Mysterious star deaths are really mergers in disguise

SOAP operas have nothing on supernovae. A charlatan star that appeared to explode earlier this year may have faked its own death to unite with a secret companion.

If so, it joins a growing cast of oddball stars suspected to be the products of stellar mergers, which have the potential to change our understanding of the universe's chemical make-up.

Stars are powered by nuclear fusion, converting hydrogen into helium in their cores. When very massive stars run out of hydrogen fuel, they start fusing heavier elements until their cores collapse and they explode. These types of exploding stars, or supernovae, scatter the elements that go on to make new cosmic bodies.

Because most of these stars detonate when they hit a set mass limit, their behaviour is fairly predictable, but occasionally we see a dying star go off the rails. Such was the case with SN 2009ip, which flared up and died down for three years before finally seen going supernova in September.

But Noam Soker of the Technion-Israel Institute of Technology in Haifa challenges the supernova interpretation of the outburst. Based on observations, he calculates that SN 2009ip generated less than 10 per cent of the kinetic energy of a typical star explosion (arxiv.org/abs/1211.5388). "The more we look at it, the stranger its behaviour for a supernova," he says.

Instead, Soker and Amit Kashi of the University of Nevada in Las Vegas argue that the outburst has more in common with V838 Monocerotis, another flaring star now thought to be the result of a merger, shown right. They think the early outbursts from SN 2009ip were caused by two large stars brushing against each other. When they merged in September, they created a new star between 100 and 120 times the mass of the sun.

Gijs Nelemans of Radboud University Nijmegen in the Netherlands says astronomers are starting to realise how common star mergers must be. "We now know there is an extraordinary diversity of strange, transient objects," he says, referring to the hundreds of fleeting celestial flare-ups that appear beyond the solar system each year. They can't all be supernovae, says Nelemans, because of their widely different durations and brightnesses.

There's still much work to be done to understand how many stars merge and their impact on the universe's composition. Not all stars may survive the merger process, and some of those that do probably don't live for long. "We must attempt a more systematic observation of merger stars throughout the galaxy," says Nelemans.

As for SN 2009ip, Soker and others will be watching closely for any distinctive elements that are only created in star explosions. "If we see evidence of radioactive cobalt, then it was a supernova and we can rule out the merger," he says.

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