Fastest Unbound Star Ever Found In The Milky Way

Almost all stars in a galaxy, such as our own Milky Way, are gravitationally bound to it, circling the center in a tidy fashion. Astrophysicists have discovered a few stars that buck this fashion, seeming to have velocities that will ultimately take them out of the galaxy all together. Now a group of astrophysicists has identified the Milky Way’s fastest unbound star yet discovered—traveling at a blazing 1200 kilometers per second (2.7 million miles per hour)—and it’s not coming from the typical source of such escapee stars. Scientists have long believed that these unbound stars come from the galactic center. 

When a pair of binary stars gets too near to the supermassive black hole at the heart of the Milky Way, the strong gravitational field tears the pair apart, pulling one star inward and throwing the other out toward interstellar 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 discovered that the star could not have come from the galactic center. Also unlike other unbound stars, US 708 is a dense, briskly spinning star rich in helium, the team reports online today in Science. 

This proposes that US 708 was once paired up in a close binary with a white dwarf, the burnt-out leftovers of an old star. In such a condition, 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 sufficient to ignite fusion inside it and it is demolished in a violent detonation known as a type Ia supernova. The team thoughts that the explosion of a white dwarf partner pushed US 708 on its intergalactic escape route (as shown in the simulation Below).


Unbound Stars

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