Our sun lies about 25,000 light years from the center of the Milky Way galaxy, roughly halfway out in the galactic disk. The new mass determination is based on the measured motions of 2,400 “blue horizontal branch” stars in the extended stellar halo that surrounds the disk. These measurements reach distances of nearly 200,000 light years from the galactic center, roughly the edge of the region illustrated above. The visible, stellar part of our Milky Way in the middle is embedded into its much more massive and more extended dark matter halo, indicated in dim red. The 'blue horizontal branch stars' that were found and measured in the SDSS-II study, are orbiting our Milky Way at large distances. From their speeds the researchers were able to estimate much better the mass of the Milky Way's dark matter halo, and found it to be much “slimmer” than thought before. Image courtesy of SDSS Collaboration, Axel Quetz, Max Planck Institute for Astrophysics, Heidelberg

The Milky Way galaxy has lost weight. About a trillion suns’ worth.

A more accurate scale rather than a galactic diet accounted for the recent slimming. This weighty discovery from the Sloan Digital Sky Survey (SDSS-II ) has broad implications for our understanding of the Milky Way.

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

The discovery, accepted for publication in The Astrophysical Journal, is based on data from SEGUE, an enormous survey of stars in the Milky Way—one of the three programs that comprise SDSS-II. 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 from an extended distribution of invisible dark matter, whose nature is still not fully understood.

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SDSS used between 50 and 100 nodes of OSG ’s FermiGrid computing cluster simultaneously to analyze the spectra which resulted in the paper for which this artwork was produced. The spectroscopic processing required approximately 32,000 CPU hours, and spanned two months calendar time.