All stars are in motion, but some have a little more oomph than others. In recent years astronomers have identified a handful of stars that are moving so fast that they will someday flee the galaxy altogether.
On their way out, these escapees may tell us a thing or two about the nature of dark matter, the mysterious stuff that makes up nearly 85 percent of all matter in the universe. “No one knows what dark matter is,” says astrophysicist Warren Brown of the Smithsonian Astrophysical Observatory. Yet astronomers know it exerts a gravitational pull, which bends the paths of objects traversing the galaxy. Outbound stars thus act as tracers to reveal where dark matter is concentrated, and physicists can use that information to test competing theories of how it behaves.
The catch is that no one is quite sure what paths these stars are on. To use them as dark matter probes, astronomers need to know the stars' full trajectories, starting from the spot where they shot outward with such force. One possibility: the stellar speedsters were ejected by supernova explosions in the galaxy. Or perhaps they drew too close to the supermassive black hole at the Milky Way's center, which flung them out like stones in a slingshot. If that were the case, each star would trace back to a known point. “If they really come from the galactic center, then that's very valuable,” says University of Maryland astrophysicist Michael Boylan-Kolchin.
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But how to tell where a star came from? Brown and his colleagues say that it boils down to a star's age, speed and position. Through a careful analysis of the spectrum of light from the newfound star HVS17 (short for hypervelocity star 17), the team determined it was 153 million years old and traveling about one million miles an hour through the outer Milky Way. A supernova-launched star would be much younger, having been ejected from its stellar cohort soon after forming by the detonation of a short-lived neighbor. The star's advanced age, then, argues for an origin near the galactic center, according to a recent study the researchers published in the Astrophysical Journal. New data from the European Gaia satellite should conclusively settle the star's origin and cast a sliver of light on dark matter.