Cosmic Times

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":

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC