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By the end of the 19th century,
it seemed that, in order to fully
understand the nature of stars, it
was necessary to systematically
record their spectra and classify
them into different types.Star classification
This immense task was undertaken
at Harvard College Observatory,
where the director Edward
Pickering employed a large team
of women to carry out the exacting
work. Here, Annie Jump Cannon
devised the stellar classification
system still used today, based
on a temperature sequence.
Cannon personally classified
some 500,000 stellar spectra.
The star catalog included not only
their position but also precise
information about their magnitude
(apparent brightness) andspectrum. This information quickly
paid dividends as astronomers
analyzed the new data. Cannon’s
colleague at Harvard, Antonia
Maury, realized that the simple
temperature sequence did not take
account of subtle variations within
each star type. Ejnar Hertzsprung
and Henry Norris Russell
independently followed this up,
leading to the discovery that stars
of the same color could be giants or
dwarfs, and the identification of the
first known white dwarf star.The physics of the stars
In an interval of some 50 years,
cutting-edge astronomy had
changed its focus. By the early
20th century, physics—the study
of matter, forces, and energy, and
how they are related—could be
applied to the sun and stars, andwould strongly influence the
future direction of astronomy.
Significant developments in basic
physics impacted on astronomy.
For example, Briton James Clerk
Maxwell published his theory
of electromagnetism in 1873,
describing electromagnetic radiation
such as light in terms of its wavelike
properties. X-rays were discovered
in 1895 and radioactivity in 1896. In
1900, German physicist Max Planck
prepared the ground for quantum
physics with a leap of inspiration,
postulating that electromagnetic
energy comes in “packets” of a
particular size, called “quanta.”
These discoveries would lead
to new ways of looking at the
skies, and shed new light on the
processes taking place within stars.
Physics and astronomy would be
inseparable from this point on. ■THE RISE OF ASTROPHYSICS
1890
1895
1896
1900
1907
1912
While investigating X-rays,
French physicist Henri
Becquerel demonstrates
the effects of the radioactive
decay of uranium.Max Planck lays the
foundation for quantum
mechanics by suggesting
that energy can only exist
in distinct sizes of “quanta.”Harvard computer Henrietta
Swan Leavitt shows how stars
called Cepheid variables can
be used to measure distances
in the universe.In experiments with
cathode ray tubes,
German physicist
Wilhelm Röntgen
discovers X-rays.The Harvard College
Observatory
produces the first
Draper Catalog
of Star Spectra.
Austrian physicist
Victor Hess shows
that powerful rays, now
called cosmic rays,
come from space.