Speedy Star 
Our galactic home, the Milky Way, is a big galaxy. It's far bigger than most other galaxies, but no one knows exactly how big. In part, that's because most of the galaxy's mass consists of dark matter, which produces no detectable energy at all. But German astronomers recently used just one speedy star to estimate the mass of the entire galaxy.
The star is about 39,000 light-years away, in the constellation Serpens, the serpent. It's in the galaxy's "halo" -- a population of ancient stars whose orbits carry them above and below the Milky Way's disk. What makes the star special is its extraordinary velocity. It's moving faster than any other halo star yet discovered: about 1.6 million miles per hour.
Despite its great speed, astronomers think the star is gravitationally bound to the Milky Way, because the star is heading toward us. To hold on to such a fast star, the Milky Way's total mass -- both visible and invisible -- must be at least 1.8 trillion times the mass of the Sun. That's twice as massive as the estimates from some other studies.
At present, though, the star's high speed is a bit uncertain. But next year, the European Space Agency will launch a spacecraft called Gaia. It'll measure precise motions and distances for a billion stars, including the high-flying one in Serpens. That should provide a better measurement of the star's speed, giving us a better estimate of just how massive our galaxy really is.
Script by Ken Croswell, Copyright 2010