A hundred billion stars here, a hundred billion stars there, pretty soon it adds up to some serious mass. Estimates of just how much mass the Milky Way contains have varied pretty significantly in the past, but one lone star may push up the upper bound for our galaxies weight.
Astronomers in Germany found the fastest moving star ever seen in our galactic halo and say that in order for the Milky Way to hold on to it, the galaxy must have a higher mass than previously thought.
Previous studies done by an international team of astronomers on 2400 stars in the galactic halo indicated the mass should be about 1 trillion times the mass of our sun. But the German team says this one star could throw out the earlier numbers. “It was immediately clear that this star must be something special and interesting,” said team astronomer Norbert Przybilla in an interview with Physics World. 
The astronomers say the Milky Way must have a mass at least 2 trillion times that of the sun. Otherwise, the star would have been flung from our galaxy, and it appears to have originated from the dark halo it’s currently in. According to the group, this number confirms what some other astronomers have found.
The star, which is flying through Serpens, is headed towards us at more than 400 miles per second (nobody panic!). That’s three times faster than the sun moves and the only stars known to move faster have been catapulted by passing too close to the supermassive black hole lurking at the center of the Milky Way.
In an interview with Physics World, the group said they hope an ESA spacecraft to be launched in 2012 will help clear things up. The spacecraft, called Gaia, will be capable of making incredibly precise distance and motion measuments for billions of stars.
Check out a preprint on arXiv