How to measure galaxies? That’s a challenge for astronomy, especially for the galaxy that calls it home. It turns out that there are several ways to know the mass of the Milky Way, and a recent study published on the arXiv preprint site summarizes the methods to provide the best answer.
One is to observe the movement of stars in galaxies. Most stars in the Milky Way revolve around the center of the Milky Way in circular orbits, just as planets revolve around the sun, and stars also revolve around the center of the galaxy. Since gravity keeps the star in orbit, the in-orbit mass can be determined from the star’s velocity and distance from the galaxy’s center. The speed and distance from the center of the known star can then be plotted to obtain the so-called rotation curve. This curve measurement of the Milky Way and other galaxies is the first evidence that galaxies are much more massive than visible stars can explain, leading to the concept of dark matter.
The problem with the rotation curve method is that only stars at a certain distance from the center of the galaxy can be measured. It is now known that most of the Milky Way’s mass is not in the center, but extends outward into a galactic halo. The mass of the halo can be estimated from the rotation curve, but the motion of globular clusters can also be observed. Globular clusters are bright, dense clusters of stars that move like individuals around a galaxy because the stars inside are gravitationally bound. They are in the sphere around the Milky Way, so measuring their motion helps gauge the mass of the halo.
To measure the outer regions of the halo, the motion of satellite galaxies, such as the Magellanic Clouds, can be observed. There are about 60 small galaxies within 1.4 million light-years from the Milky Way. Since they are located outside the galactic halo, their orbital motion is determined by the mass of the Milky Way. The only downside to this method is that, with only a few dozen orbiting galaxies, the results won’t be very accurate.
All of the above methods calculate the mass of the Milky Way through orbital motion. However, some methods do not rely on orbital motion, such as observing the tidal plumes of dwarf galaxies. Some globular clusters and dwarf galaxies are too close to the center of the Milky Way and are torn apart by tidal forces. The remnants of these galaxies form stellar streams, such as the Sagittarius stream. By calculating the motion of these stellar streams, the mass of the galaxy can be estimated.
Another way is to look at stars leaving the Milky Way. Occasionally a star will pass by another star, gaining enough speed to escape the Milky Way. Since the escape velocity depends on the mass of the galaxy, the statistical measurement of the escaped stars can obtain the mass of the galaxy.
Finally, check out the Local Group of galaxies, including Andromeda and satellite galaxies. The Local Group is gravitationally isolated from more distant galaxy clusters, so looking at the Local Group’s equilibrium state provides insight into the overall mass and the mass of the Milky Way.
Each method has advantages and disadvantages. The research team calculated the average value of various methods and found that the optimal value of the mass of the Milky Way is 1 trillion solar masses.
(This article is reproduced with the authorization of Taipei Planetarium; Source of the first image: NASA)
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