Expert: Courtney Seligman - 9/25/2008

Question
Hi Courtney, how did early astronomers first know that the Solar System was not the center of the Galaxy?  

thanks

Answer
This was discovered by Harlow Shapley in 1918. Prior to that, it was thought that the Sun lay near the center of a large, disc-shaped stellar structure, extending a thousand or so light years above and below the plane of the disc, and several thousand light years in most directions, in the plane of the disc. This structure was 'discovered' by Herschel in the 1700's, by counting the number of stars of various brightnesses in different directions, and presuming that if you could see fainter and fainter stars (virtually without end), you must be looking at stars that were further and further away.

By the early 1900's, astronomers knew that beside this structure, there was a large collection of "globular" clusters which could not be centered on the Sun, because almost all of them were on one side of the sky. By studying the periods and brightnesses of Cepheid variables in globular clusters, Shapley showed that they formed a roughly spherical system centered on a point in the plane of our local disc of stars, but nearly 50,000 light years away, in the direction of the constellation of Sagittarius. The Milky Way, which is the visible expression of the concentration of stars in the local disc of stars, is "fatter" in that direction, and Shapley suggested that the disc was actually much larger than we could directly determine, and had its center at the center of the sphere of globular clusters.

The new, larger version of the stellar universe was called a galaxy, after the Greek word galaxias, referring to milk, because we see it from the inside as the Milky Way. At first, this was presumed to be the entire Universe. It would be another decade and a half before the existence of other galaxies was proven.

As it happened, Shapley overestimated the size of the galaxy, because although it was becoming clear that there were large amounts of dark interstellar material blocking our view in some directions, it wasn't clear that such "fog" existed in virtually all directions. The result of not knowing this is that the stars he used to estimate the distance of the globular clusters looked fainter than they should have, and not being aware of this, he assumed they were further than they really were. We now believe that the center of the Galaxy is only about 25,000 light years away. But there has otherwise been little change in our understanding of its structure in the 90 years since Shapley determined it.

Courtney Seligman
Professor of Astronomy
Long Beach City College