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A ‘Speedy Little Star’ May Be on Course to Escape Our Galaxy
The so-called hypervelocity object, which is either a low-mass star or a brown dwarf, is traveling through the Milky Way at around a million miles an hour.
In his spare time, Tom Bickle, an astronomy student in Southampton, England, likes to blast heavy metal while combing through time-lapses of the night sky, hunting for traces of a hypothesized ninth planet and other hidden objects lurking in the outskirts of our solar system.
It was on one such occasion that he stumbled across something strange: a faint blob moving across his computer screen.
“I knew immediately that it was unusual,” Mr. Bickle said.
Professional astronomers followed up on the observation. The object is either a low-mass star or an object known as a brown dwarf, and it is hurtling through space at a million miles per hour. At that speed, it could be traveling fast enough to break free from the gravitational clutches of the Milky Way.
“It was right when that number came out that we realized we had something spectacular,” said Adam Burgasser, a physicist at the University of California, San Diego who led a study of the observation published this month in The Astrophysical Journal Letters. “We were very excited.”
The discovery has the potential to shed light on the oldest — and some of the fastest — stars in our galaxy, known as halo stars.
“One of the ways we find old stars is that we know they’re moving in very strange orbits,” Dr. Burgasser said. Most stars in the vicinity of our sun orbit around the disk of the Milky Way in a circle. But halo stars often have trajectories that are ovular, or tilted away from the galactic plane.
That’s because they most likely formed before the Milky Way settled into its current structure, Dr. Burgasser said. “The fast speeds of halo stars are really a signature of their different origins,” he added.
More than a dozen “hypervelocity” stars — which zip across the galaxy at more than 900,000 miles per hour, twice the speed of our sun — have been discovered so far. But all of them are close to, or greater than, the sun’s mass.
By contrast, the newly found object, cataloged as CWISE J1249+3621 by astronomers, is only 8 percent of the sun’s mass. That is right at the classification boundary between a star and a brown dwarf (also known as a “failed star,” because it lacks enough mass to fuse hydrogen).
According to Dr. Burgasser, it’s not clear that objects with such low mass could have formed in the early history of the Milky Way. And its extreme speed suggests that it might have an unusual origin.
One alternative theory for how CWISE J1249+3621 got its kick is that it was once locked in orbit around a white dwarf, or the leftover core of a star that exploded. Impact from such a supernova could have accelerated a companion to such high speeds.
Another possibility is that the object was part a star cluster that encountered a pair of black holes, an interaction that violently spit the object out of its system.
Three amateur observers, including Mr. Bickle, are credited with finding CWISE J1249+3621 as part of a project called Backyard Worlds: Planet 9. Participants search for moving sources in images taken by NASA’s Wide-field Infrared Survey Explorer and its extended mission, which shut off in July.
“You’d think you could write a software package to do this,” Dr. Burgasser said. But the human eye, he added, “is much better and much faster at finding these faint little moving stars than any algorithm we’ve tried.”
The researchers used data from existing sky surveys and additional observations with the Keck II telescope in Hawaii to confirm the speed of the newly discovered object. But more information about its chemistry will be needed to solve the mystery of where it came from. The chemical makeup of the oldest objects in the galaxy, for example, should resemble the composition present in the early Milky Way; an object blasted away by a supernova, on the other hand, will be rich in nickel.
Dr. Burgasser isn’t worried about catching the object before it dashes off into intergalactic space. One million miles an hour may seem like a ludicrous speed to us on Earth, but at that rate it travels only 1.5 light-years per millennium.
“Space is big,” he said. “We can afford to take our time.”
Katrina Miller is a science reporter for The Times based in Chicago. She earned a Ph.D. in physics from the University of Chicago. More about Katrina Miller
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