Astronomers reveal recent clue as to why giant stars explode

The mystery of how giant stars spend their last moments before exploding in radiant death just got just a little closer to being solved, partly by researchers who challenged the concept that implied models are sufficient for studying dying stars.

In a paper published Tuesday in The Astrophysical Journal, astronomers on the University of Texas, Austin describe a recent model for what they think happens when among the biggest stars within the cosmos explode as supernovae at the top of their lives, before turning into neutron stars or black holes.

When stars eight times as massive because the Sun or larger burn through their hydrogen fuel, they start fusing ever more heavy elements resembling oxygen, silicon, and eventually iron. At that time, unable to sustain the thermonuclear reactions at their core that push against their very own massive gravity, they collapse in on themselves and explode in a supernova.

Despite burning through most of their hydrogen, some supermassive stars retain an outer layer of hydrogen gas up until the time they explode. But a certain variety of supernovae, Type IB, involves stars which have lost this outer layer of hydrogen.

Scientists like University of Texas, Austin professor of astronomy Craig Wheeler would really like to know the way those stars lose that hydrogen.

“What we’re trying to know is what makes these massive stars blow up,” he said. “Understanding what mass ejection they do shortly before they explode is one piece of that, in some sense, even yet greater story of how the huge stars evolve and die.”

In 2014, a star 40 million light years away within the spiral galaxy NGC 7331 went supernova, and observers quickly classified it as a Type 1B, as there was no evidence of hydrogen in the sunshine coming from the explosion. But as Wheeler and his colleagues watched the sunshine from SN 2014C over the approaching months and years, they were puzzled — they began seeing evidence of hydrogen where there had been none.

The supernova SN 2014C within the distant NGC 7331 galaxy as seen in visible and X-ray light

(Nasa)

“When you take a look at a supernova that when it blows up has no hydrogen in it. After which just a little bit later, months, a yr, even a decade later, starts showing evidence for hydrogen emission, that implies that it used to have hydrogen it blew it off into the encircling medium,” Dr Wheeler said. “Then the supernova explodes and collides with that previously rejected hydrogen.”

What Dr Wheeler and his colleagues suggest within the paper is that SN 2014C took place in a binary star system and that the companion of the star that exploded helped draw off the outer layer of hydrogen from the dying star before it exploded. The stellar winds of each stars then pushed the hydrogen out into the space across the pair in a “common envelope.”

But that’s not a recent idea. What Dr Wheeler believes is vital is the form of the common envelope — his findings show it needs to be a disk.

“There’s an old joke that each one physicists first assume that cows are spherically symmetric,” Dr Wheeler said, the purpose being that physicists often make simplifying assumptions about phenomena to start on an issue. Within the case of SN 2014C, he said, a lot of the models assumed that the common envelope across the two stars was roughly spherical, and that the supernova blast expanded spherically.

But when Dr Wheeler and his team crunched all of the numbers, they found contradictions in the information as long as they tried to place them right into a symmetrical, spherical model.

“It needed to be asymmetric ultimately, and so we’ve argued that [the hydrogen is] coming off in a disk, and that helps us to place all of the information together in a coherent picture,” he said. “Not only that it may be asymmetric, however it absolutely rattling well have to be asymmetric, is certainly one of the principal messages of our paper.”

Material orbiting massive objects as a disk isn’t unusual in astronomy, Dr Wheeler points out. The matter whirling around black holes before slipping inside is compressed right into a disk, planets form out of disks of dust around stars, and galaxies themselves are, to an extent, disk-like.

But many physicists, Dr Wheeler said, have used spherical assumptions when using computers to model Type Ib supernovae to calculate the “simplest thing they may get away with that explored what they were taking a look at.”

“We’re going to need to do a pause on that,” he added.

And while questions of how astronomers model supernovae may appear arcane to put readers, Dr Wheeler points out that they’re pieces in a much larger puzzle that covers almost all the cosmos. Supernovae are what generate the heavier elements like iron, silicon, and carbon vital for Earth-like planets and Earth-like life, while the neutron stars and black holes these massive explosions end in tug and twist at the material of reality itself.

Higher understanding the right way to model supernovae will help scientists higher understand the universe, and Dr Wheeler is arguing that this asymmetric model may apply to many supernovae across the universe, and never just SN 2014C. He’s given it a number of thought, but he readily admits he may very well be unsuitable, and is looking forward to the scientific community’s response to the paper.

“I believe that our colleagues are going to have to take a seat back and, you understand, rub their chins and take into consideration this and assimilate that into their picture,” Dr Wheeler said. “There’s a gathering in Munich next fall that I’m unsure I’m going to have the option to go to, but they’ll all be there and it might be great fun to have this conversation.”