The Milky Way Is Weighed

The Milky Way galaxy weighs about 1 trillion times as much as our sun, according to a new estimate.

Previous estimates had ranged from 750 billion solar masses to up to 2 trillion. Lately, researchers have been leaning toward the higher figure. But now astronomers have used a more refined method to conclude that our galaxy's mass is slightly less than 1 trillion solar masses.

The galaxy's mass is a mix of stars, gas, dust and mysterious dark matter.

The new estimate is based on a large sample of stars in the galactic halo, a relatively sparse sphere of stars that surrounds our galaxy's main disk. The speeds of stars in the halo reveal the mass of the galaxy by allowing astrophysicists to infer the amount of gravity required to keep those stars in orbit.

"The galaxy is slimmer than we thought," said Xiangxiang Xue of the National Astronomical Observatories of China, who led the international team of researchers. "That means it has less dark matter than previously believed, but also that it was more efficient in converting its original supply of hydrogen and helium into stars."

The finding, based on data from the Sloan Digital Sky Survey (SDSS-II), has broad implications for our understanding of the Milky Way, the researchers said in a statement.

"The total mass of the galaxy is hard to measure because we're stuck in the middle of it," explained collaborator Timothy Beers of Michigan State University. "But it is the single most fundamental number we have to know if we want to understand how the Milky Way formed or compare it to distant galaxies that we see from the outside."

Previous estimates had been based on 500 objects or fewer. The new math was based on data for 2,400 stars.

The larger data set "gives us a huge statistical advantage," said study team member Hans-Walter Rix of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.

The research will be detailed later this year in the Astrophysical Journal.

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