Trumpler and Shapley

Hershcel 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?

Are we near the center of the Milky Way galaxy?

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. The 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). Using the variable stars known as RR Lyrae stars as distance indicators, Shapley determined the distribution of globular clusters in the Milky Way galaxy (right) and demonstrated that the Sun was not in the center of the Milky Way, it was in the disk of the 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. and an animation of the Cepheid variables in M100.

Lecture 1
Lecture 2