Supernova may offer new view of early universe
The star's 'monster' explosion, observed last September, is the biggest ever seen.
Dinosaurs were just beginning to thump across Earth when an enormous star exploded in a galaxy not too far away. Its light finally reached Robert Quimby's telescope last September in what researchers now call the brightest, most powerful supernova they have ever seen.
It also appears to be far different from any they have seen before. The team of researchers argues that they may have witnessed for the first time a stellar endgame similar to blasts that obliterated many of the universe's first stars more than 12.6 billion years ago.
These first stars – hundreds of times more massive and millions of times brighter than the sun – are thought to have lived briefly and exploded violently. They exploded so violently that, instead of collapsing upon themselves and creating neutron stars or black holes like most supernovae, these stars were annihilated. In the process, they jetted their star dust into the cosmos and seeded the young universe with a range of chemical elements forged in their furnaces. It's these elements that became the building blocks for new generations of stars, planets, and eventually, organic life.
The mystery is why Supernova 2006GY exploded at all, since conditions in the universe have changed so dramatically from that early period, according to the team of US researchers, who made their case at a briefing in Washington Monday.
That's not the only mystery, other researchers say.
"We don't actually know the mechanism by which this star exploded," acknowledges Mario Livio, an astrophysicist with the Space Telescope Science Institute in Baltimore. But, he adds, data from the supernova seem to rule out other plausible explanations, leaving the early-star mechanism as the last hypothesis standing. This "tantalizing possibility ... makes this supernova even more intriguing," he says.
The supernova appears near the core of a spiral galaxy 240 million light-years away, in the constellation Perseus. Dr. Quimby discovered it with a telescopic munchkin: an 18-inch automated telescope and software he developed as a graduate student at the University of Texas in Austin. The scope runs by itself all night, looking at galaxy clusters for signs of stellar deaths. "When I get up in the morning, if I see a dot" in an image "where there wasn't one before, that's a supernova," he says.
Trying to avoid anything interesting
Typically, he conducts his own follow-up studies. But at the time his telescope captured the explosion last year, he was in the midst of writing his PhD thesis. "I was actively trying not to find anything interesting to distract me," he recalls. When the image popped up, "I said, 'Yeah, it's a supernova. I'll just tell other people about it' " and let them do the follow-up work.
Enter Nathan Smith and colleagues from the University of California at Berkeley. The team suspected this was no ordinary supernova. Its brightness was one tip-off. So was its longevity. Usually, a supernova's light – which briefly outshines its galaxy – peaks and then fades over several weeks back into a galaxy's background light.
"This one rose for something like 70 days toward its peak and then stayed there, or declined only slightly, for several months," explains Alexei Filippenko, a Berkeley astronomer and member of the research team, which includes Quimby and Craig Wheeler of the University of Texas. Their work has been submitted for publication to The Astrophysical Journal.
The team has lost sight of the supernova because its host galaxy now lines up too close to the sun to observe. "But the last time we saw it, it was still really, really bright," Dr. Filippenko says.
Add up all the energy needed to sustain that brightness for so long, he continues, and it works out to be 100 times more powerful than a normal supernova. "This was a truly monstrous explosion," adds Dr. Smith. "We've never seen that before."
Some researchers have suggested that the blast is a normal supernova whose progenitor star lived in a hydrogen-rich neighborhood. The long-lived light would result as the copious number of atoms ejected by the blast collided with the hydrogen. But that would result in large X-ray emissions. So astronomers aimed NASA's Chandra X-ray Observatory at the supernova and found that its X-ray emissions were far too faint to match that explanation.
Others suggested that the bright event might be a supermassive black hole at the galaxy's center acting up. But the supernova falls just outside the galaxy's core.
This leaves the door open to an exotic form of supernova that theorists invoke to explain the end times for many of the universe's earliest stars.
Typically, a star at least 10 times more massive as the sun burns off the heavier chemical elements it creates in its fusion furnace. When the only fuel left is iron, burning stops. The outward "push" of the star's radiation against the orb's gravity vanishes. The star collapses, squeezing the iron core until it rebounds and blasts away the remaining outer material. What's left is a dense, spinning ball known as a neutron star, or in some cases, a black hole.
How a star obliterates itself
But theorists suggest that some of the early stars – from 140 to 260 times the sun's mass – are so hot in the center that the gamma rays they emit spontaneously change into electrons and their antimatter counterparts, positrons.
As a result of this conversion, the star starts to collapse. It experiences runaway thermonuclear reactions that form heavier elements but also lead to an explosion that leaves nothing behind but shards of elements heavier than hydrogen and helium. Such blasts would achieve energies some 100 times higher than typical supernovae. And radiation from decaying radioactive nickel, created in vast quantities in the blast, would keep the supernova's lamp lit far longer than usual.
This mechanism is controversial, Filippenko says. Given the far different conditions in the local, older universe – which would embrace Supernova 2006GY – the supernova should not have gone off at all. Invoking the early-universe explanation came "out of desperation," he says, because no other explanation fit the data.
Still "it's the best candidate to date," says Dr. Livio.