Scientists calculate the mass of our Milky Way galaxy using the weight of dark matter

Graphic view of our Milky Way Galaxy. The Milky Way Galaxy is organized into spiral arms of giant stars that illuminate interstellar gas and dust. The Sun is in a finger called the Orion Spur. Overlaid is a graphic of galactic longitude in relation to our Sun. Credit: NASA/Adler/U. Chicago/Wesleyan/JPL-Caltech

Astronomers said on Thursday that they have accurately calculated the mass of Milky Way for the first time, using a new set of data that include the weight of dark matter. 

In a collaboration between NASA and the European Space Agency's Gaia observation Spacecraft, a team of experts calculated the mass of our galaxy to be around 1.5 trillion solar masses. Previous estimates had put the mass of Milky Way ranging between 500 billion and 3 trillion solar masses. 

The uncertainty mainly stemmed from differing methods used to measure the dark matter - which doesn't absorb or reflect any radiation and is thought to make up about 85 percent of the entire matter of cosmos. 

Measurements from the NASA/ESA Hubble Space Telescope and the ESA Gaia mission have been combined to improve he estimate of the mass of our home galaxy the Milky Way: 1.5 trillion solar masses. Credit: NASA/ESA Hubble

"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 Laura Watkins, from Germany-based European Southern Observatory. 

To get around this, the team measured the velocity of globular clusters - dense grouping of stars that orbit the Galaxy at enormous distances. 

"The more massive a galaxy, the faster its clusters move under the pull of its gravity", said N. Wyn Evans, from the University of Cambridge's Institute of Astronomy. 

On the left is a Hubble Space Telescope image of a portion of the globular star cluster NGC 5466. On the right, Hubble images taken ten years apart were compared to clock the cluster's velocity. A grid in the background helps to illustrate the stellar motion in the foreground cluster (located 52,000 light-years away). Notice that background galaxies (top right of center, bottom left of center) do not appear to move because they are so much farther away, many millions of light-years. Credits: NASA, ESA and S.T. Sohn and J. DePasquale (STScI)

Most of the previous measurements have found the speed at which a cluster is approaching or receding away from earth, that is, the velocity along our line of sight.

Instead this time, researchers were able to use data collected by Gaia probe and NASA's Hubble telescope, to observe the sideways motion of these clusters. From this, they were able to calculate their total velocity and in turn their total mass.

Apart from our solar system, the Milky Way is home to around 400 billion stars and an estimated 100 billion planets.