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The fastest star in the galaxy propelled by thermonuclear supernova

A speeding star is the fastest such object in the Milky Way -- and researchers believe that a supernova sent it on its way.

Artist's impression of a star flung out of a galaxy by a supernova. In reality, the supernova would have long since subsided. ESA/Hubble

Hurtling through space at a speed of 1,200 kilometres per second (about 2.7 million mph), star US 708 is the fastest star in the Milky Way. Its speed is so fast that it is classified as an "unbound" or hypervelocity star -- so fast that its speed exceeds the escape velocity of gravity, and fast enough to one day leave the galaxy.

"At that speed, you could travel from Earth to the moon in 5 minutes," said University of Hawaii at Manoa astronomer Eugene Magnier, who participated in the research, which was led by Stephan Geier of the European Southern Observatory.

US 708 isn't just the fastest hypervelocity star to date -- it also has a few other features that mark it as unusual. Unlike the half-dozen or so other HVSs identified to date, it is a rapidly rotating, compact helium star -- a star that is the remnant of a formerly massive star that has lost its hydrogen envelope.

This is characteristic of a star that has interacted with a close companion -- a binary star.

Usually, the extreme speeds of HVSs is thought to be the result of a close encounter with the black hole at the centre of the Milky Way -- yet US 708's trajectory makes this very unlikely. Instead, the team believes that the star's origin in a binary system holds the answer.

Before one star in a binary system goes supernova -- the white dwarf of the pair -- it sucks material from its companion star -- hence US 708's missing hydrogen. This allows the star to accumulate mass and explode in a spectacular display -- what is known as a Type Ia supernova. According to theoretical models, this could blast the companion star away at high speeds -- although this has never been observed directly.

The existence of US 708, however, indicates that there may be some truth to this model.

"Several types of stars have been suspected of causing the explosion of a white dwarf as supernova of type Ia," Geier said. "Until now, none of them could be confirmed. Now we have found a delinquent on the run bearing traces from the crime scene."

According to the paper, this research provides not just evidence that the fastest HVSs are accelerated via supernova; it's also an important step forward in understanding Type Ia supernovae in general.

The full study, "The fastest unbound star in our Galaxy ejected by a thermonuclear supernova," can be found online in the journal Science.