Image of the Pinwheel Galaxy (M101) taken before the supernova explosion (indicated with arrow) by the Hubble Space Telescope (left)and at the supernova's maximum brightness, using images from Las Cumbres Observatory Global Telescope Network 0.8m Byrne Observatory Telescope and the Palomar Transient Factory. Image of the Pinwheel Galaxy (M101) taken before the supernova explosion (indicated with arrow) by the Hubble Space Telescope (left)and at the supernova's maximum brightness, using images from Las Cumbres Observatory Global Telescope Network 0.8m Byrne Observatory Telescope and the Palomar Transient Factory. (J Fulton (LCOGT), PTF & the Space Telescope Science Institute. )

The early stages of an extraordinarily bright type of supernova have been witnessed for the first time, providing evidence about what kind of star it came from.

The type 1a supernova was discovered just 11 hours after it exploded on August 24 in the nearby Pinwheel Galaxy, only 21 million light years from Earth, reported an international team of scientists. They described their findings in a pair of papers in the scientific journal Nature.

Supernova story

Astronomer Peter Nugent said far more information can be deduced from data from the early stages of a supernova than later on, just as it's far easier to figure out how fireworks explode if you witness them travelling from the ground than if you only see the last sparks fade to smoke in the night sky. In the latter case, he said, you can't tell how long the explosion took or where it came from.

In the case of the supernova's early moments, the researchers were able to plot the increase in light over time and extrapolate back to the beginning of the explosion. In that way, they were also able to figure out the size of the shockwave and therefore the star that produced it.

Based on the characteristic colours of the light, they could also watch how different chemical elements were scattered by the explosion.

The sight of carbon and oxygen helped confirm that the supernova originated in a carbon oxygen white dwarf. The researchers could also see that some of the oxygen was travelling at 20,000 kilometres per second — much faster than other chemical components in the expanding fireball, suggesting that some of the oxygen forms clumps, as some supernova theories predict.

They could also see that scientific theories and models that predict a layered structure for the fireball, with lighter elements such as carbon and oxygen on the outside and heavier elements such as nickel at the centre of the explosion, are wrong. In this stellar explosion, the lighter and heavier elements appeared to be very well mixed, the report said.

It was the first time this type of supernova has been caught early enough to let researchers figure out when it exploded and what it looked like in the first moments after its explosion.

Based on the data, the researchers determined that the supernova was caused by the thermonuclear explosion of a white dwarf star made mainly of carbon and oxygen, fuelled by a companion star that wasn't much bigger than our own sun.

That is consistent with previous theories about this type of supernova, but astronomers had never before had direct evidence to back those theories up.

Peter Nugent, an astronomer at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, found the supernova among data gathered by the Palomar Transient Factory. The automated telescope in California surveys a large portion of the sky twice every night in search of new, bright objects that have recently appeared in the sky.

University of Toronto researcher Nicholas Law, who set up the telescope to do the survey and co-authored the new paper, was "absolutely critical" to the results, said Nugent, who was the lead author of one of the two papers.

After the supernova's discovery, the progression of the explosion was monitored by other telescopes around the world. Nugent credited the find to a "bit of luck," noting that an explosion like this in a galaxy so nearby only happens about once every 25 years or so. The fact that it was so near Earth made it detectable early after its explosion.

Nugent said the Palomar telescope flags about 100 objects per night, and researchers sort through those manually, typically finding about one or two that look interesting because they might be an early-stage supernova or one in a nearby galaxy.

Nugent was working with co-author Joshua Bloom, another astronomer at Lawrence Berkeley National Laboratory, on the night of the discovery.

"He found a really nice supernova about five seconds before I found this one," Nugent recalled. "And I'm like, 'Oh, that's great. Here's one that's a lot, lot better.' He said, 'Oh darn, I would have gotten to that one next!'"