Amassing Evidence

Ana Bonaca

Yesterday I talked about how we can measure the size and shape of our galaxy even though much of it is obscured by gas and dust. As I pointed out, we can only observe a small fraction of the stars in the Milky Way. But then how do we know how many stars there are in our galaxy? The short answer is we don’t, but we can get an idea by measuring the mass of our galaxy.

Typically the way the mass of our galaxy is measured is to measure the speed of stars and other objects in the Milky Way. This gives us a mass of about 100 – 400 billion solar masses. Not all of that is due to stars, but it gives us a basic idea of the number of stars in our galaxy. But recently a more accurate measure has been determined, and it was done by observing globular clusters. Globular clusters are dense, spherical cluster of stars.¹ Their shape comes from the fact that they are clustered under their own gravity. They are not located in the plane of our galaxy, but instead are found in all directions in a kind of halo around our galaxy. They were first used by Harlow Shapley to determine the basic structure of our galaxy in the early 1900s.

The thing about globular clusters is that they aren’t completely gravitationally bound. Through various gravitational interactions, a few stars escape a globular cluster at a regular basis (a process called evaporation). As a result, these clusters leave a trail of stars behind as they slowly orbit our galaxy. In this recent work the team looked at these trails and found that they had variations of density within them. This means the rate of evaporation varied over time. When they analyzed these variations, they found a pattern, and this pattern is dependent upon the overall mass of our galaxy. Basically that means the evaporation is affected not only by gravitational interactions within the globular cluster, but also interactions between the cluster and our galaxy. So the team used this data to determine the mass of our galaxy. What they found was that within a radius of 60,000 light years the mass is about 210 billion solar masses.

It should be emphasized that this is only the mass of our galaxy within a 120,000 light year diameter, which is about the diameter of the visible galaxy. Beyond this region our galaxy is dominated by dark matter, which makes up most of the total mass of our galaxy. So we can say that the Milky Way has roughly 200 billion stars.