Exactly how massive is the Milky Way?

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Artist’s impression of the Milky Way galaxy. Credit: Andrew Z. Colvin

How do you weigh a galaxy? It’s an astronomical challenge, especially if it’s the galaxy you call home. It turns out that there are several ways to get a handle on the mass of the Milky Way and a recent study published in arXiv the prepress server summarizes these methods to present the best value.

One method is to observe the motion of stars in the galaxy. Most of the stars in the Milky Way follow a roughly circular path around the galactic center. Just as planets orbit the sun, stars orbit the galaxy. Since gravity is the force that holds stars in their orbit, you can use a star’s speed and distance from its center to determine the mass in its orbit. Not all stars have circular orbits, but on average they do. So you can plot velocity versus distance from center for known stars and get what’s known as a rotation curve. Measurements of this curve in the Milky Way and other galaxies were the first evidence that galaxies had much greater mass than could be explained by visible stars, leading to the idea of ​​dark matter.

One of the problems with the rotation curve method is that we can only measure stars at a certain distance. We now know that most of our galaxy’s mass is not concentrated in the center, but rather extends outward in a galactic halo. We can estimate the mass of the halo from the rotation curve, but we can also observe the motion of globular clusters.

Globular clusters are bright clusters full of stars. Because the stars within a globular cluster are gravitationally bound, these clusters move around the galaxy as a single object. They are located in a sphere surrounding the Milky Way, so measuring their motion helps us measure the mass of the galactic halo.

Various methods of massing the Milky Way. Credits: Bobylev and Baikova

To measure the outer region of the galactic halo, we can observe the motion of satellite galaxies such as the Magellanic Clouds. There are about 60 small galaxies within about 1.4 million light years of the Milky Way. Not all of them are in orbit around our galaxy, but many are. Since they are located outside our galactic halo, their orbital motions are determined by all the mass of our galaxy. The only downside to this approach is that with only a few dozen galaxies in orbit, the result isn’t particularly accurate.

All of these approaches calculate the mass of the Milky Way from its orbital motion. There are some methods that don’t rely on orbital motion. One of them is to look at the tidal plumes of dwarf galaxies. In the history of our galaxy, there are some globular clusters and dwarf galaxies that have strayed too far from the central region of the Milky Way and have been torn apart by tidal forces. The remnants of these galaxies form a stream of stars, such as the Sagittarius stream. By calculating the motion of these flows we can estimate the galactic mass.

Another approach is to watch stars leave our galaxy. Occasionally a star will have a near miss with another star and gain sufficient velocity to escape our galaxy. Since escape velocity depends on galactic mass, a statistical measure of escaping stars provides a mass for the galaxy.

Finally, we can observe the local group of galaxies. This includes the Andromeda galaxy and its satellite galaxies. Our local group is gravitationally isolated from more distant galaxy clusters, so observing the equilibrium state of the local group gives us an idea of ​​its overall mass and the mass of the Milky Way.

Each of these approaches has its own advantages and levels of accuracy. None of them have the final say on their own. In this latest work, the team took a statistical average of various methods and derived what we might call the best value for the mass of our galaxy. The value they determined was a trillion solar masses, plus or minus a few hundred billion solar masses.

More information:
VV Bobylev et al, Review of Current Estimates of the Mass of the Galaxy, arXiv (2023). DOI: 10.48550/arxiv.2305.18408

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