In 1918 Harlow Shapley used his calibrated variable star period-luminosity relation to find distances to 93 globular clusters. Globular clusters are spherical clusters of 100,000's to several million stars (looking like a glob of stars) in very elliptical orbits around the center of the galaxy. Two globular clusters are shown below: Messier 5 (in Serpens Caput constellation) and 47 Tucanae (in the southern constellation Tucana).
In the side view picture of the Galaxy at the beginning of this section and below, the globular clusters are represented by the small circles congregating around the bulge of the Galaxy (they have been enlarged over a 100 times to make them visible). Shapley found a strong concentration of globular clusters in the direction of the constellation Sagittarius. In a continuation of the process started by Copernicus almost 500 years before, Shapley announced that our solar system is not at the center of the Galaxy!
If we were at the center of the Galaxy, we would see globular clusters equally distributed all around us. Instead, they congregate around some other point that Shapley calculated to be 32,600 light years from our solar system. He did not know about the dust in the interstellar medium. That discovery was made about a decade later. Taking into account the effects of dust extinction on the brightnesses of stars and also that there are different types of Cepheids, astronomers reduced the solar system's distance to the center of the Galaxy to about 26,000 light years. More recent trigonometric parallax measurements of star formation regions using VLBI measurements has fine-tuned that to about 27,000 light years (see also this link). Our solar system's position is marked with a cross in the pictures of the Galaxy at the beginning of this section.
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last updated: June 9, 2010