The Energizer bunny has nothing on HE 0437-5439. This massive blue star should have burnt out tens of millions of years ago, but it's still going strong. The mystery has puzzled astronomers ever since they discovered the big star, which has zoomed well beyond the boundaries of the Milky Way, in 2005. They knew that the black hole at the center of our galaxy must have ejected it; otherwise it could never have traveled so far so fast. But because the journey took 100 million years, and hot blue stars only live about a third that long, the star should have gone kaput long ago. The explanation, astronomers report  this week in The Astrophysical Journal Letters, is that HE 0437-5439 wasn't always a blue star. According to their calculations, it actually started out as a couple of run-of-the-mill yellow stars like our sun, which can live for billions of years. But millions of years after the black hole ejected them, the two stars merged. So HE 0437-5439 has trekked most of its 200,000 light-years as a youthful pair of suns—but now, as an aged blue giant, it's quickly dying.
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