, in

1877. At age 5, his parents
      encouraged him to observe
      a transit of Venus across the
      sun’s disc, which inspired an
      interest in astronomy. He was
      awarded a doctoral degree
      by the astronomy department
      of Princeton University for an
      analysis of the way that Mars
      perturbs the orbit of the asteroid
      Eros. From 1903 to 1905, he
      worked at the Cambridge
      Observatory, England, on star
      photography, binary stars, and
      stellar parallax. In 1905, he was

appointed as an instructor

in astronomy at Princeton

University, and in 1911, he

became professor of astronomy

there. He was also director

of Princeton University

Observatory from 1912 to 1947.

Key works

1927 Astronomy: A Revision of

Young’s Manual of Astronomy;

Volume 1: The Solar System;

Volume 2: Astrophysics and

Stellar Astronomy

1929 On the Composition of the

sun’s Atmosphere

White dwarfs

Main sequence

(Dwarfs)

Giants

Supergiants

ABSOLUTE MAGNITUDE

-10

-5

0

+5

+10

+15

TEMPERATURE (°C)

20,000 10,000 5,000 2,500