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Astronomers determine weight of our home galaxy, the Milky Way

Photo Credit: NASA JPL
Data based on observations made by the Gaia mission and Hubble telescope suggest that the Milky Way is as heavy as 1.5 trillion Suns
by TR Pakistan

Scientists have discovered that the Milky Way is as heavy as 1.5 trillion Suns and has a radius of 129,000 light-years from the galactic center.

This measurement has been found by combining new data from the European Space Agency (ESA) Gaia mission with observations made with the NASA/ESA Hubble Space Telescope.

The mass of the Milky Way is one of the most important measurements astronomers can make about our galactic home but previous estimates ranged from 500 billion to 3 trillion times the mass of the Sun. This huge uncertainty arose primarily from the different methods used for measuring the distribution of dark matter — which makes up about 90 percent of the mass of the galaxy.

“We just can’t detect dark matter directly,” explains Laura Watkins, who led the team performing the analysis, in a statement. “That’s what leads to the present uncertainty in the Milky Way’s mass — you can’t measure accurately what you can’t see!”

Given the elusive nature of the dark matter, the team had to weigh the Milky Way by measuring the velocities of globular clusters — dense star clusters that orbit the spiral disc of the galaxy at great distances.

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“The more massive a galaxy, the faster its clusters move under the pull of its gravity,” said Professor Neil Wyn Evans of the University of Cambridge. “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. 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 astronomers used Gaia’s second data release, which included measurements of globular clusters as far as 65,000 light-years from Earth, as a basis for the study.

“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, who led the Hubble measurements.

Gaia’s measurements of 34 globular clusters was combined with observations made from Hubble. This allowed 12 more globular clusters, as far as 130,000 light-years from Earth, to also be added to the study. Since the Hubble Space Telescope has been observing some of these objects for a decade, it was also possible to accurately track the velocities of these clusters.

Roeland P. van der Marel from the Space Telescope Science Institute, noted that it would not have been possible to calculate the Milky Way’s mass without the use of these two space telescopes.

According to astronomers, a more accurate determination of the mass for the Milky Way could help answer a lot of questions and give us a clearer understanding of where our galaxy sits in a cosmological context.