Rogue Stars: The Miscreants of Our Galaxy

A young star speeding away from the Milky Way is in fact an alien visitor, astronomers have confirmed. The wayward object is one of several rogues that are giving astronomers a glimpse into the volatile nature of our galaxy and others.

Astronomers have found about 10 stars hurtling away from our galaxy, at speeds that exceed its gravitational grasp. While most stars rush through space at speeds on the order of hundreds of kilometers per second, these aptly-named "hypervelocity stars" are rocketing away at least twice as fast.

Most of these speedy stars are thought to be exiles from the center of our galaxy, flung out into intergalactic space by the powerful forces of the massive black hole at the center of our galaxy. Their violent creation is giving astronomers insight into the almost impenetrable world at the center of the Milky Way, the mysteries of our nearby galactic neighbors, and the nature of intergalactic space.

Volatile origins

Hypervelocity stars were first theorized to exist in 1988. The theory was that binary star systems at the galaxy's center would occasionally wander too close to the massive black hole looming there, which would disrupt their orbital dance. While one of the pair was captured by the black hole, the other would be sent rocketing off at an incredible speed.

"That's the only way you can accelerate a star to go thousands of kilometers per second," said astronomer Alceste Bonanos of the Carnegie Institution for Science, a member of the team that made the discovery of the alien star's origins.

Of the billions of stars in the Milky Way, only a tiny fraction are thought to be shot out from the center like this. This explains why they weren't found until 2005, Bonanos says, "because there aren't very many."

Astronomers looked at the spectra of stars at the most outer reaches of the Milky Way and found a few that "were going very, very fast, which isn't normal," Bonanos said.

By examining the age of these exiled stars, astronomers concluded that they seem to have had time to come from the center of our galaxy.

The galaxy's center is shrouded in gas and dust and normally hard for astronomers to peer into, Bonanos said. Gas clouds usually act as excellent stellar nurseries, but the violent tidal forces from the black hole were thought to prevent any nearby stellar births.

The rogue stars seem to contradict that idea, as they seem to have come from the vicinity of the black hole, Bonanos told LiveScience.

Except for one, which is an alien passerby.

'Alien' traveler

Of these 10 strange stars, one, dubbed HE 0437-5439, seemed a bit stranger than the rest.

"This one is different from the other nine," said study team member Mercedes Lopez-Morales, also of the Carnegie Institution.

Based on its current position, the star would have to be 100 million years old to have come from the center of the Milky Way. But it is only 35 million years old.

Bonanos and Lopez-Morales took a closer look at the elemental composition of the star and found that it seemed to be a visitor from our small galactic neighbor, the Large Magellanic Cloud (LMC).

"Stars in the LMC are known to have lower elemental abundances than most stars in our galaxy," Bonanos explained, which seemed to fit HE 0437-5439's make-up.

But while the elemental profile matched, there's one big conundrum: The LMC "is not known to have a massive black hole that could eject it," Bonanos said.

The usual tell-tale signs of a big black hole, such as strong X-ray and radio signals, are missing. Astronomers aren't sure if dwarf galaxies like the LMC have huge black holes in their center, so "this star might be a hint for something important," Bonanos said.

Collision course?

Another strange consequence of these roving stars is the contradiction they provide to the long-held notion that intergalactic space is pretty much empty.

'There seem to be all these stars flying around between galaxies," Bonanos said. If stars are shot out from our galaxy, they are likely propelled from others, she says, though we are unlikely to be able to see them because stars are too hard to individually identify from the distance of most galaxies.

It is predicted that thousands of hypervelocity stars have been spit out by the Milky Way's black hole, Bonanos said, though many are still hurtling through the galaxy.

So far all of the hypervelocity stars found are moving away from us, but they could be shot out of the galaxy's center in any direction, up or down from the galactic plane, or even toward us.

But there's no need to worry about a stellar roadrunner knocking into Earth, or any other planet or star, Bonanos says.

"There's a lot of empty space" in the solar system, she says, so these speeding stars will likely have a clear path out of the neighborhood.

Andrea Thompson
Live Science Contributor

Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.