The Fastest Unbound Stars in the Universe
A schematic showing a late stage in the merger of two supermassive black holes and the consequent ejection of some of the orbiting stars. These stars, which could be numerous and detectable, might reach velocities that are a significant fraction of the speed of light, and could traverse large portions of the universe in their lifetimes.
Stars do not stand still. They move through the heavens with motions determined by their encounters with other stars and stellar systems. Stellar velocities in the Milky Way are typically a few hundred thousand miles per hour. However, some peculiar stars do move at much greater velocities, in particular the so-called hypervelocity stars, the fastest known case of which moves at about one and one-half million miles per hour. These stars are thought to have been ejected from the vicinity of the supermassive black hole at the center of our galaxy during a close passage. Indeed, the existence of such rapidly moving stars strengthens the evidence for the existence of a massive black hole at the galactic nucleus.
The speeds of hypervelocity stars are fast, but they are thousands of times slower than the speed light, and relativistic effects can be neglected for them. CfA astronomers James Guillochon and Avi Loeb have found a mechanism that can produce even higher stellar velocities. If one of two orbiting stars breaks up or explodes as a supernova, the other star will be ejected with a velocity that is limited to its previous maximum orbital velocity. But if there is a third massive star in the system, the ejected star's velocity can be enhanced. The astronomers show that when a pair of orbiting, supermassive black holes in a galactic center merge, any tightly bound stars orbiting them can be liberated in this enhanced way, and flung out at speeds of as much as thirty percent of the speed of light.
The scientists demonstrate that there could be many of these relativistic, hypervelocity stars in the cosmos. Moreover, they show that their mechanism is the only possible method to accelerate significant numbers of stars to these speeds, so that if any are detected a merger event would be established. Not least, the astronomers note that these semi-relativistic hypervelocity stars can cross large swaths of the observable universe in their lifetime, and hence can serve as new cosmological messengers that should be detectable with a new generation of telescopes.
"The Fastest Unbound Stars in the Universe," James Guillochon and Abraham Loeb, ApJ 806, 124, 2015.