Fast-moving star duo is heading out of the Milky Way

Something catapulted a pair of stars from the outer rim of our galaxy, but astronomers aren’t sure what. A binary star known as PB 3877 is rocketing away at about 2 million kilometers per hour — possibly fast enough to escape the galaxy’s gravitational pull — and all the usual explanations for such speedy stars fall short. Astrophysicist Péter Németh of the University of Erlangen-Nuremberg in Germany and colleagues report the discovery in the April 10 Astrophysical Journal Letters.

Many galactic escapees get kicked out after a close brush with the supermassive black hole in the Milky Way’s center. But PB 3877, first noticed in 2011 and currently about 18,000 light-years away in the constellation Coma Berenices, has been nowhere near that behemoth. A supernova could be responsible; it has happened before (SN Online: 3/5/15). But PB 3877 is two stars traveling together. A supernova would have torn the two apart. Németh and colleagues propose that the duo may be left over from a smashup between the Milky Way and a smaller galaxy. If that’s the case, then there might be others like PB 3877 lurking in the galactic outskirts.