Textbooks may need to be rewritten as evidence emerges that most Type Ia supernovae are caused by colliding stars. One study providing such evidence also suggests that these explosions start off with surprising variation, even when their peaks look similar.
Type Ia supernovae are particularly exciting to astronomers because they are usually thought to be equally luminous, allowing us to calculate their distance using their brightness.
However, this theory was based on measurements at the peak of the light curve, and the way they fade away. The explosions' first hours have been a mystery, because with one exception we have never been looking in the right direction to detect supernovae early enough.
SN2013dy was detected just 2.4 hours after it exploded, but the next fastest discovery was at 11 hours, until a team of astronomers discovered that they could use the Kepler spacecraft for a secondary purpose and detect very young supernovae, finding three in the process.
The Kepler space telescope sought planets passing in front of stars, checking target stars once every 30 minutes. Some astronomers realized, according to team member Dr Brad Tucker of Australian National University, that “This could be a really good way to look for supernovae occurring in the background.”
Tucker and his co-authors downloaded Kepler's images of galaxies containing supernovae that otherwise would have been dumped, revealing regular updates on the supernovae's brightenings.
Although two of the three discoveries look very similar in the Nature paper reporting the result, subtle differences early on were observed, while the third supernova is substantially fainter.
Moreover, the Kepler observations show no signs of a companion star either observed before the explosion or getting in the way of the blast thereafter. This adds to a growing pool of evidence questioning the idea that Ia supernovae occur when a white dwarf draws enough mass from a companion to pass the Chandrasekhar limit. Instead, all three explosions appear to be the result of the merger of two white dwarfs, previously thought to be a much rarer trigger.
Models of white dwarf mergers give more scope for diversity than models that use single degeneracy, adding to a growing body of evidence that Type Ias are not all the same.
Tucker notes that models of stellar evolution don't predict enough white dwarfs evolving close together in the early universe to account for the explosions we see if mergers are the main cause. “This is why why I really like this result, because it tells us there is a lot of physics we don't know,” Tucker says. He hopes that the exceptionally early observations of the explosions will sharpen models of how supernovae occur. “It's very hard for theorists to blow up a type Ia supernova in the lab.”
Tucker is at the same institution as Brian Schmidt, who co-discovered dark energy by using Type Ias to measure the acceleration of the universe. Tucker was keen to note, "The accelerating universe will not now go away - they will not have to give back their Nobel prizes.” Instead he says that differences between the explosions could explain the 10% uncertainty observed in measurements of the rate of acceleration.