In 2005, astronomers discovered a star unlike any other in the Milky Way. Most of the billions of stars in the galaxy, including our sun, travel at an average speed of about 800,000 kilometers, or 500,000 miles, per hour. But this star? It was moving three times faster than that, hurting across space at 2.5 million kilometers, or 1.6 million miles, per hour.
Since then, astronomers have discovered about 20 of these fast-moving stars, known as hypervelocity stars. They’ve also been trying to pin down their origin. The leading theory comes from a 1988 paper that predicted the existence of hypervelocity stars, years before the first one was observed. It suggests a hypervelocity star starts out as one-half of a binary star system that gets too close to the supermassive black hole at the Milky Way’s center. The black hole captures one star, pulling it into a tight orbit, and pushes away the other, slingshotting it across the galaxy at enormous speeds.
But Douglas Boubert, a Ph.D. student at the Institute of Astronomy at Cambridge, isn’t sold on this theory. “It became the best explanation because it was the only explanation for these stars,” Boubert says.
Boubert and his colleagues offer another explanation, published Tuesday in the Monthly Notices of the Royal Astronomy Society, based on telescope observations and computer simulations. The Milky Way’s hypervelocity stars, he says, don’t originate in the Milky Way at all. They are, instead, runaways from a smaller galaxy orbiting our own called the Large Magellanic Cloud. Each hypervelocity star was part of a binary system in this dwarf galaxy until its twin exploded. The force of the supernova ejected the surviving star at such great speeds that it was able to escape the gravitational pull of the dwarf galaxy and eventually find a new home in ours.