Researchers may have found the weight of the Milky Way

Astronomers weigh the galaxy with Hubble and Gaia

Now, by combining new data from the European Space Agency (ESA) Gaia mission with observations made with the NASA/ESA Hubble Space Telescope, astronomers have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129 000 light-years from the galactic centre.

Previous estimates of the mass of the Milky Way ranged between 500 billion and 3 trillion times the mass of the Sun.

"We know from cosmological simulations what the distribution of mass in the galaxies should look like, so we can calculate how accurate this extrapolation is for the Milky Way", said Laura Watkins of the European Southern Observatory in Garching, Germany.

This globular cluster, NGC 4147, seen with the NASA/ESA Hubble Space Telescope, was one of many which were used by astronomers to measure the total mass of the Milky Way.

But researchers decided they could figure out the Milky Way's velocity by looking at some of the 157 globular clusters, or really densely packed groups of stars, orbiting the galaxy's center.

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

'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.

Measuring the total mass of our home galaxy is a tough puzzle. However, only a small percentage of this is contributed by the approximately 200 billion stars in the Milky Way and includes a 4 million solar mass supermassive black hole at the center. Gaia was designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and to track their motions. "A lot of the time, we try to understand the universe by putting it in context of the Milky Way". An global team of astronomers from the European Southern Observatory (ESO) in Garching near Munich, the British University of Cambridge, the Johns Hopkins University Center for Astrophysical Sciences, and the Space Telescope Science Institute in Baltimore, USA could at least approximately determine the weight of the Milky Way.

Taking this data, they were able to combine it with measurements of globular clusters as far away as 130,000 light years from Earth that Hubble has taken over the last decade, giving the team an enormous amount to data to work with. That means there's a pretty large margin of error in the estimate, meaning the true mass of the Milky Way may be somewhere between 0.79 and 2.29 trillion solar masses-but the current estimate is a good start. When data from Hubble and Gaia was combined, it allowed the researchers to estimate the distribution of mass in the Milky Way out to 1 million light-years from Earth. Tony Sohn, an astronomer at the Space Telescope Science Institute. Since Hubble has been observing some of these objects for ten years, it was also possible to accurately track the velocities of these clusters as well. Most large galaxies have such hangers-on, and the Milky Way is no exception. "We see the Milky Way as it is now, not as it was in the past", Watkins says. But as Phil Plait at Bad Astronomy blog notes, measuring the mass of the Milky Way is much more hard because we are inside it and can't get the big picture, literally.

'We were lucky to have such a great combination of data, ' explained Roeland van der Marel of the Space Telescope Science Institute in the US.



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