Fastest star to flee our Galaxy discovered

Scientists using powerful ground-based telescopes said Thursday they have discovered a star that's blasting out of the Milky Way with a speed much faster than any other runaway star in the Galaxy.

Unlike other known unbound stars, the star known as US 708, moving at about 1,200 kilometers per second, was ejected from an extremely tight binary by a supernova explosion, they reported in the U.S. journal Science.

So far, only a few so-called hypervelocity stars are known to travel with speeds so high that they will escape its gravity to wander intergalactic space.

Normally, the supermassive black hole at the center of the galaxy acts as the slingshot that gets hypervelocity stars moving so fast.

But US 708 is a different story, Stephan Geier of the European Southern Observatory and his colleagues concluded after reconstructing this hypervelocity star's trajectory using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii.

"It is the fastest unbound star in our Galaxy," they wrote in their paper. "In reconstructing its trajectory, the Galactic center becomes very unlikely as an origin, which is hardly consistent with the most favored ejection mechanism for the other HVSs (Hypervelocity stars)."

In addition to its extreme speed, US 708 has another peculiar property in marked contrast to other hypervelocity stars: it is a rapidly rotating, compact helium star, the researchers said.

Thus, US 708 could have originally resided in an ultracompact binary system, transferring helium to a massive white dwarf companion which ultimately grows big enough to trigger a special kind of stellar explosion called a Type Ia supernova.

The surviving companion, US 708, was spun up considerably in the process, eventually ejected from the disrupted binary, they said. Endite