Govert Schilling
January 11, 2013

Suppose you stepped on the scales one morning to find that you weighed only half as much as the day before. You’d check the scales, right? In fact, a weight loss of cosmic proportions is exactly what happened when Alis Deason recalibrated the scales used to weigh our Milky Way galaxy. “We find the Milky Way is only half as massive as generally assumed,” says Deason, an astronomer at the University of California, Santa Cruz, who presented her new estimate here at the 221st meeting of the American Astronomical Society.

Determining the mass of the Milky Way is tricky, partly because most of it comes from unseen dark matter. Scientists usually measure the rotation speed of the galaxy (out to some 45,000 light-years from the center) and combine the result with theoretical ideas about the way the dark matter is distributed. Using this technique, a team led by Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, derived a total mass of a few trillion times the mass of the sun, a result they published in 2009. Still, Reid says, “estimating the total Galaxy mass is complicated,” and much uncertainty remained.

Deason and her colleagues took a different approach. In a study to be published in Monthly Notices of the Royal Astronomical Society, they first searched for very distant stars in the Milky Way’s halo: a huge ball of space almost a billion light-years across, in which old stars swarm around the galaxy’s center like mosquitoes around a lamppost. The spread of velocities of these distant halo stars reveals how much mass the Milky Way contains, she explains.

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