The Milky Way Is Lighter Than We Thought

New measurements suggest that our galaxy is on the slimmer side, at a mere 700 billion times the mass of the sun.

Want to start a fight between astrophysicists who study the Milky Way? Ask them to tell you how much the galaxy weighs.

Now, a new modeling method by researchers at McMaster University in Ontario, Canada, may help put those debates to rest.

According to the new work, presented May 31 at a Canadian Astronomical Society conference in Winnipeg, the Milky Way contains the same amount of mass as 700 billion suns—and that puts it on the slimmer side of the scale. At the same time, our galaxy seems to contain slightly more dark matter than previously calculated. This mysterious invisible substance is thought to exist in a cloud around the Milky Way.

Figuring out how much the Milky Way currently weighs can allow cosmologists to get a better handle on our galaxy’s past and future.

“Understanding our galaxy’s mass puts it into a better cosmological context,” says study leader Gwendolyn Eadie, a doctoral candidate at McMaster. For starters, the rate at which stars in any given galaxy form, exist, and die seems to be tied to the overall mass of the galaxy.

“People who study the evolution of galaxies look at how the mass relates to its evolution,” says Eadie. “If we have a better handle on what the mass of the Milky Way is, we can understand how it and other galaxies form and evolve.”

Moving Through Dark Matter

Previous estimates of how much matter the Milky Way contains vary wildly. Some studies report it holds the equivalent of 1 trillion suns, while others say it’s merely 100 billion.

Those measurements all include the types of matter we can observe or detect directly—dust, planets, moons, stars, and some of the dwarf galaxies orbiting the Milky Way—as well as the galaxy’s dark matter halo.

Invisible except for its gravitational effects on other objects, dark matter is exceptionally tricky to measure, and Eadie has been working on the problem since she started graduate work studying ancient groups of stars known as globular clusters.

Eadie ultimately devised a method to measure our galaxy’s dark matter using the known motions and velocities of 89 globular clusters that exist around the Milky Way.

She used globular clusters because they are dispersed at different distances throughout the galaxy, and because they are relatively large, well defined, and easier to track over time than individual stars. As these clusters orbit the galactic center, dark matter pushes and pulls on them in predictable ways.

Put together with the known masses of visible objects in the galaxy, her model created a “mass profile” of the Milky Way, which estimates the mass contained within any distance from the galactic center.

At 700 billion suns, Eadie’s final estimate agrees more closely with the “lighter galaxy” camp, which judges that some of the outer-edge objects, including some large, distant globular clusters and diffuse dwarf galaxies, aren’t truly bound by the Milky Way’s gravity and thus not part of its overall total mass.

And since the stellar mass of the galaxy is currently estimated at around 60 billion suns, and dust and gas make up about one to three percent of the rest, Eadie’s results suggest that as much as 88 percent of the mass in the Milky Way is comprised of dark matter.

Hefty Home

While Eadie’s work isn’t the first to try to estimate the mass of the galaxy, her study combines a wide variety of data sources to yield one of the most thorough analyses to date, says Alan McConnachie, a research officer and instrument specialist at the National Research Council of Canada's Herzberg Institute for Astrophysics.

“Figuring out how fast, and in what direction, globular clusters are moving is pretty hard. Combining all of these data together in a consistent model for the Milky Way is a real challenge,” McConnachie says.

“People sometimes are a bit surprised that we don't have a better idea of how heavy the Milky Way is, given that it’s the galaxy we live in,” he adds. “This work is a big step toward being able to claim with confidence that we know how massive our home actually is.”

For Eadie, the result is inspiration to continue astrophysicists’ efforts to pinpoint the nature of dark matter.

“On the one hand, the visible Milky Way in the night sky is extensive—that stream of stars and dust across the dark sky is beautiful, magnificent, and enormous,” Eadie says. “But the idea that the stuff I'm seeing is only about one-fifth of what's out there inspires me to figure out what we're missing.”

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