Question

a) How did interstellar extinction mislead astronomers into believing that we are at the center of our galaxy?

b) Describe how Harlow Shapley proved that the Sun is not at the center of our galaxy.

Answer 1

Part(a) : Interstellar dust decreases our visibility of stars, limiting our ability to see anywhere near the edge of the Galaxy in any direction along the galactic plane. Astronomers estimated our galactic position from counting stars in different directions along the galactic plane. Since they could see only a relatively short distance in any direction, they saw no significant trends in star numbers, only approximately equal star counts in all direction. Knowing nothing of interstellar dust, they therefore falsely concluded that we are located near the center of the Galaxy.

Part(b) : Harlow Shapley determined that the sun was not at the center of our galaxy by studying Cepheid’s in globular clusters, which are huge spherical groups of closely packed stars. Shapley used the relation between the period of Cepheid’s and their observed brightness to map the relative distances of clusters from us and from each other. The clusters were widely and randomly distributed both below and above the plane of the Milky Way. They also appeared to be concentrated in one small area that was in the direction of the constellation Sagittarius. Shapley argued that such a distribution would make sense if the Galaxy had the shape of a flattened disk with the clusters grouped around the galactic center. This proved that the solar system was displaced from its accepted central position by a considerable distance.

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