Most stars in a galaxy, such as our own Milky Way, are gravitationally bound to it, orbiting the center in an orderly fashion. Astronomers have found a few stars that buck this trend, seeming to have velocities that will eventually take them out of the galaxy all together. Now, a team of astronomers has identified the Milky Way’s fastest unbound star yet found—traveling at a blistering 1200 kilometers per second (2.7 million miles per hour)—and it’s not coming from the usual source of such escapee stars. Researchers have long thought that these unbound stars come from the galactic center. When a pair of binary stars gets too close to the supermassive black hole at the heart of the Milky Way, the intense gravitational field tears the pair apart, dragging one star inward and flinging the other out toward intergalactic space. The team looking at the unbound star US 708 used the W. M. Keck telescope in Hawaii as well as new and archived observations of it from the Pan-STARRS1 survey telescope in Hawaii to work out its true velocity. As well as documenting its huge speed, they found that the star could not have originated from the galactic center. Also unlike other unbound stars, US 708 is a compact, rapidly spinning star rich in helium, the team reports online today in Science. This suggests that US 708 was once paired up in a close binary with a white dwarf, the burnt-out remains of an old star. In such a situation, the white dwarf’s gravity sucks material from its companion (its hydrogen outer layers in the case of US 708) until the dwarf grows big enough to ignite fusion inside it and it is destroyed in a violent explosion known as a type Ia supernova. The team believes that the explosion of a white dwarf partner propelled US 708 on its intergalactic escape route (as depicted in the simulation above).
Video credit: NASA, ESA and P. Ruiz Lapuente (University of Barcelona); Cut and colored by S. Geier