Astronomers are using Gaia and the Hubble Space Telescope to make the most precise measure of the Milky Way’s mass to date. The new result puts our galaxy on par with — if not more massive than — Andromeda Galaxy.

The mass of the Milky Way has long been debated, to the point that we don’t even know where it stands in the Local Group of galaxies. Is it the heavyweight champion, or does our sister galaxy, Andromeda, outweigh us?

Laura Watkins (Space Telescope Science Institute) and colleagues have used data recently released by the European Space Agency’s Gaia satellite, as well as roughly ten years of Hubble Space Telescope observations, to peg the motions of 46 tightly packed bunches of stars. Known as globular clusters, their orbits help pin down the Milky Way's mass.

Milky Way globular clusters
This artist’s impression shows a computer generated model of the Milky Way and the accurate positions of the globular clusters used in this study surrounding it.
ESA / Hubble, NASA / L. Calçada

Our galaxy’s gravitational pull determines the clusters’ movements, explains coauthor N. Wyn Evans (University of Cambridge, UK). If our galaxy is more massive, the clusters will move faster under the stronger pull of its gravity. The key is to understand exactly how fast the clusters are moving.

Many previous measurements have measured the speed at which a cluster is approaching or receding from Earth. “However,” Evans says, “we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated.”

The team finds a mass equivalent to 1.5 trillion Suns. The results will appear in the Astrophysical Journal (preprint available here).

A Tricky Scale

Milky Way Halo
The Milky Way's disk of stars (labeled here as "thin disk") are relatively insignificant to the galaxy's massive dark matter halo.
NASA / ESA / A. Feild

Astronomers have been fussing over the mass of the Milky Way the way parents fuss over their newborns. Understandably so: Just as a baby’s weight serves as an indicator of more important things, like their growth and well-being, the heft of our galaxy affects everything from our understanding of its formation to the nature of dark matter.

But while the pediatrician will usually tell you your baby’s weight to within a percent (equivalent to a tenth of an ounce if you’re in the U.S.), the Milky Way’s mass is known only to within a factor of two. Imagine putting your newborn on the scale, only to have the needle waver between 5 and 10 — is baby failing to thrive? Or doing just fine? The uncertainty would render the result meaningless.

On the galactic scale, of course, there are a few more zeroes involved: Over the years, astronomers have found that the Milky Way’s mass is somewhere between 0.5 trillion and 3 trillion Suns. There are plenty of reasons for the large range. First, studying our galaxy is difficult because we’re inside of it; things like dust or the galactic plane of stars can block our view. Second, even when astronomers trace the orbits of objects — such as globular clusters — measuring their motion across the sky is trickier. It takes many years of observations to nail down their so-called proper motions. That’s what Watkins and her colleagues have done, using dedicated Hubble programs that have monitored stellar motions over roughly 10 years, as well as the second data release from the Gaia mission that has been monitoring stars since 2014.

By far the trickiest part of the problem, though, is that much of the mass astronomers are trying to measure can’t be seen. The bulk of the Milky Way is in dark matter, not stars. Moreover, the Milky Way’s dark matter halo may extend 1 million light-years out from the galaxy’s center. Even if astronomers follow the orbit of a globular cluster around the galaxy, it will only reveal the mass inside its orbit. The farthest globular cluster in Watkins’s study is out at 130,000 light-years. To measure the mass beyond that distance, the astronomers must make some assumptions about the nature and shape of the dark matter halo.

A More Exact Mass

Globular cluster NGC 4147
The globular cluster NGC 4147 is about 60,000 light-years from Earth.
ESA / Hubble / NASA / T. Sohn et al.

Nevertheless, the new measurement is so precise that it has helped narrow things down. “Together with another analysis of similar data by Posti & Helmi, [this study] has tipped the scale towards a heavier Milky Way,” says Ana Bonaca (Harvard-Smithsonian Center for Astrophysics), who was not involved in the study. “Thanks to these studies, we now know that a very low value for the mass of the Milky Way is unlikely.”

For astronomers, this new mass estimate will be most relevant for understanding the Milky Way’s swarm of satellite galaxies. For the rest of us: Phew — we’re not smaller than Andromeda after all!

There’s still work to be done, though. The ideal tracer would be in the outer halo, Bonaca notes, out beyond 300,000 light-years. The trick is finding something that far out that we can still see, such as globular clusters, dwarf galaxies, or even streams of stars that the Milky Way’s gravity has torn from an infalling cluster or dwarf. Watkins and colleagues for their part think it’s likely that Gaia will continue to estimate the motions of many more globular clusters. No doubt, researchers will continue to narrow down the Milky Way’s mass using this and other methods for some time to come.


Image of Russell Sampson

Russell Sampson

March 18, 2019 at 7:21 pm

It should be noted that in the abstract of the ApJ preprint the value for the HST and Gaia derived virial mass of the Milky Way is 1.54 x 10^12 solar masses with an uncertainty of +0.75 and -0.44 x 10^12 solar masses. This produces an uncertainty envelope of between 2.29 and 1.10 x 10^12 solar masses. This is quite close to the range from the old estimates as cited in the abstract as "below 10^12 solar masses to above 2 x 10^12 solar masses".

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Monica Young

March 19, 2019 at 10:42 am

Hi Russell, You raise a good point. If you're able to get into the paper's introduction, the old range covered 0.5 - 3.0 x10^12 solar masses. So, 1.1 - 2.3 x 10^12 is still a significant improvement! Most importantly, it's the lower mass estimates that this newer study rules out.

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March 25, 2019 at 12:33 am

Comparison is made between the Milky Way and M31 (Andromeda) but no figures are given. Presumably we know M31's mass very well, since we can see the whole thing, but saying that the Milky Way is "bigger" cries out for some size for M31.

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Monica Young

March 27, 2019 at 8:31 am

Unfortunately, M31's mass also has some degree of uncertainty, and the study presented here focused on the Milky Way, not on M31. Different methods can give different answers, so to compare the Milky Way's mass to that of M31, we'd want a second study using the same methods presented here but focusing on M31. However, some previous studies had found surprisingly low mass estimates for the Milky Way, low enough that our galaxy would for sure be less massive than Andromeda; with this study ruling out those lower masses, that's no longer the case and we can say that we're probably on par with, if not more massive than Andromeda.

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March 25, 2019 at 8:29 am

Based on above, can we get a mass of the baryonic matter in the Milky Way?

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