How Much Does the Milky Way Weigh?

Astronomers weigh the galaxy with Hubble and Gaia

In a new trick, data from the European Space Agency's Gaia mission was combined with observations from the NASA Hubble Space Telescope to allow indirect measurements of the dark matter's mass.

The result was that our home galaxy weighs around 1.5 trillion solar masses within a radius of 129,000 light years around the galactic center.

Previous estimates of the mass of the Milky Way ranged from 500 billion to 3 trillion times the mass of the Sun. According to scientists, huge uncertainty in those estimates arose due to the use of different methods for measuring the distribution of dark matter within the galaxy. 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. They measured the velocity 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" explains Professor Wyn Evans from the University of Cambridge.

The Hubble and Gaia can record the sideways motion of the globular clusters, from which a more reliable speed (and thereby gravitational acceleration) can be calculated.

Variation comes from disagreements and discrepancies in techniques used to measure and weigh dark matter, which make sup to 90 per cent of the galaxy. 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 mission aims to create a 3D map of Milky Way and to provide details about its composition and formation. Its second data release includes measurements of globular clusters as far as 65 000 light-years from Earth.

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The Hubble Space Telescope is a project of worldwide cooperation between ESA and NASA.

ESA's Gaia satellite (pictured) and Hubble provided data on 46 globular clusters, masses of hundreds of thousands of stars that swarm around the Milky Way's centre.

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.

That extrapolated figure gives a mass for the Milky Way as a staggering 1.5 trillion solar masses (with one solar mass being the mass of our Sun). Gaia was launched on 19 December 2013 and is located at the L2 Lagrange point - the same location that the upcoming NASA/ESA/CSA James Webb Space Telescope will have. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well. "A lot of the time, we try to understand the universe by putting it in context of the Milky Way".

And understanding the total mass of our galaxy, and how its stars' mass stack up against its dark matter, can help astronomers answer important cosmological questions about how galaxies like ours evolve. But as Phil Plait at Bad Astronomy blog notes, measuring the mass of the Milky Way is much more hard because we are inside it and can't get the big picture, literally. Amazingly, only a few percent of this is down to the approximately 200 billion stars in our galaxy; the rest we still can't see, just infer.

'We were lucky to have such a great combination of data, ' explained Roeland van der Marel of the Space Telescope Science Institute in the US.

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