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Astronomers have pinpointed two massive stars orbiting close to each other that could merge into a "super" sun, 100 times bigger than our own.
The "binary" star system, in a galaxy orbiting the Milky Way, has been captured by Nasa scientists using satellite and ground-based telescopes.
It is one of the most "extreme" systems of its type and, at less than 3 million years old, is relatively young. The stars, around 165,000 light years from Earth and labelled LH54-425 by astronomers, contain about 62 and 37 times the mass of our Sun.
As they age and swell, scientists believe they will transfer massive amounts of mass to each other.
Eventually they are likely to merge, producing a single huge star to rival one of the largest found in the Milky Way: the Eta Carinae binary system.
"The merger of two massive stars to make a single super star of over 80 suns could lead to an object like Eta Carinae, which might have looked like LH54-425 one million years ago," said George Sonneborn, of Nasa.
"Finding stars this massive so early in their life is very rare. These results expand our understanding of the nature of very massive binaries, which was not well understood.
"The system will eventually produce a very energetic supernova."
Rosina Iping, of the Catholic University, Washington, said: "These stars are evolving in the blink of an eye compared to the Sun, which has looked pretty much the same for over 4 billion years.
"But this binary looks totally different from Eta Carinae even though there is maybe only one million years difference in age. Will it produce something like Eta Carinae? We don't know."
Nasa speculated this month that Eta Carinae may be about to explode. It devised the theory after deciding the brightest stellar explosion ever recorded could be a new type of supernova.
Violent explosions of extremely massive stars now seem to have been relatively common in the early universe and a similar explosion might be ready to go off in the Milky Way, astronomers said.
"This was a truly monstrous explosion, a hundred times more energetic than a typical supernova," said Nathan Smith of the University of California at Berkeley, who led a team of astronomers from California and the University of Texas in Austin.
"That means the star that exploded might have been as massive as a star can get, about 150 times that of our Sun. We've never seen that before."
The discovery of the supernova, SN 2006gy, provided evidence that the death of such massive stars was "fundamentally different" from theoretical predictions.
The star that produced SN 2006gy apparently expelled a large amount of mass prior to exploding. Eta Carinae too has been losing mass.
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