For decades, astronomers have wondered if Eta Carinae, a massive binary star that shines 5 million times brighter than the sun, was unique, as nothing like it had been found in the Milky Way galaxy, or beyond.
But scientists now know that Eta Carinae, located about 7,500 light years from Earth, is not alone. A study using archived Hubble and Spitzer space telescope imagery found five Eta Carinae "twins" in nearby galaxies, astronomers said at the American Astronomical Society meeting in Kissimmee, Fla., on Wednesday.
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Aside from its girth, Eta Carinae's most distinctive feature is an expanding envelope of gas and dust, the result of a massive eruption in the 1840s that spewed the equivalent of more than 10 times the mass of the sun into space.
What triggered Eta Carinae's eruption remains a mystery, but scientists used telltale fingerprints of its distinctive dust cloud to find five more supermassive stars that experienced similar explosions.
"We're looking for a rare evolutionary phase in very rare stars -- the rarest of the rare objects," astronomer Rubab Khan, with NASA's Goddard Space Flight Center in Greenbelt, Md., told reporters.
Eta Carinae's twins are far way, up to 26 million light years away -- too far for telescopes to pick out individual stars. Instead, Khan and colleagues developed an optical and infrared blueprint to hunt for similar stars.
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The technique compares the amount of ultraviolet and visible light, which is dimmed by the dust cloud, with heating in the dust caused by the light being re-absorbed at longer, mid-infrared wavelengths.
Scientists then tried to find matches with the amount of dust observed around Eta Carinae. Two similar stars were found in the galaxy M83, located about 15 million light years away, and one each in NGC 6946, M101, and M51, located between 18 million and 26 million light-years away.
Khan said each Eta Carinae twin is believed to be a supermassive star buried in gas and dust equivalent to five to 10 times the mass of the sun.
"Eta Carinae is not unique ... It happens in nature. However, it's very, very rare. This is the first time we can quantitatively say just how rare Eta Carinae is," Khan said.
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The discovery is expected to help scientists better understand how massive stars form, evolve and die.
The research was published in the Dec. 20 edition of The Astrophysical Journal Letters and presented at the AAS conference.