Sidebar: "Great Debate" Resolved
The primary message of this article is that theories in astronomy, as
any other science, are based on evidence. When there are two leading
theories for the same phenomenon, astronomers must look objectively at
the available evidence for both theories to determine which is more
plausible. A secondary message is that science knowledge changes over
time when new evidence comes to light.
Prior to Hubble's measurement of the distance to the Andromeda Nebula,
the nature of "spiral nebulae" was not definitively known. By 1920,
there was a growing number of scientists who believed that the spiral
nebulae were actually systems of stars outside our own galaxy, which
could not be resolved because of their great distances. However, other
scientists still believed that these spiral nebulae were clouds of gas
within the Milky Way.
To shed light on this debate in the astronomical community, two
astronomers on either side of this issue – Harlow Shapley and Heber
Curtis – held a debate on April 26, 1920 at the Smithsonian Museum
of Natural History.
Shapley argued that the whole of the Universe was contained in the Milky
Way, which was about 300,000 light years in size. The spiral nebulae
were clouds of gas held within the Milky Way galaxy. One theory about
the spiral nebulae was that they were solar systems in the process of
forming. In addition, astronomer Adriaan van Maanen claimed to have
observed rotation in the spiral nebulae. Shapley argued that the speeds
for such rotation would have to be too high if the nebulae were outside
the Milky Way. (Note: this claim of observed rotation in the spiral
nebulae is certainly wrong, though at the time it was an accepted
observation.)
Curtis argued that the Sun was part of a smaller system of stars
measuring about 30,000 light years across. He believed that the spiral
nebulae were island universes – systems of stars external
to our own Milky Way. He demonstrated that the spectrum from these
spiral nebulae were indistinguishable from the spectrum from the Milky
Way, suggesting that they are similar in nature.
It was generally accepted that Shapley had won the debate with more
observations supporting his claims. However, we now know that Curtis
was correct on the nature of spiral nebulae. As technologies and
observations improve, our understanding of the Universe around us also
improves.
Other resources
The following web pages have more information on "The Great Debate":
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