13 super-fast, Alien Stars are invading the milky way

13 super-fast, Alien Stars are invading the milky way

Have you been lucky enough to see a shooting star? Not a micrometeorite flaring to a crisp in Earth's atmosphere - an actual star, zooming at millions of miles and hour.

Astronomers call these "hyper velocity stars," and they represent the fastest-moving stars in our galaxy. These rogue stars move so speedily that they are gravitationally unbound from the Milky Way; instead of orbiting the galaxy's centre like our sun and billions of others do, many hyper velocity stars seem to blaze forward on an unstoppable path out of the Milky Way entirely. Some may end up drifting aimlessly through intergalactic space. Others might one day plunge through the hearts of distant, alien galaxies like cosmic expats.

"These new hyper velocity stars are very different from the ones that have been discovered previously," the study's lead author Lauren Palladino, of Vanderbilt University, said in a statement.

"The original hyper velocity stars are large, blue stars, and appear to have originated from the galactic centre," Palladino added. "Our new stars are relatively small — about the size of the sun — and the surprising part is that none of them appear to have come from the galactic core."

On the fast track

Palladino discovered 20 potential hyper velocity stars while using a massive stellar census, the Sloan Digital Sky Survey, to map the path of sunlike stars in the Milky Way.

Leaving the galaxy takes a phenomenal amount of energy. Stars must reach speeds 1 million mph (1.6 million km/h) faster than the 600,000 mph (970,000 km/h) at which objects already speed around the Milky Way.

Most hyper velocity stars were once part of a binary pair that strayed too close to the super massive black hole at the centre of the galaxy. As one star spirals inward toward the black hole, the other is flung outward, fast enough to leave the galaxy. Scientists have discovered 18 giant blue stars on their way out that could have travelled such a route.

"It's very hard to kick a star out of the galaxy," collaborator Kelly Holley-Bockelman, also of Vanderbilt, said in the statement.

The team presented their newly discovered class of stars at the American Astronomical Society meeting in Washington, D.C., earlier this month. The results were published in the Jan. 1 edition of The Astrophysical Journal.

Stars ejected by the black hole have a different composition from that of the newly discovered stars. The 20 new stars have the same makeup as normal disk stars do, so the team doesn't think these newly discovered stars came from the galaxy's core, halo or some other exotic place.

"None of these hyper velocity stars come from the centre, which implies there is an unexpected new class of hyper velocity star — one with a different ejection mechanism."

Stellar mystery kick

Precise calculations require measurements taken over decades, so some of the stars may not actually travel as fast as they appear to, Palladrino said. To minimise errors, the team performed several statistical tests.

"Although some of our candidates may be flukes, the majority are real," she said.

What might have provided the needed galaxy-fleeing kick, however, is still a mystery.

"The big question is, what boosted these stars up to such extreme velocities?" Holley-Bockelmann said. "We are working on that now."