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A massive star in a nearby galaxy is on the run. It’s moving faster than any other known star — fast enough to escape its home galaxy.
The star is a member of M31, the Andromeda galaxy. Under dark skies, the galaxy is visible to the unaided eye as a hazy smudge of light. Right now, it’s about half-way up the eastern sky as night falls. At a distance of two-and-a-half million light-years, it’s the farthest object that’s easily visible to the eye alone.
Astronomers discovered the runaway while looking for red supergiants in M31. These stars are much bigger, heavier, and brighter than the Sun. They’re also nearing the ends of their lives, which is one reason they’re so puffed up.
One of the supergiants is moving in our direction far faster than any other star in the survey. The astronomers estimated that its overall speed relative to the rest of M31 is about 650,000 miles per hour.
The star probably was born with a cluster of other massive stars. Gravitational interactions among the stars may have kicked this one out of the cluster. The kick was so powerful that the star may be moving fast enough to escape from M31.
Or it would escape if it lived long enough. The star is expected to explode as a supernova in the next million years or so — long before it can escape the galaxy. But the blast probably will leave a dead, heavy core. It will continue to race away from M31 — and may eventually escape into intergalactic space.