How Do You Weigh a Galaxy?

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Title: Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
Authors: Laura L. Watkins et al.
First Author’s Institution: University of Chicago
Status: Published in ApJ
We can’t put it on a digital scale, we can’t hang it on a balance and compare it against something else, so how does one measure the mass of our home galaxy? The authors of today’s paper use measurements of globular clusters in the halo of the galaxy taken from the Gaia satellite to estimate a mass for the Milky Way.
What Is Our Galaxy Made of and Why Should We Weigh It?
Our galaxy contains four major parts: the bulge, the disk (which contains the thin disk and the thick disk), the bar, and the halo (see Figure 1). The first three components are made up of baryons, particles that make up protons and neutrons and therefore most of the things ...