Astronomers weigh the galaxy with Hubble and Gaia


"Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy" said Tony Sohn (Space Telescope Science Institute, USA), who led the Hubble measurements.

Previously, the Milky Way was estimated to be between 500 billion and 3 trillion solar masses; this new estimate is in the middle of the range, which was determined several decades ago using a variety of observations. This large difference is primarily due to the different methods used for measuring the distribution of dark matter - which makes up about 90% of the mass of the galaxy.

"We just can't detect dark matter directly", said Laura Watkins, from the Germany-based European Southern Observatory.

How they did it: The team observed the velocities of globular clusters, which are clusters of stars that orbit the center of the galaxy. Previous investigations used the speed of clusters relative to Earth to figure out the mass. "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".

Scientists announced today that they have greatly improved their estimate of the mass of the Milky Way galaxy with its most precise measurement yet. What scientists do know about its contents is that there is a 4-million-solar-mass supermassive black hole anchored at the centre of the galaxy. Gaia was designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and to track their motions. 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.

The project involved global collaboration between scientists working for NASA and ESA on the Hubble and Gaia telescopes. To achieve that, they combined new data from the ESA Gaia mission with observations made with the NASA/ESA Hubble Space Telescope. Scientists expect Gaia to eventually provide accurate positional and radial velocity measurements and to produce a kinematic and stereoscopic census of approximately one billion stars in the galaxy and throughout the Local Group. Its second data release includes measurements of globular clusters as far as 65 000 light-years from Earth. The latest answer claims to be the most accurate estimate yet, placing the Milky Way somewhere in between at 1.5 trillion solar masses, a healthy number for most galaxies with the same brightness.

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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).

Hanging over the scruff of Ursa Major's neck some 12 million light-years from Earth, a cluster of young stars known as the Cigar Galaxy is puffing epic amounts of wind into deep space. 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. "The Milky Way is the closest galaxy to us so it is the one we can study in most detail".

The Milky Way isn't exactly the easiest thing to study, even if it's our galaxy.

A long time ago gravity drew two galaxies together into the gorgeously chaotic state we see above with NGC 6052. "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".

"We were lucky to have such a great combination of data".