‘Impossible’ star explosions made by gas and solar wind pile-upNew Scientisr
By Jesse Emspak 4 September 2017
Shockingly bright Mark Garlick/Science Photo LibraryNovae – the bright explosions originating from white dwarf stars – have puzzled astronomers because they often shine brighter that should be possible without blowing themselves apart. How they do it comes as a shock.
Ray Li at Michigan State University and his colleagues studied the gamma ray emission and visible light from a nova called ASASSN 16ma. They concluded that the extra power comes from “shocks” – the gas that a nova initially blows out getting slammed from behind by faster blasts of gas.
A nova starts with a white dwarf. White dwarfs are the remains of stars up to 1.4 times the mass of the sun. When the star runs out of fuel it can no longer generate energy and support its own weight. The star collapses and becomes so dense that a single cubic centimeter can weigh 1000 metric tonnes – surface gravity is hundreds of thousands of times that of Earth. If the white dwarf has a companion star, it can pull gas from that star and crush it until that material fuses on the surface and explodes – a runaway reaction that we see as a nova.
Novae can’t be too luminous, though, because at a certain point, theoretically, they should just blow themselves apart. Yet many novae are brighter than theory would suggest.
This is because the nova first ejects gases at a few hundred kilometers per second, says the team. Soon after that, the white dwarf’s solar wind follows – but that’s moving 10 times as fast. The outer envelope of gas is full of charged particles that rebound off that faster gas as it slams into them from behind.
The resulting acceleration of the particles gives off gamma rays, and adds energy to the outer gaseous envelope. On top of that, X-rays emitted by the white dwarf light up the gas. This means that the bulk of the energy comes not from the surface of the white dwarf but from the outer gas envelopes.
“Traditionally, people believe that the fusion on the white dwarf surface is the only energy source for the visible light in a nova,” Li says. “However, in ASASSN-16ma, the gamma rays and the optical emission are strongly correlated, suggesting that they have the same origin – the shocks.”
The shocks themselves were already known about, says Laura Chomiuk, who took part in the study, but “nobody thought they were very important”.
The team now wants to observe more novae to see if the hypothesis holds up, says Chomiuk. This will take some time because stellar explosions don’t happen on a regular schedule. Novae, she says, are still poorly understood. “We’re still trying to understand why some of our novae are so luminous and some not,” she says.
Journal reference: Nature Astronomy, DOI: 10.1038/s41550-017-0222-1
https://www.newscientist.com/article/2146213-impossible-star-explosions-made-by-gas-and-solar-wind-pile-up/amp/
