In a nearby galaxy, huge stars 100 times bigger than the Sun and a million times brighter live out their lonely existences with no obvious explanation for how they came to be. Now we know: they're running away from home.
Located 160,000 light-years away in the Large Magellanic Cloud, the third nearest galaxy to the Milky Way, these massive stars represented a cosmic mystery for astronomers. These stars, relatively young at only two million years old and occupied for most of their existence in furiously ejecting about half their mass as solar wind, would have required huge concentrations of stellar material to form in the first place. This couldn't have happened in their current location, which is an extremely barren area of their galaxy.
Vasilii Gvaramadze of the Sternberg Astronomical Institute at Moscow State University now has an answer, at least for one of these young massive stars. He's determined the star BI 237 is careening through the Large Magellanic Cloud at eighty miles per second on a more or less straight trajectory. That means it's possible to trace its path back to its birthplace, which turns out is precisely the sort of place astrophysicists would expect: the LH 82 star cluster.
Astronomers have long considered the possibility that these huge stars formed elsewhere and were then somehow sent off on a runaway course - something we've observed in our own galaxy - but the Large Magellanic Cloud was thought to be too far away for the velocity of the stars to be measured. Gvaramadze found a novel way around this problem, using a known characteristic of runaway stars in the Milky Way to identify similar ones in the distant galaxy.
Specifically, such stars produce bow shocks, which are parabolic formations created by the mixture of stellar wind emanating from the star with the interstellar gas up ahead. He was able to find the infrared signature of just such a bow shock on star BI 237, confirming its runaway status and providing a direction home to its origin point.
As star clusters form, the most gigantic stars tend to concentrate together in the center of the menagerie. When such huge stars are brought in close proximity, they begin orbiting each other in extremely eccentric ways, and it's possible for some of the stars to pick up enough additional speed to break free of the mutual orbits and shoot straight out of the cluster. This is likely what happened with BI 237. The hope is that further analysis of other isolated massive stars will reveal similar bow shocks, providing a definitive answer to one of astrophysics's most puzzling riddles.