Peeks into early life of supernovas show how to blow up a star

There’s more than one way to explode a star. Four recently seen type 1a supernovas show off some of this diversity. One supernova hints at the gas of a partner star fueling the explosion; colliding white dwarfs — the exposed cores of dead stars — probably triggered the other three.

The observations, described in a pair of papers in the May 21 Nature, give astronomers a rare look at the first few days of a stellar detonation.

Shrapnel from supernova iPTF14atg appeared to run into another star within hours of the explosion, Yi Cao, an astrophysicist at Caltech, and colleagues report. The presence of this close stellar partner supports the idea that a relatively large star provoked a white dwarf to explode. The companion star may have dumped its gas on the white dwarf until the added weight sparked a thermonuclear detonation.

The blast of gas then slammed into the companion star, creating a shock wave and an ultraviolet glow. The UV flash tipped Cao and colleagues off to the presence of an accomplice.

Astrophysicist Robert Olling of the University of Maryland in College Park and colleagues, however, saw no such bump in the light from three other supernovas captured by the Kepler space telescope. Kepler can’t see UV light. But if there was another star lurking nearby, Olling says, the telescope should have seen something in the first few days after the supernova. The lack of a second flash supports another leading idea for what triggers the explosion — the collision of two white dwarfs locked in mutual orbit.

Evidence for different triggers “doesn’t mean there’s any problem,” says Ryan Foley, an astrophysicist at the University of Illinois at Urbana-Champaign. “They’re looking at very different physical systems.” Olling’s are run-of-the-mill type 1a supernovas that astronomers use to measure distances to other galaxies, Foley says. The explosion that Cao detected, however, “is a weird supernova.” It’s much less luminous and the debris doesn’t fly away as fast.

Perhaps a star drizzling gas onto a white dwarf ends in a subpar supernova. A collision between white dwarfs, on the other hand, might lead to something with a little more oomph.

“If you think the asteroid that wiped out the dinosaurs was bad,” Olling says, “imagine two white dwarfs crashing into each other.”

Sorting out how many supernovas are caused by one trigger or the other is tricky because type 1a supernovas are rare. On average, only one goes off in a galaxy every 100 years, Olling says. “You have to look at lots of galaxies for a long time to find just a handful.”

Olling is doing just that. He and his colleagues are using the revived Kepler telescope to scan tens of thousands of galaxies for type 1a supernovas while conducting simultaneous observations from the ground. They haven’t found any yet, he says, but he hopes to build up a large enough sample so that researchers can start to better understand what’s driving these powerful explosions.