I noticed Sidney van den Bergh’s
letter on Edwin Hubble and Harlow Shapley in
the September 2004 issue of Physics Today (page 15), and I believe a clarification about the
“Different Views” of these two
observational astronomers is in order. Any reader not
familiar with the history of galactic astronomy would
be deprived of a most important footnote, namely the
distinguishing definitions of the universe in the early
20th century.
Van den Bergh is essentially
correct when he states that the discovery of “the
existence of galaxies beyond the outer limits of our
Milky Way system” is much to the credit of these
men. However, he does not qualify that into the 1920s
there were two diverging theories about the extent
of the universe: the old theory that defined the Milky
Way galaxy as the universe, including all the
“spiral nebulae” observed for over a
century and denoted as gas clouds, within the Milky Way boundary; and the new
one, that these other spirals were, on the contrary,
external individual galaxies, coined as “island
universes.”
Shapley was the central Figure of
the former opinion based on his conclusions while
observing globular clusters at Mount Wilson during the
years 1914 to 1917. He declared that these
clusters—and essentially all others—were
confined within the Milky Way boundary. To the north,
at Lick Observatory of the University of California,
Berkeley, Heber D. Curtis had been studying for some 10
years the spiral nebulae that Shapley assumed without
investigation to be gas nebulae, at distances similar
to those of the clusters Shapley had studied. Curtis
concluded they were spiral galaxies, and he became the
chief proponent of the external universe view that the
Milky Way was only one of many galaxies in a much
larger universe. The scale of the universe became the
central theme of the 1920 meeting of the National
Academy of Sciences in Washington, DC. What has been
called the “Great Debate” during that
meeting was presentations by Shapley for the old theory
and Curtis for the new. But it was never a debate;
Shapley did not want to get into what Curtis projected
enthusiastically to be a “scrap” over the
two views. Shapley presented a safe astronomy lecture,
whereas Curtis successfully made his point with a
powerful, comprehensive research presentation.
The real debate was in the seesaw
of papers that characterized the two views thereafter
into the 1920s. Hubble’s work at Mount Wilson
began in 1919, where he used the recently completed and
largest reflector in the world, the Hooker 100-inch, to
also concentrate on the spiral nebulae. He and Shapley
had very personal and professional differences that
developed in the short time they were both at Mount
Wilson. Hubble became convinced that the nebulae of the
controversy were galaxies, and his proof came in early
1924 when he determined that the so-called
“Andromeda Nebula” was a separate galaxy
far beyond the Milky Way.
So Shapley’s universe was
profoundly smaller than the one Hubble revealed as an
emerging cosmos—though the general applications
of Shapley’s work deserve all the credit of
astronomical history.
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