128
See also: The sun’s spectrum 112 ■ Analyzing starlight 113 ■ The star catalog
120–21 ■ The characteristics of stars 122–27 ■ Refining star classification 138–39I
n the late 19th and early 20th
centuries, Edward Pickering
and his assistants carried
out extensive work classifying
star spectra. They cataloged the
range of light wavelengths coming
from a star, which, among other
information, contains dark absorption
lines. These lines indicate the
presence of particular elements
in the star’s atmosphere that are
absorbing those wavelengths.
One of Pickering’s assistants,
Antonia Maury, developed her own
classification system, taking into
account differences in the width of
absorption lines in star spectra. She
noticed that some spectra, which she
denoted as “c,” had sharp, narrow
lines. Using Maury’s system, Danish
astronomer Ejnar Hertzsprung saw
that stars with “c-type” spectra were
far more luminous than other stars.Bright and dim red stars
Hertzsprung figured out that what
Maury had identified as “c-type”
stars were radically different from
other types in the same category.
For example, within the M-classcategory, or red-colored stars,
he noticed that the “c-types”
were highly luminous, high-mass,
comparatively rare stars—today,
these are called red giants or red
supergiants, depending on their
size. The remaining majority of
non-“c-type” M-class stars were
low-mass, faint stars that are now
known as red dwarfs. A similar
distinction of two main kinds also
applied to K-class (orange) stars. ■THERE ARE
TWO KINDS
OF RED STAR
ANALYZING ABSORPTION LINES
IN CONTEXT
KEY ASTRONOMER
Ejnar Hertzsprung
(1873–1967)BEFORE
1866 Angelo Secchi creates
the first classification of stars
according to their spectral
characteristics.1880s At the Harvard College
Observatory, Edward Pickering
and Williamina Fleming
establish a more detailed
classification system.1890s Antonia Maury develops
her own system for classifying
star spectra, taking into
account differences in width
and sharpness of spectral lines.AFTER
1913 Henry Norris Russell
creates a diagram, similar
to one made by Hertzsprung,
that plots the absolute
magnitude (intrinsic brightness)
of stars against spectral class.
This later becomes known as a
Hertzsprung−Russell diagram.A typical red giant has a diameter
about 50 times that of the sun, and
150 times that of a typical red dwarf.
However, a red giant has only about
8–10 times the mass of a red dwarf.Red giantRed dwarfSun