125
around the the world, noticed more
variety in the detail. She devised a
more complex system of 22 groups
designated by Roman numerals,
each divided into three subgroups.
Pickering was concerned that
applying such a detailed system
would delay the task of compiling
the catalog. However, Maury’s
approach to stellar classification
proved a crucial step toward the
creation of the Hertzsprung–Russell
diagram in 1910, and consequent
discoveries about stellar evolution.
Cannon joined the Harvard
College Observatory staff in 1896
and began working on the next
part of the catalog, which was
published in 1901. With Pickering’s
approval, to make classification
clearer and easier, she reverted
to Fleming’s spectral classes
designated by letters, but she
changed the order.
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
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
THE RISE OF ASTROPHYSICS
Classifying the
stars according
to their spectra
reveals their
age and size.appearance of its spectrum and
made her classes a temperature
sequence from hotter to cooler.
On this, Cannon followed Maury’s
lead. Some of Fleming’s letters
were dropped because they were
unnecessary, so the final sequence
became O, B, A, F, G, K, M, based
on the presence and strength of
certain spectral lines, especially
those due to hydrogen and helium.
Students of astronomy still learn it
by remembering the mnemonic,
“Oh Be A Fine Girl, Kiss Me,”
attributed to Henry Norris Russell.Harvard system
Cannon’s 1901 system laid the
foundations for the Harvard
Spectral Classification system.
By 1912, she had extended it to
introduce a range of more precise
subclasses, adding 0 to 9 after
the letter, with 0 the hottest in the
class and 9 the coolest. A few new
classes have been added since.
The Harvard system essentially
classifies stars by temperature and
takes no account of the luminosity
or size of the star. In 1943, however,
luminosity was added as an ❯❯A star’s spectrum can
reveal its temperature,
luminosity, and
composition.The spectra of stars cover a wide
range of star types.