Artistic impression of PB3877's position with respect to the Sun and the Milky Way. Thorsten Brand

Stars might form in galaxies but not all of them stay there. We know of about two dozen that have been able so far to escape the clutches of the Milky Way. They are known as hypervelocity stars, and now astronomers have observed the most unusual yet.

A team of astronomers from the Friedrich-Alexander University in Germany have detected the first hypervelocity binary star. The system, called PB3877 (or J1211), has a hot compact star and a cooler companion. They are both traveling with a velocity of 571 kilometers (354 miles) per second, and they are located 18,000 light-years from Earth.

PB3877 had already been reported as a hypervelocity star, but deeper observations performed with the Keck telescope on Mauna Kea in Hawaii showed that it was not alone on its high-speed trip.

“When we looked at the new data, much to our surprise, we found weak absorption lines that could not come from the hot star,” Thomas Kupfer, co-author of the paper, said in a statement.

“The cool companion, just like the hot primary, shows a high radial velocity. Hence, the two stars form a binary system, which is the first hypervelocity wide binary candidate.”

The binary nature of the system is very important. Hypervelocity stars are believed to have gained their velocity either thanks to a kick from a powerful supernova or by a close encounter with a supermassive black hole. These explanations can’t be applied to this hypervelocity binary, as such a strong gravitational interaction would separate the stars.

“PB3877 may be an intruder from another galaxy,” lead author Péter Németh suggests in the paper published in the Astrophysical Journal Letters.

“In that case, its prolonged gradual acceleration would not harm its integrity. 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 discovery of this system also has theoretical consequences. The past and the future of this system depends on the amount of dark matter present in the Milky Way, so by obtaining the precise orbital properties of PB3877 astronomers could test different dark matter models.

Follow-up observations will continue to investigate the mysterious origin and curious properties of the system. “By finding further stars or binaries on similar orbits would indicate an external origin. Therefore, our quest for similar strangers will continue,” concluded Németh.