This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

Our Location

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This material (including images) is copyrighted!. See my copyright notice for fair use practices. Select the photographs to display the original source in another window. The ground-based telescope pictures here are from the Anglo-Australian Observatory (AAO---used by permission).

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!

globulars show us the direction to the Galaxy center

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. Now taking into account the effects of dust extinction on the brightnesses of stars and also that there are different types of Cepheids, astronomers find the solar system's distance to the center of the Galaxy is about 26,000 light years. Our solar system's position is marked with a cross in the pictures of the Galaxy at the beginning of this section.

Our Motion

Further evidence of Shapley's discovery is seen in the motion of the Sun about the center. Globular clusters on one side of the celestial sphere have redshifts and globular clusters on the other side of the celestial sphere have blueshifts. Stars and H II regions near the Sun have small doppler shifts. From this you can conclude that the galactic disk is rotating in an organized fashion but the globular clusters are not. The 21-cm line emission from neutral atomic hydrogen is now used to map the motion of the disk. Objects in the disk orbit about a common center that is 26,000 light years away in the direction of Sagittarius.

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last updated: 25 May 2001

Is this page a copy of Strobel's Astronomy Notes?
Author of original content: Nick Strobel