Open Cluster NGC 3603

Trumpler and Shapley

Globular Cluster M15

Are We Really Near the Center of the Milky Way Galaxy?

Herschel, and later Kapteyn counted stars in various directions to determine the shape of the Milky Way and our position in the Milky Way but came up with the wrong answer.

Why?

We are not at the center of the Milky Way galaxy. Herschel and Kapteyn were unaware of the existence of dust in the interstellar medium (ISM).




What is our Location in the Milky Way Galaxy?

This issue was resolved by Shapley through observations of the ancient star clusters known as globular clusters. Globular clusters are believed to have formed very early in the history of the Milky Way and reflect the initial shape of the cloud which formed the Galaxy (and, as such, are distributed in a spherical distribution about the center of the Galaxy). To the right is a figure that shows the 119 globular clusters in the Milky Way that are within 15,000 parsecs (~50,000 light years) within the center of the Milky Way. Globular clusters are useful for this exercise because they exist above and below the disk of the Milky Way galaxy and so can be easily seen to large distances.


To the left is a figure that shows the 119 globular clusters in the Milky Way that are within 15,000 parsecs (~50,000 light years) within the center of the Milky Way Globular clusters are useful for this exercise because they exist above and below the disk of the Milky Way galaxy and so can be easily seen to large distances. Shapley observed these globular clusters in a method identical to that used by Herschel and Kapteyn when they counted stars. However, unlike Herschel and Kapteyn, Shapley was able to determine the distance to the globular clusters and not simply assume that they all had the same luminosity.

RR Lryrae in M4

Using variable stars known as RR Lyrae stars as distance indicators, Shapley determined the distribution of globular clusters in the Milky Way galaxy and demonstrated that the Sun was not in the center of the Milky Way, it was in the disk of the Galaxy. Based on his observations determined that the Sun was approximately 1/2 of the way out in the disk Milky Way galaxy.


Comment--Variable stars will be an important topic later as they serve as among the most reliable of distance indicators. The most important class of variable stars are the Cepheid variables which are evolved stars which are undergoing pulsations and periodically varying in luminosity. The key observational fact about them is that their intrinsic average luminosities are related to their pulsation periods For example, see Cepheid variables in M100.

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