The Milky Way Galaxy (the one we’re in) contains an estimated 200 billion stars. But that’s just the tip of the iceberg, the Galaxy is surrounded by vast amounts of an unknown material called “dark matter” (matter that we can’t normally detect because it doesn’t interact with the electromagnetic spectrum). 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 more precise measure of the Galaxy’s total 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 an equivalent mass of 1.54 trillion solar masses, most of it locked up in dark matter.
This new mass estimate puts the 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 margin of error 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.
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 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 the study.
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.
By combining Gaia’s measurements with measurements from Hubble, the scientists could better pin down the Milky Way’s mass in a way that would be impossible without both space telescopes,