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.
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