How a black hole can kick stars out of the galaxy

An utterly ridiculous artist’s impression of a hypervelocity star getting kicked out of the Milky Way. [Credit: Ben Bromley, University of Utah]

Most stars in the Milky Way orbit the galactic center in sedate paths, but not hypervelocity stars. These rare objects are speeding out of the galaxy, thanks to a close encounter with the Milky Way’s central black hole. Not only that, hypervelocity stars might even be able to tell us something about the dark matter halo: the shroud of invisible mass that surrounds the bright star-crowded part of the galaxy. My latest column in The Daily Beast talks about the latest hypervelocity star discovery.

So why are hypervelocity stars so fast? Many stars are in binary systems, locked in mutual orbit with another star. If a binary strays too close to the galaxy’s massive central black hole, the complex gravitational interaction between the two stars and the black hole can split the system apart. In that case, one star ends up in a closer orbit, while the other gets a big boost of speed—maybe even fast enough to escape from the galaxy. [Read more…]

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