Studying the Milky Way as a whole is not an easy task since we are inside it. But astronomers are resourceful and with the combined power of the Hubble Space Telescope and the Gaia observatory, they have been able to obtain a new and very accurate measurement of the mass of our galaxy.
The Milky Way weighs roughly 1.5 trillion solar masses within a radius of 129,000 light-years from the galactic center. This and previous mass estimations have not been easy to obtain. The galaxy is not just made up of stars and planets, there's gas and dust in it as well. But the most difficult part to estimate is dark matter. And that makes up most of the Milky Way's mass.
“We just can’t detect dark matter directly,” team leader Dr Laura Watkins, from the European Southern Observatory, said in a statement. “That’s what leads to the present uncertainty in the Milky Way’s mass – you can’t measure accurately what you can’t see!”
To estimate the value, the team used globular clusters, dense groups of stars that orbit galaxies outside the main body. Given that they are located far away from the galaxy's core, they are an excellent tracer of its mass.
“The more massive a galaxy, the faster its clusters move under the pull of its gravity,” co-author N. Wyn Evans, from the University of Cambridge, added. “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.”
That sideways velocity measurement requires lots of precise observations, and that’s what Gaia and Hubble bring in. The Gaia telescope was designed by the European Space Agency to construct the most precise map of the Milky Way and measure the velocity of millions of stars. The researchers used data from Gaia, which included information on globular clusters as far as 65,000 light-years from Earth. This was then combined with legacy observations from Hubble, which has studied clusters as far as 130,000 light-years away for a decade.
“We were lucky to have such a great combination of data,” explained co-author Roeland P. van der Marel of the Space Telescope Science Institute. “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.”
The accurate mass of the Milky Way will help scientists as they attempt to answer some of our deepest questions about the universe. The findings from this study are currently available online and will be published in The Astrophysical Journal.