Scientists solve weighty matter of Milky Way mass

Rodiano Bonacci
Marzo 11, 2019

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.

Data from the Gaia mission is key with measurements of these globular clusters extending up to 65,000 light-years away from Earth, while the observations from Hubble added data from globular clusters as far as 130,000 light-years from the planet. The vast majority of the weight of our galaxy is dark matter. The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe.

Pervious estimates put the mass of the Milky Way ranging between 500 billion and three trillion times the mass of the Sun.

The Milky Way, the galaxy which contains Earth's solar system, is home to up to 400 billion stars and an estimated 100 billion planets.

NASA said the new mass estimate puts the galaxy "on the beefier side compared to other galaxies in the universe", which range from 1 billion solar masses to 30 trillion solar masses. The combined mass and distance of globular clusters make them excellent tracers, or reference points, for measuring the mass of the Milky Way.

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.

The more massive a galaxy, the faster its globular clusters move under the pull of gravity, according to a forthcoming paper in The Astrophysical Journal. The measurement is by far the most accurate one of the galaxy, thanks to the data provided by NASA/ESA Hubble Space Telescope and ESA's Gaia mission. "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".

"We were lucky to have such a great combination of data", explained Roeland P. van der Marel (Space Telescope Science Institute, USA). Gaia, for example, will scan the sky for a total of around nine or 10 years before the mission is complete, and that data will allow scientists to measure stellar motions more accurately, and in turn, develop a clearer picture of galactic mass.

"Having motions for more objects would give better accuracy", she told Gizmodo. Those clusters orbit near the center of our galaxy.

Future Gaia data could offer more insight into the shape of this halo, which will help to refine astronomical models.

We may finally know how much the Milky Way weighs.

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