The Milky Way Loses Weight

Have you ever been surprised at your annual weigh-in at the doctor's office to find that your bathroom scale at home was wrong? Or, bought a new scale that had a difference of opinion with your old one? That's what has happened with our very own Galaxy, the Milky Way. "The Galaxy is slimmer than we thought," said Xiangxiang Xue of the Max Planck Institute for Astronomy in Germany and the National Astronomical Observatories of China, who lead a research team using the Sloan Digital Survey to measure the mass of the stars in the galaxy. "We were quite surprised by this result," said Donald Schneider, a member of the research team, from Penn State. The researchers explained that it wasn't a Galactic diet that accounted for the galaxy's recent slimming, but a more accurate scale.

The Milky Way Slims Up

The researchers used the motions of distant stars to make the new determination of the Milky Way's mass. They measured the motions of 2,400 "blue horizontal branch" stars in the extended stellar halo that surrounds the disk of the galaxy. These measurements reach distances of nearly 200,000 light years from the Galactic center, roughly the edge of the region illustrated in the image above. Our Sun lies about 25,000 light years from the center of the Galaxy, roughly halfway out in the Galactic disk. From the speeds of these stars, the researchers were able to estimate much better the mass of the Milky Way's dark-matter halo, which they found to be much 'slimmer' than thought before.

The discovery is based on data from the project known as SEGUE (Sloan Extension for Galactic Understanding and Exploration), an enormous survey of stars in the Milky Way. Using SEGUE measurements of stellar velocities in the outer Milky Way, a region known as the stellar halo, the researchers determined the mass of the Galaxy by inferring the amount of gravity required to keep the stars in orbit. Some of that gravity comes from the Milky Way stars themselves, but most of it comes the distribution of invisible dark matter, which is still not fully understood.

The most recent previous studies of the mass of the Milky Way used mixed samples of 50 to 500 objects. They implied masses up to two-trillion times the mass of the Sun for the total mass of the Galaxy. By contrast, when the SDSS-II measurement within 180,000 light years is corrected to a total-mass measurement, it yields a value slightly under one-trillion times the mass of the Sun.

"The enormous size of SEGUE gives us a huge statistical advantage," said Hans-Walter Rix, director of the Max Planck Institute for Astronomy. "We can select a uniform set of tracers, and the large sample of stars allows us to calibrate our method against realistic computer simulations of the Galaxy." Another collaborator, Timothy Beers of Michigan State University, explained, "The total mass of the Galaxy is hard to measure because we're stuck in the middle of it. But it is the single most fundamental number we have to know if we want to understand how the Milky Way formed or to compare it to distant galaxies that we see from the outside."

All SDSS-II observations are made from the 2.5-meter telescope at Apache Point Observatory in New Mexico. The telescope uses a mosaic digital camera to image large areas of sky and spectrographs fed by 640 optical fibers to measure light from individual stars, galaxies, and quasars. SEGUE's stellar spectra turn flat sky maps into multi-dimensional views of the Milky Way, Beers said, by providing distances, velocities, and chemical compositions of hundreds of thousands of stars.