1918

Size of the Milky Way

Harlow Shapley (1885–1972), Edwin Hubble (1889–1953)

During the decade since Cepheid Variable stars were found in 1908 to be useful as measuring sticks for determining cosmic distances, a number of astronomers worked to determine distances to spiral nebulae, globular clusters, and other enigmatic objects in order to get a handle on whether they were inside or outside of the Milky Way. Indeed, the size of the Milky Way itself was the subject of intense debate, with many astronomers believing that it essentially was the universe, while many others believed it to be just one of many separate “island universes,” as the distant nebulae had been dubbed by the eighteenth-century philosopher Immanuel Kant.

The first astronomer to make an experimental estimate of the size of our galaxy was the American astronomer Harlow Shapley, who studied the distribution of globular clusters in the sky. Cepheid variable stars had been used to determine the distance to a nearby globular cluster, so Shapley assumed that they were all the same size and used the changing apparent diameter of other clusters to estimate their distances. By 1918 he had determined that the globular clusters form a sort of halo around the platelike disk of our galaxy, enabling him to estimate that the Milky Way is about 300,000 light-years across, with the Sun not centered but offset by about 50,000 light-years (so much for heliocentrism). This was astonishingly larger than many had thought the galaxy to be, and it convinced Shapley that there were no island universes: the globular clusters and spiral nebulae must all be bordering or within the Milky Way.

It turned out that Shapley’s estimate of the size of the Milky Way was about three times too large, mostly because his assumption that all globular clusters are the same size is not really valid. The disk of our galaxy is actually about 100,000 light-years across and about 1,000 light-years thick (with a somewhat thicker central bulge), and the Sun is offset from the center by about 27,000 light-years. Shapley was right about the globular clusters being in and near the Milky Way in a diffuse halo, but he was wrong about the spiral nebulae. As shown by Edwin Hubble and others in subsequent decades, spirals and many other forms of “nebulae” are actually separate galaxies, some like our own, some not, but all are millions to billions of light-years away.

A Hubble Space Telescope photo of the face-on spiral galaxy NGC 1309. With bright bluish regions of new star formation in its spiral arms, and a central region of older, yellowish stars, this is probably close to what our Milky Way galaxy would look like if we could view it from far above.