— The brightest supernova ever seen appears as a dark spot (arrow) in this negative infrared image taken by the Palomar 5-metre telescope. The other dark spot at centre is the host galaxy’s core (Image: E Ofek et al/Caltech/Palomar Observatory)
A supernova intrinsically two to three times brighter than any previously recorded has been observed, and its characteristics suggest it did not form like others of its class.
It appears to have been forged in a collision between two stars, adding fuel to a long-running debate about what causes the type Ia explosions that are a crucial tool in cosmology.
The prevailing view of type Ia supernovae is that they result from a dense stellar corpse called a white dwarf that slowly collects matter from an ordinary companion star. Eventually the white dwarf reaches a mass threshold called the Chandrasekhar limit, triggering an explosion that completely destroys it.
This mass cut-off is thought to make all such supernovae explode with about the same intrinsic brightness, allowing astronomers to calculate their distance based on how bright they appear through telescopes. In fact, it was observations of type Ia supernovae that led to the surprising discovery in 1998 that some mysterious entity, dubbed dark energy, was causing the universe's expansion to speed up.
But some astronomers have argued that type Ia's are actually due to two white dwarfs merging. The combined mass of the two objects is above the Chandrasekhar limit, leading to the explosion.
Evidence for this hypothesis came in 2002 from a supernova called 2002ic. It had some characteristics of type Ia's, but unlike others of its type, it also showed clear signs of hydrogen in its light spectrum. Some researchers said that could be explained by a white dwarf colliding with the core of a red giant star a dying, Sun-like star that bloats up and starts expelling its outer layers before becoming a white dwarf itself.
But other astronomers countered that 2002ic was a disguised type II supernova. These explosions occur when a massive ordinary star collapses to form a neutron star or a black hole.
Since then, three other supernovae with similar characteristics to 2002ic have been found. Now, a fourth has been detected, and it offers the best evidence yet for the merger scenario at least for some type Ia's, according to a new study led by Eran Ofek of Caltech in Pasadena, US.
The supernova was discovered on 18 September 2006, and was named 2006gy. After correcting for light absorbed by dust between Earth and the supernova, it appears to have been about three times brighter than any previously observed stellar explosion.
Unlike the other unusual type Ia's, such as 2002ic, the September supernova has been traced to a galaxy dominated by old stars. This suggests it was not a type II event, which requires a massive star that has a very short lifetime, the researchers argue.
Study co-author Avishay Gal-Yam, also of Caltech, says most type Ia supernovae may originate in such mergers. To explain why typical type Ia's do not show hydrogen in their spectra, he argues the white dwarfs manage to eject the red giants' hydrogen as they spiral into them.
But he admits that there is still room for doubt about the merger model. "When you have one example, you are always a little cautious," he told New Scientist. The merger model would receive a boost "if we have another one like this and it also happens in a galaxy made of old stars", he says.
Mario Livio of Johns Hopkins University in Baltimore, Maryland, US, who has studied both the merger and accretion models, agrees that the September supernova "looks as if it might have come from some sort of a merger".
But he says this scenario probably accounts for only a very small fraction of type Ia supernovae. Calculations of white dwarf mergers suggest that such catastrophic events would convert carbon in the two objects into heavier elements. That would theoretically lead to the creation of a black hole or a neutron star rather than a type Ia supernova, he says.
Because merger-generated explosions are likely so rare, they should not affect the interpretation of cosmological studies that rely on type Ia supernovae including measurements of dark energy, he says.