The weight of the Milky Way has finally been discovered

Olive Hawkins
March 10, 2019

'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'.

Earlier estimates of the mass of our home galaxy have ranged from 0.5 trillion to 3 trillion solar masses. According to scientists, huge uncertainty in those estimates arose due to the use of different methods for measuring the distribution of dark matter within the galaxy.

"We just can't detect dark matter directly".

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.

Astronomers used the data from these two telescopes to measure the motion of 46 "globular clusters" - the distinct groups of stars orbiting the centre of Milky Way.

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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.

Scientists have previously been able to measure the speed at which a globular cluster approaches or recedes from Earth along our line of sight but this information alone is quite limiting. They combined data collected by NASA's Hubble telescope and the European Space Agency's Gaia observation satellite. This allowed them to calculate the total velocity, and consequently the galactic mass out to almost one million light-years from Earth.

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. Gaia was specially designed to create a precise 3-dimensional map of astronomical objects throughout the Milky Way and track their motions and made challenging all-sky measurements that included several globular clusters. Most of the remaining mass is locked up in dark matter, an invisible and mysterious substance that acts like scaffolding throughout the universe and keeps the stars in their galaxies. An worldwide 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. It's hard to see it all at once, buried as we are within one of its spiral arms. The Milky Way's mass of 1.5 trillion solar masses is fairly normal for a galaxy of its brightness. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well. Accurately determining the mass for the Milky Way gives us a clearer understanding of where our galaxy sits in a cosmological context. Tony Sohn, an astronomer at the Space Telescope Science Institute. Astronomers use the knowledge we can get of the Milky Way to learn about more distant galaxies that we can't study in such detail. "We're inside the Milky Way, stuck about halfway out from the center, and everything we learn about it we learn from right here".

"Not knowing the precise mass of the Milky Way presents a problem for a lot of cosmological questions", said Roeland van der Marel, a researcher at the US-based Space Telescope Science Institute (http://www.stsci.edu/), on Thursday. "So astronomers make simulations of how the Milky Way might have formed and evolved over billions of years and then look to see which simulations end with a galaxy looks most like the Milky Way".

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