By Victoria Gill

Science reporter, BBC News

The explosion may be caused by a heavier "thief" star stealing helium from its neighbour

Astronomers have put forward opposing explanations for what could be a new type of exploding star or supernova. Supernova 2005E was initially picked up by telescopes back in 2005 and has been carefully examined by scientists. They now report, in the journal Nature, that the explosion does not match known types of supernova. In the same issue of the journal, however, another research team offers a different explanation for a very similar stellar phenomenon. Until now, two main types of supernova had been documented. The first (type Ia) is caused by the violent thermonuclear explosion of an old, dead star - or a white dwarf. Type II supernovae are triggered when a young, massive star runs out of nuclear fuel and collapses under its own weight. Our study has rescued the standard theory of stellar evolution

Professor Koji Kawabata

Hiroshima University In this case, the researchers say that the amount of material hurled out from SN 2005E was too small for it to have come from an exploding young giant. And its location - far from the busy "stellar nurseries" where new stars form - suggested that this was an older star that had had time to move away from its birthplace. The material being blasted into space by SN 2005E also contained unusually high levels of the elements calcium and titanium. Dr Hagai Perets, who led the study, began his examination of the strange supernova whilst working at the Weizmann Institute of Science in Rehovot, Israel. He is now based at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US, and said: "It was clear that we were seeing a new type of supernova." But another research team, led by Professor Koji Kawabata from Hiroshima University in Japan examined a supernova called SN 2005cz, which had very similar properties. Professor Kawabata and his team argued that this event was in fact a collapsing giant. "These properties are best explained by a core-collapse supernova at the low-mass end of the range of massive stars that explode," he and his colleagues wrote in their paper. They say that this star represents a boundary between stars that end their lives with a gigantic supernova explosion and those that do not explode. "Our study has rescued the standard theory of stellar evolution," said Professor Kawabata. "This supernova was faint and gone quickly, [which] is probably a main reason why we have not [seen] this kind of supernova before". Thief star Practically every chemical element in the Universe, other than hydrogen and helium, is made in stars

Dr Mark Sullivan, Oxford University Dr Perets' team carried out simulations that revealed the strange event appeared to involve two stars - a pair of white dwarves - and that one of them was "stealing helium" from the other. They suggested that, once the mass of accumulated helium reached a critical point, the thief star became very hot and dense and a nuclear explosion occurred, producing other elements, such as calcium and titanium. "The donor star is probably completely destroyed in the process, but we're not quite sure about the fate of the thief star," said Dr Avishay Gal-Yam, also from the Harvard-Smithsonian Center. Mark Sullivan, an astronomer from the University of Oxford in the UK, said the possibility of a new class of supernova was "very exciting". "We've known about the two main types of supernovae for decades, so to find something different, and with a new explosion mechanism, obviously changes our view of the way that stars explode and chemical elements get recycled," he told BBC News. He continued: "Practically every chemical element in the Universe, other than hydrogen and helium, is made in stars." Dr Sullivan explained that the only way for these elements to get from the stars in which they are made into us is when they get recycled in supernova explosions. "A new type of supernova explosion gives us new insights into how some of these elements get recycled and end up in our Solar System," he added.



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