Published: Sun, March 10, 2019
Science | By Michele Flores

Hubble & Gaia accurately weigh the Milky Way

Hubble & Gaia accurately weigh the Milky Way

It is no surprise then, that previous estimates of the Milky Way's mass has ranged anywhere from 500 billion to 3 trillion times the mass of the Sun.

Measuring the total mass of our home galaxy is a tough puzzle. They measured the velocity of globular clusters - dense star clusters that orbit the spiral disc of the galaxy at great distances.

To weigh the galaxy, the team augmented Gaia measurements for 34 globular clusters out to 65,000 light-years, with Hubble measurements of 12 clusters out to 130,000 light-years that were obtained from images taken over a 10-year period.

The number that NASA came up with is that the Milky Way weighs in at 1.5 trillion solar masses. Using data from NASA's Hubble Space Telescope and the European Space Agency's Gaia satellite, astronomers have determined the most accurate measurement of its mass: Our vast galaxy clocks in at 1.5 trillion solar masses. And Watkins tells Dvorsky at Gizmodo that Gaia, which is expected to map the sky for another decade, will continue to reveal more globular clusters and help astronomers continue refining the weight estimate of the galaxy. "Not knowing the precise mass of the Milky Way presents a problem for a lot of cosmological questions". A tiny, tiny fraction is the monstrous supermassive black hole at the center of the galaxy, coming in at four to five million solar masses, plus the stuff like the planets, and you and I. The remaining 90-odd per cent is dark matter. Still, astronomers would like to have a precise measure of the Galaxy's mass to better understand how the myriad galaxies throughout the Universe form and evolve. The clusters move because they feel a gravitational force - they only know the total force they feel, not how much of that force came from one type of object and how much came from another.

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Globular clusters are isolated, densely packed, spherical collections of ancient stars, that orbit the centre of our galaxy.

The study was presented in the paper "Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions", which will be published in The Astrophysical Journal. The larger the galaxy, the quicker its globular clusters travel around the pull of gravity.

The worldwide team of astronomers in this study consists of Laura L. Watkins (European Southern Observatory, Germany), Roeland P. van der Marel (Space Telescope Science Institute, USA, and Johns Hopkins University Center for Astrophysical Sciences, USA), Sangmo T. Sohn (Space Telescope Science Institute, USA), and N. Wyn Evans (University of Cambridge, UK). While previous measurements have been along the line to sight to globular clusters, astronomers know the speed at which a globular cluster is approaching or receding from Earth. The heaviest are 30 trillion solar masses. Most large galaxies have such hangers-on, and the Milky Way is no exception. Dark matter makes up 90 percent of the galaxy's mass, but European Southern Observatory's Laura Watkins reveals that it is not possible to see and observe dark matter directly, which made it hard to get an accurate figure for the scientists.

When the Hubble and Gaia measurements are combined as anchor points, like pins on a map, astronomers can estimate the distribution of the Milky Way's mass out to almost 1 million light years from Earth. 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.

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