Rogue Stars: The Miscreants of Our Galaxy

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

Astronomershave found about 10 stars hurtling away from our galaxy, at speeds that exceedits gravitational grasp. While most stars rush through space at speeds on theorder of hundreds of kilometers per second, these aptly-named "hypervelocitystars" are rocketing away at least twice as fast.

Most ofthese 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 blackhole at the center of our galaxy. Their violent creation is giving astronomersinsight into the almost impenetrable world at the center of the Milky Way, themysteries of our nearby galactic neighbors, and the nature of intergalacticspace.


Hypervelocitystars were first theorized to exist in 1988. The theory was that binary starsystems at the galaxy's center would occasionally wander too close to the massiveblack hole looming there, which would disrupt their orbital dance. Whileone of the pair was captured by the black hole, the other would be sentrocketing off at an incredible speed.

"That'sthe only way you can accelerate a star to go thousands of kilometers persecond," said astronomer Alceste Bonanos of the Carnegie Institution forScience, a member of the team that made the discovery of the alien star'sorigins.

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

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

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

Thegalaxy's center is shrouded in gas and dust and normally hard for astronomersto peer into, Bonanos said. Gas clouds usually act as excellent stellarnurseries, but the violent tidal forces from the black hole were thought toprevent any nearby stellar births.

The roguestars seem to contradict that idea, as they seem to have come from the vicinityof the black hole, Bonanos told LiveScience.

Except forone, which is an alien passerby.


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

"Thisone is different from the other nine," said study team member MercedesLopez-Morales, also of the Carnegie Institution.

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

Bonanos andLopez-Morales took a closer look at the elemental composition of the star andfound that it seemed to be a visitor from our small galactic neighbor, the LargeMagellanic Cloud (LMC).

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

But whilethe elemental profile matched, there's one big conundrum: The LMC "is notknown to have a massive black hole that could eject it," Bonanos said.

The usualtell-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 hugeblack holes in their center, so "this star might be a hint for somethingimportant," Bonanos said.


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

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

It ispredicted that thousands of hypervelocity stars have been spit out by the MilkyWay's black hole, Bonanos said, though many are still hurtling through thegalaxy.

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

But there'sno need to worry about a stellar roadrunner knocking into Earth, or any otherplanet or star, Bonanos says.

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

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Andrea Thompson

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.