Billions of stars and various accumulations of matter form our cosmic home. Can you estimate the total mass of our Milky Way? This is possible indirectly, namely on the movement of globular clusters in the outer area of our galaxy. The combination of data from the Hubble Space Telescope and ESA’s Gaia satellite has resulted in the most precise mass determination of our home galaxy to date. The Milky Way “weighs” around 1.5 trillion solar masses. Information about mass can provide insights into the development of our galaxy, but also of the universe as a whole, say the astronomers.
The Milky Way is our cosmic home – and yet astronomers know it less well than many of our neighboring galaxies. Our position in the middle of this cluster of stars complicates many measurements and observations that would actually require an “outside view”. This is one of the reasons why the exact mass of our home galaxy is still not exactly known. Because it does not only result from the mass of the roughly 200 billion stars and that of the supermassive black hole in the center of the galaxy. The lion’s share of 90 percent of the Milky Way’s mass is made up of an invisible component – dark matter. “But we cannot measure dark matter directly,” explains lead author Laura Watkins from the Space Telescope Science Institute in Baltimore. “That is what makes the uncertainty about the mass of the Milky Way galaxy.” Earlier estimates ranged from 500 billion to three trillion solar masses, depending on the method.
Globular clusters as a measuring aid
But Watkins and her team have combined two of the most powerful tools in astronomy to get more accurate data on the Milky Way: the Hubble space telescope and the European Space Agency’s Gaia satellite. The Gaia satellite has already created the most accurate and comprehensive 3D map of the Milky Way to date, including the movement of numerous globular clusters. Although the Hubble space telescope has a much narrower field of view, it can look further out into space. This enables it to track the movement of globular clusters that are up to 130,000 light years away. From the speed of movement of these star clusters, astronomers can infer the distribution and mass of dark matter in the Milky Way.
“Every mass distribution creates a gravitational potential that sets objects in motion. By measuring this movement, we can calculate back how large the underlying gravitational potential is and thus also the mass, ”explain the astronomers. However, previous measurements of this type could only determine the movement of globular clusters in our line of sight – how they move toward or away from us. Gaia and Hubble, on the other hand, also followed the sideways movement of the globular clusters – and that makes the measurements much more accurate. “By combining Gaia readings for 34 globular clusters and Hubble data for twelve more distant star clusters, we can limit the mass of the Milky Way in a way that would be impossible without these two space telescopes,” says Watkins’ colleague Roeland P. van der Marel .
1.5 trillion solar masses
With the help of this data, the researchers now arrive at a Milky Way mass of 1.5 trillion solar masses in a radius of around 129,000 light years around the galactic center. This value is therefore roughly in the middle of the earlier estimates and corresponds relatively well to the mass that can be expected for a galaxy of this luminosity, as the astronomers report. This value is important not only to get to know our home galaxy better, but also beyond: “The exact determination of the mass profile of the Milky Way is important for our understanding of the dynamic history of the Local Group and the satellites of the Milky Way,” explain Watkins and her colleagues. “In addition, the mass of galaxies is closely linked to the formation and growth of structures in the universe. The determination of the Milky Way mass therefore gives us a clearer picture of where our galaxy stands in the cosmological context. “
Source: NASA, Laura Watkins (Space Telescope Science Institute, Baltimore) et al., Astrophysical Journal