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A
merican astronomer
Annie Jump Cannon was
the early 20th-century’s
leading authority on the spectra
of stars. When she died in 1941,
Cannon was described as “the
world’s most notable woman
astronomer.” Her great contribution
was to create the basis of the
system for classifying the spectra
of stars that is still in use today.
Cannon worked at the Harvard
College Observatory, as part of the
team of “Harvard Computers,” a
group of women employed by the
director Edward C. Pickering to help
compile a new stellar catalog. The
college’s catalog, begun in the
1880s with funding from the widow
of astrophotographer Henry Draper,
used new techniques to collect
data on every star in the sky
brighter than a certain magnitude,
including obtaining the spectra of
as many stars as possible. In the
1860s, Angelo Secchi had set out
a provisional system for classifyingTHE CHARACTERISTICS OF STARS
stars according to their spectra.
Pickering’s team modified this
system. By 1924, the catalog
contained 225,000 stars.Early approaches
Williamina Fleming, the first of
Pickering’s female computers,
made the earliest attempt at a more
detailed classification system, by
subdividing Secchi’s classes into
13 groups, which she labeled with
the letters A to N (excluding I), then
adding O, P, and Q. In the next
phase of the work, fellow computer
Antonia Maury, working with better
data received from observatoriesIN CONTEXT
KEY ASTRONOMER
Annie Jump Cannon
(1863–1941)BEFORE
1860 Gustav Kirchhoff shows
that spectroscopy can be used
to identify elements in starlight.1863 Angelo Secchi classifies
stars using their spectra.1868 Jules Janssen and Joseph
Norman Lockyer discover
helium in the solar spectrum.1886 Edward Pickering begins
compiling the Henry Draper
Catalogue using a photometer.AFTER
1910 The Hertzsprung−Russell
diagram reveals the different
sizes of stars.1914 US astronomer Walter
Adams records a white dwarf.1925 Cecilia Payne-
Gaposchkin finds that stars
are composed almost entirely
of hydrogen and helium.Each substance sends out its
own vibrations of particular
wavelengths, which may be
likened to singing its own song.
Annie Jump CannonThe seven main classes of star,
categorized according to spectra and
temperature, are, from left to right:
O, B, A, F, G, K, and M, with O the
hottest and M the coolest.