Scientists solve weighty matter of Milky Way mass

James Marshall
March 8, 2019

Our galaxy has been put on a set of mathematical scales by scientists who have discovered the weight of the Milky Way.

The entire galaxy is 1.5 trillion times greater (1.5 multiplied by ten to the power of 12) than this. "That's what leads to the present uncertainty in the Milky Way's mass - you can't measure accurately what you can't see".

The Hubble and Gaia observations are complementary. The faster the clusters move, the more massive the galaxy.

'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. Gaia has been creating a 3D map of the galaxy's astronomical objects and tracking their movements since its mission began in 2014.

To get around this, the team measured the velocity of globular clusters - dense groupings of stars that orbit the galaxy at enormous distances. It made exacting all-sky measurements that include many globular clusters.

When the Gaia and Hubble 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. A team of researchers from ESO, the Space Telescope Science Institute, the Johns Hopkins University Center for Astrophysical Sciences and the University of Cambridge combined observations from the NASA/ESA Hubble Space Telescope and ESA's Gaia satellite to study the motions of globular star clusters that orbit our Galaxy. "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". The space telescope uses that to its advantage by measuring fainter stars and distant clusters.

The clusters are key because they contain the oldest stars, born just a few hundred million years after the big bang. They formed prior to the construction of the Milky Way's spiral disk, where our Sun and solar system reside. Using an airborne infrared telescope, the astronomers counted pixels of dust in the faraway galaxy to estimate that roughly 50 million to 60 million suns' worth of mass were caught up in the Cigar Galaxy's mighty celestial wind.

The worldwide team of astronomers in this study are Laura Watkins (European Southern Observatory, Garching, Germany), Roeland van der Marel (Space Telescope Science Institute, and Johns Hopkins University Center for Astrophysical Sciences, Baltimore, Maryland), Sangmo Tony Sohn (Space Telescope Science Institute, Baltimore, Maryland), and N. Wyn Evans (University of Cambridge, Cambridge, United Kingdom).

The Hubble telescope (pictured) provided data for the study. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. Tony Sohn, an astronomer at the Space Telescope Science Institute.

"One of the main objectives of this research was to evaluate how efficiently the galactic wind can drag along the magnetic field", study co-author Enrique Lopez-Rodriguez, a Universities Space Research Association scientist, said in a statement. But that's less than half the total mass-the majority comes from dark matter. In addition, each galaxy is home to countless stars - some of which shine brightly in the foreground of the Hubble image.

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