Image showing PB3877's location on the outskirts of the Galaxy. Credit: Thorsten Brand

An unknown hypervelocity binary star that could be an intruder to the Milky Way has been discovered, marking the first wide binary star to be observed travelling at such a high speed.

PB3877 was discovered using Sloan Digital Sky-Survey data in 2011.

Follow-up observations using the Keck II telescope revealed the nature of the binary system.

The star is travelling at nearly the escape velocity of the Galaxy, meaning the speed that is required to break free of the Milky Way’s gravitational restrains.

The surface of the hotter of the two stars is over fives times hotter than the Sun, while its companion is a thousand degrees cooler than the Sun.

The binary pair is 18,000 lightyears away, on the outskirts of the Galaxy.

About two dozen hypervelocity stars are known, but this is the first wide binary observed travelling at such a high speed.

The study challenges the common theory that hypervelocity stars reach such high speeds by being ejected by the supermassive black holes at the centres of galaxies.

“From our calculations we can exclude the Galactic centre as the place of origin, because its trajectory never came close to it,” says team member Eva Ziegerer.

“Other ejection mechanisms, such as stellar collisions and a supernova explosion have been proposed, but all of them would lead to the disruption of a wide binary.”

“We studied hypervelocity stars since 2005, the year of discovery of the first three,” says Ulrich Heber, a member of the team at the Friedrich Alexander University that made the study.

“In the meantime about two dozen have been found, but all are single, none has a companion directly visible in its spectrum.”

One of the theories put forward by the team is that PB3877 could be an intruder from another galaxy.

“In that case its prolonged gradual acceleration would not harm its integrity," says study leader Péter Németh.

"The outskirts of our Galaxy contain various stellar streams that are believed to be the remnants of dwarf galaxies that were torn to shreds by the strong tidal force of the Milky Way.”

The origin of the binary star remains a mystery, as does its fate.

The future of the star could rest on how much dark matter is in the Milky Way, and so the binary could be an important piece in solving the mystery behind dark matter.

Meanwhile, the team intends to continue observations to decipher PB3877’s origins.

“By finding further stars or binaries on similar orbits would indicate an external origin.

Therefore, our quest for similar strangers will continue,” says Németh.