The Milky Way contains an estimated 200 billion stars. But that’s just the bare tip of the iceberg — the Galaxy is surrounded by vast amounts of an unknown material called dark matter. Astronomers know it exists because, dynamically, the Milky Way would fly apart if dark matter didn’t keep a gravitational lid on things. Still, astronomers would like to have a precise measure of the Galaxy’s mass to better understand how the myriad galaxies throughout the Universe form and evolve. A team of researchers from ESO, the Space Telescope Science Institute, the Johns Hopkins University Center for Astrophysical Sciences and the University of Cambridge combined observations from the NASA/ESA Hubble Space Telescope and ESA’s Gaia satellite to study the motions of globular star clusters that orbit our Galaxy. The faster the clusters move under the entire Galaxy’s gravitational pull, the more massive it is. The team concluded the Milky Way has a mass of 1.54 trillion solar masses, most of it locked up in dark matter.

The new mass estimate puts our Milky Way Galaxy on the beefier side, compared to other galaxies in the Universe.

The lightest galaxies are around a billion solar masses, while the heaviest are 30 trillion, or 30,000 times more massive. The Milky Way’s mass of 1.5 trillion solar masses is fairly normal for a galaxy of its brightness.

Previous estimates of the Milky Way’s mass ranged from 500 billion to 3 trillion solar masses. This huge uncertainty arose primarily from the different methods used for measuring the distribution of dark matter — which makes up about 90% of the mass of the Galaxy.

“We just can’t detect dark matter directly. That’s what leads to the present uncertainty in the Milky Way’s mass — you can’t measure accurately what you can’t see,” said Dr. Laura Watkins, an astronomer at ESO.

Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the velocities of globular clusters — dense star clusters that orbit the spiral disk of the Galaxy at great distances.

“The more massive a galaxy, the faster its clusters move under the pull of its gravity,” said Dr. N. Wyn Evans, from the University of Cambridge.

“Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, 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 scientists used Gaia’s second data release — which includes measurements of globular clusters as far as 65,000 light-years from Earth — as a basis for their study.

“Globular clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our Galaxy,” said Dr. Tony Sohn, an astronomer at the Space Telescope Science Institute.

Observations from Hubble allowed faint and distant globular clusters, as far as 130,000 light-years from Earth, to be added to the study. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well.

“We were lucky to have such a great combination of data. By combining Gaia’s measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way’s mass in a way that would be impossible without these two space telescopes,” said Dr. Roeland P. van der Marel, also from the Space Telescope Science Institute.

The team’s results will be published in the Astrophysical Journal.

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Laura L. Watkins et al. 2019. Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions. ApJ, in press; arXiv: 1804.11348