All of these measurements would not have been accomplished without the data from Gaia and Hubble. The vast majority of the weight of our galaxy is dark matter.
When those measurements were combined as anchor points, astronomers were able to estimate the distribution of the Milky Way's mass out to almost 1 million light-years from Earth. The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe.
Interestingly, astronomers determined that only a tiny percentage of the galaxy's mass could be attributed to the approximately 200 billion stars in the Milky Way.
Previous estimates of the mass of the Milky Way ranged from 500 billion to 3 trillion times the mass of the Sun. "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 Roeland van der Marel of the Space Telescope Science Institute in the US, Eurekalert.org reports. "Compared to other galaxies with similar brightness, the Milky Way's mass is fairly typical".
As the dark matter is hard to calculate, scientists have to weigh the Milky Way by measuring the velocities of globular clusters - dense star clusters that orbit the spiral disc of the galaxy at great distances.
Globular star clusters are essentially isolated islands, each containing hundreds of thousands of stars - including the oldest-known stars, dating back to a few hundred million years after the big bang - that orbit the centre of the galaxy.
"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", Evans added. "However, we were able also to measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated", Wyn Evans said. Data from Gaia was pulled from a 22-month period, while data from Hubble was recorded over a 10-year span, allowing the astronomers to see the movements of these objects across reasonably large timescales.
"We were lucky to have such a great combination of data".
Watkins is happy with the new technique and the results, but she admits there's still room for improvement. Also, Watkins' team used only 46 clusters in this work.
Future Gaia data could offer more insight into the shape of this halo, which will help to refine astronomical models.