How Much Does the Milky Way Weigh?

Measurements from the Gaia satellite and Hubble Space Telescope show our galaxy tips the scales at about 1.5 trillion solar masses

Globular Clusters
The positions of the globular clusters used to estimate the mass of the Milky Way. ESA/Hubble, NASA, L. Calçada

Astronomers have recalculated how massive our home galaxy, the Milky Way, is using new data from the NASA's Hubble Space Telescope and the European Space Agency's Gaia satellite. The results are in: Our galaxy weighs about 1.54 trillion solar masses, according to a new study scheduled for publication in the Astrophysical Journal.

Ashley Strickland at CNN reports that a surprisingly small amount of that mass comes from the 200 billion stars in the Milky Way and the large, 4-million-solar-mass black hole they all swirl around. The rest is composed of dark matter, the undetectable stuff that makes up 80 percent of the universe.

What is dark matter? We literally don’t know; researchers hypothesize that it could be tons of undetectable brown dwarf stars, supermassive black holes or exotic theoretical particles like neutralinos.

Understanding the mass of our galaxy, no matter what it is composed of, is important in astronomy. George Dvorsky at Gizmodo reports that not knowing the mass of our home spiral makes it difficult to calculate how it is interacting with nearby galaxies, like Andromeda. Knowing the mass of our own galaxy would also help us better understand how it evolved, giving us more insight into how other galaxies came to be.

“We want to know the mass of the Milky Way more accurately so that we can put it into a cosmological context and compare it to simulations of galaxies in the evolving universe,” co-author Roeland van der Marel, an astronomer at the Space Telescope Science Institute in Baltimore, says in a press release. “Not knowing the precise mass of the Milky Way presents a problem for a lot of cosmological questions.”

Previously, estimates for the mass of the Milky Way were all over the map, ranging from a paltry 500 billion solar masses to 2 or 3 trillion masses. The new numbers puts it right in the middle.

“We were surprised that our value fell in the middle of the very wide range of previous estimates,” study leader Laura Watkins of the European Southern Observatory tells Dvorsky. “A lot of the most recent studies had tended to favor lower values. So this value was on the high end of the most recent work.”

While the Milky Way is a bit brawnier than researchers expected, it’s more or less an average mass for a galaxy of its size. So far, researchers have found galaxies as light as 1 billion solar masses and as heavy as 30 trillion solar masses, which is about 20 times as massive as our own.

Researchers have techniques for estimating the mass of distant galaxies that we see through our telescopes by using the velocity at which they spin. But as Phil Plait at Bad Astronomy blog notes, measuring the mass of the Milky Way is much more difficult because we are inside it and can’t get the big picture, literally. “To be fair, it’s like trying to understand your house but not being allowed to leave your closet,” he writes. “We’re inside the Milky Way, stuck about halfway out from the center, and everything we learn about it we learn from right here.”

But researchers decided they could figure out the Milky Way’s velocity by looking at some of the 157 globular clusters, or really densely packed groups of stars, orbiting the galaxy’s center. So the team looked at 34 distant clusters measured over 22 months by ESA’s Gaia star survey satellite ranging from 6,500 to 70,000 light years away. They also examined 12 other clusters observed by the Hubble telescope, some up to 130,000 light years away, taken over a 10-year period. The movements of those clusters over time gave the researchers enough data to estimate the rotation of the entire galaxy, which they could use to calculate its mass.

“The more massive a galaxy, the faster its clusters move under the pull of its gravity” co-author N. Wyn Evans of the University of Cambridge says in another press release. “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.”

Plait reports that the team had to estimate the mass of the galaxy beyond the 130,000 light year mark, especially the halo of dark matter that is believed to surround it. That means there’s a pretty large margin of error in the estimate, meaning the true mass of the Milky Way may be somewhere between 0.79 and 2.29 trillion solar masses—but the current estimate is a good start. And Watkins tells Dvorsky at Gizmodo that Gaia, which is expected to map the sky for another decade, will continue to reveal more globular clusters and help astronomers continue refining the weight estimate of the galaxy.