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International Year of Astronomy - Annie Jump Cannon

Annie Jump Cannon, in her later years, sitting at her desk at the Harvard College Observatory. The glass stand placed in front of her was used to visually examine photographic plates of stellar spectra.  Smithsonian Institution Archives
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Annie Jump Cannon

"A life spent in the routine of science need not destroy
the attractive human element of a woman's nature."

Annie Jump Cannon is most famous for inventing the Harvard
Classification Scheme of stars according to their temperatures.

In the 1890s, Cannon became a member of a group of women
hired by the director of the Harvard College Observatory, Edward Pickering, to
reduce data and carry out astronomical calculations. Several members of this
group, collectively known as the "Harvard Computers", became famous astronomers
of their day and their pioneering work is considered fundamental in the modern
field of stellar astrophysics. Pickering's long-term project consisted of
obtaining photographic plates of stellar spectra of as many stars as possible,
and to index and classify them accordingly. A star's spectrum is obtained by
decomposing the star's light into colors after it passes through a prism.

Most stellar spectra show a number of dark lines, called absorption
lines, which can be used to determine the chemical composition of the star as
well as its temperature. Each element is responsible for a unique set of
absorption lines in the spectrum, thought of as that element's fingerprint,
which can be measured very accurately in a laboratory. Then, if a particular
element's set of lines is observed in a stellar spectrum, that element must be
present in that star. The spectral absorption line pattern also depends on the
temperature of the star-in general, the strength of the hydrogen lines as well
as the number of other lines present are indicative of a star's temperature.

Cannon cataloged nearly 400,000 stars into the categories O,
B, A, F, G, K, and M (P and Q were used for planetary nebulae and objects with
peculiar spectra, but these are no longer included among the stellar classes),
continuing and vastly improving the work of her colleagues Williamina Fleming
and Antonia Maury. Cannon also published catalogs of variable stars, including
300 that she discovered. Her career spanned more than forty years during which
she received numerous recognitions-several of which had never been given to a
woman before. At Harvard, she was named Curator of Astronomical Photographs,
but it was only in 1938, two years before her retirement, that she obtained a
regular Harvard appointment as the William C. Bond Astronomer.

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