‘Milky Way weighs about 1.5 trillion solar masses’

Our Milky Way galaxy weighs in at about 1.5 trillion solar masses, measurements from the Hubble Space Telescope and the European Space Agency’s Gaia mission have shown, scientists say. The mass of the Milky Way is one of the most fundamental measurements astronomers can make about our galactic home.

However, despite decades of intense effort, even the best available estimates of the Milky Way’s mass disagree wildly.

Now, by combining new data from ESA’s Gaia mission with observations made with Hubble, astronomers have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129, 000 light-years from the galactic centre.

Previous estimates of the mass of the Milky way ranged from 500 billion to three 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 per cent of the mass of the galaxy.

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

Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the velocities of globular clusters—dense star clusters that orbit the spiral disc of the galaxy at great distances.

“The more massive a galaxy, the faster its clusters move under the pull of its gravity,” said N Wyn Evans from the University of Cambridge, UK. “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,” Evans said.