Hypervelocity Star Flung Out of Milky Way May Have Unusual Origin

Stars are heavy. Heavy things require more energy to move. Therefore, accelerating solar masses to roughly 568km/s (or 353 miles/s) requires a staggering amount of it. Scientists have typically believed that the handful of hypervelocity stars we’ve found — the term refers to stars moving significantly faster than the typical ‘background’ rate for stars in the Milky Way galaxy — were flung out towards the void between the galaxies by Sagittarius A*, the supermassive black hole at the center of our galaxy. With a diameter of an estimated 60 million kilometers across, Sagittarius A* is really good at throwing things (gravitationally speaking).

But in at least one star’s case, Sagittarius A* doesn’t appear to be the culprit. LAMOST-HVS-1, a star with 8.3 solar masses, was flung out of the inner disk of the Milky Way some 33 million years ago, with a velocity between 551-587km/s. That’s far faster than most stars — the Sun, for example, moves at a velocity of ~220km/s, or less than half that of LAMOST-HVS-1. There are two ways a star is thought to gain this kind of velocity: Either it was part of a binary pair and survived the supernova of its partner, or it takes part in complex gravitational interactions between multiple bodies as part of a star cluster.

Supernova ejection is simpler — at least, to the extent that anything involving the exploding mass of a star can be described as simple. The gravitational ejection model allows for the presence of multiple stars or various configurations of stars and black holes. What’s interesting about LAMOST-HVS-1 is that while we thought this star had been flung from Sagittarius A*, it actually appears to come from the Norma spiral arm of the Milky Way. But there are no known massive clusters of stars in this area that would actually produce the high velocities observed.

And the story gets stranger. LAMOST-HVS-1 isn’t just a hypervelocity star, it’s a huge hypervelocity star that’s no longer bound to the Milky Way at all. With an estimated mass roughly eight times higher than the Sun, it would’ve taken an incredible amount of energy to throw the star from the Norma spiral arm (it appears to have originated from an area near the galactic center, though still distinct from it).

“This discovery dramatically changes our view on the origin of fast-moving stars,” study co-author Monica Valluri, an astronomy professor at the University of Michigan, said in a statement. “The fact that the trajectory of this massive, fast-moving star originates in the disk rather than at the galactic center indicates that the very extreme environments needed to eject fast-moving stars can arise in places other than around supermassive black holes.”

The team doesn’t have a model yet for exactly how the gravitational interactions in LAMOST-HVS-1’s original backyard could have flung it into the galactic void in the first place, but they’re hoping future research will gather more data on the area where the star came from. It’s a sign that the gravitational interactions around stars can produce unusual effects we once assumed could only be caused by supermassive black holes.

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