Runaway star bound for disaster
By Amanda Jermyn

Once upon a time, about 4.8 million years ago, a star with the romantic name of S5-HSV1 lived with a smaller companion star in a binary system near Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. But one day, these stars strayed too close to the black hole, and in the ensuing gravitational battle, the smaller companion star was captured by the monster black hole, while S5-HSV1 was hurled away at extremely high speed.

We know all this because an international team of astronomers led by Ting Li of the Carnegie Observatories recently discovered S5-HSV1 whizzing away from the center of our galaxy at the dazzling speed of almost four million miles an hour. The speeding star is currently located in the Grus constellation about 29,000 light-years from Earth. The discovery was made with the Anglo-Australian Telescope as part of the Southern Stellar Stream Spectroscopic Survey (S5). Using data from the European Space Agency’s Gaia satellite, which has identified the positions and movements of 1.3 billion stars in the Milky Way, the researchers were able to trace the star’s movements back to the center of the galaxy.

Back in 1988 astronomer Jack Hills proposed the three body interaction in which one star in a binary system (two stars orbiting each other) is captured by a black hole’s gravity and the other is ejected, but this is the first time the mechanism has been observed. According to Sergey Koposov of Carnegie Mellon University, first author on this study, “This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never had an unambiguous association of such a fast star with the Galactic center.”

S5-HSV1 is a main sequence star, meaning it is still undergoing hydrogen fusion in its core, and it’s still relatively young, only about 500 million years old. It’s about 2.35 times the mass of the Sun, and shines quite brightly. It is a hypervelocity star, meaning that it moves more than twice as fast as most other stars. In fact, it’s the fastest main sequence star ever found in the Milky Way. According to Douglas Boubert, an astronomer at Oxford University and a member of the research team, “S5-HVS1’s velocity is so high that it will inevitably leave the Galaxy and never return.” Astronomers estimate that this will happen in about 100 million years. This star will keep on racing through the universe, far from its origins in the Milky Way, until, about a billion years from now, it finally blows up and dies. Sadly, not all fairy tales have happy endings.