What would fling a star out of the galaxy at over 1 million miles per hour? Warren Brown (Harvard-Smithsonian Center for Astrophysics) and colleagues have some thoughts on that, based on their own and other studies in Europe that have so far identified five stellar exiles, a group now called 'hypervelocity stars.' "These stars literally are castaways," says Brown. "They have been thrown out of their home galaxy and set adrift in an ocean of intergalactic space." Brown's team went after galactic escapees in a targeted manner, using computer models that showed such stars would be forced into their current trajectories by interactions in the galactic core. The idea is this: a binary star swings too close to the black hole at the galaxy's center. Its gravity tears the duo apart, in the process capturing one of the stars and ejecting the other one at high velocity. Evidence exists not only in the exiled stars themselves but in the other half of the binary pairs that once contained them;...

Catching up on news from the recently concluded American Astronomical Society meeting, I want to be sure to mention RAVE -- the Radial Velocity Experiment -- which is an all-sky spectroscopic survey of as many as a million stars passing near the Sun. Observations of stars in other galaxies show that only dark matter can explain their movement since there is not enough visible mass to keep them gravitationally bound. Early results from RAVE confirm that dark matter dominates the mass of the Milky Way as well. The survey will help to firm up our picture of the Milky Way's own dark matter and, by extension, its mass. "One important early application of RAVE aims to measure just how much stuff there is in our Milky Way galaxy — the collection of stars, gas and dark matter that is the home of our sun," said Rosemary Wyse (Johns Hopkins). "Newton's Law of Gravity allows us to figure out from the orbital motions of stars how much mass is holding them together. Faster motions need more...

If you go back 35 years, what we now call 'dark matter' was even more controversial than it is today. What was the stuff, if it really existed, and should we consider it a dim version of what we see around us elsewhere in the galaxy? Or was it something truly exotic, not yet explicable but traceable in the effects it had on galactic evolution? Whatever it was, we seemed to need it to explain a key observation: stars in the outer regions of the galaxy move faster than they ought to based on the observed mass of stars in the Milky Way. Today we still don't know what dark matter is, but an intriguing new study again reminds us of its extensive influence. As reported at the ongoing meeting of the American Astronomical Society, the Magellanic Clouds -- a pair of galaxies that orbit the Milky Way -- seem to be interacting with the galaxy's dark matter, creating a previously observed warp in the galactic disk. Using a new map of hydrogen gas emissions, researchers have found that the motion...