When we’re talking about moving through space, the velocities get pretty high. The Earth itself is hurtling through the void at around 30 kilometers every second. But a newly discovered star called US 708 takes the cake. It’s traveling at an incredible 1,200 kilometers per second (2.7 million miles per hour).
Stars like our Sun are bound to the galaxy by gravity just like the planets in our Solar System are bound to the Sun. Stars typically travel between tens and a few hundred miles per hour with only a few hypervelocity stars breaking the mold. US 708 is one of them. This star was discovered by a team of astronomers using the Keck II and Pan-STARRS1 telescopes in Hawaii. And it’s really moving.
At 1,200 kilometers per second, a speed at which you could travel from the Earth to the Moon in about five minutes, it easily has enough velocity to escape the gravity of our Milky Way galaxy altogether. This raises a big question: how did a star gain this much speed?
This isn’t the only star with enough energy to escape our galaxy, but it’s one of very few; there are only a half-dozen or so known super fast moving stars out there. And they’re speed is usually the result of a close encounter with the supermassive black hole at the center of a galaxy. This is not the case with US 708, however. It’s the only hypervelocity star we know of whose speed is due to its being ejected from a tight binary couple by a supernova explosion.
US 708 is a compact helium star, a remnant of a once-massive star that lost its hydrogen envelope over time. It’s also rotating pretty rapidly. Astronomers suspect it was formed as a result of an interaction with a companion and was originally part of binary system. US 708’s hydrogen would have bled off into its companion star, a transfer that would have triggered the companion star’s demise in a type Ia supernova. US 708, the surviving star, would have been violently blown away from its now-absent companion, gaining incredible speed in the process.
Finding this star adds new observational evidence to the link between helium stars and supernovae, helping us get closer to understanding some of the largest thermonuclear explosions ever.
IMAGE: ESA/Hubble, NASA, S. Geier