New measurements reveal slimmer Milky Way
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 28, 2008

A more accurate measuring scale has refined the weight of the Milky Way by a factor of one trillion, a discovery that has broad implications for our understanding of the Milky Way.

The discovery is based on SEGUE (Sloan Extension for Galactic Understanding and Exploration), an enormous survey of stars in the Milky Way and one of three programs that comprise SDSS-II (Sloan Digital Sky Survey). Using SEGUE measurements of stellar velocities in the outer Milky Way, a region known as the stellar halo, the researchers determined a revised mass of the Galaxy, of just under one trillion times the mass of the Sun, 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.

Astronomers used the motions of 2,400 distant stars out to 180,000 light years from the galactic centre to measure the mass of the Milky Way Galaxy. These stars reside in the galactic halo that surrounds the main visible element of the galaxy. Image: SDSS Collaboration/Axel Quetz/Max Planck Institute for Astrophysics, Heidelberg.

"The galaxy is slimmer than we thought," said Xiangxiang Xue of the National Astronomical Observatories of China. "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.”

To trace the mass distribution of the Galaxy, the SEGUE team used a carefully constructed sample of 2,400 "blue horizontal branch" stars – stars that have evolved past the red giant stage and which now burn helium in their cores – whose distances and velocities can be determined from their measured brightness out to 180,000 light years away from the Sun.

The most recent previous estimate of the mass of the Milky Way – up to two trillion times the mass of the Sun – was based on mixed samples of 50 to 500 objects. By contrast, the SDSS-II measurement yields a value slightly under one trillion times the mass of the Sun.

"The enormous size of SEGUE gives us a huge statistical advantage," says Hans-Walter Rix, director of Max Planck Institute for Astronomy. "The large sample of stars allows us to calibrate our method against realistic computer simulations of the Galaxy."

The total mass of the Galaxy is hard for astronomers to measure because we are stuck in the middle of it, but it is one of the single most important numbers that we have to know to understand how the Milky Way formed, and to compare it to distant galaxies that we see from the outside.