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The Butterfly Nebula. In its center is a binary star system made up of two stars in a death spiral. Credit: NASA

Supernova survivor stars are telling us how they got out the inferno alive

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Jun 13, 2021, 7:40 PM EDT

Stars in binary systems are in it for life. Meaning, if one goes supernova, the other is going to have to take the heat.

If there is anything you don’t want to be when a ginormous ball of burning hydrogen explodes, it’s a star in the wrong place at the wrong time. The surviving star will end up getting inflamed in the most literal sense (since it already is a ball of flames). Finding these cosmic final girls has been more difficult than you would think for something that just climbed out of an inferno, and there are some unusual things about them that scientists want to investigate.

What is even weirder is that these stars can not only survive the trauma, but eventually go back to what they were as if nothing ever happened. This is what OzGrav researcher Ryosuke Hirai of Monash University and his team of scientists have now found out.

“We think it’s important to not only find companion stars after supernovae, but to monitor them for a few years to decades to see if [they shrink] back,” Hirai, who recently published a study in Monthly Notices of the Astronomical Society, said in a press release.

Most stars were once thought to go it alone. We might be used to the idea since the objects in our solar system orbit one star, but studies have found that at least half of the stars out there develop and live out their lives in binary systems and orbit each other until one burns the last of its hydrogen and either turns into a white dwarf or—if it it large enough and hot enough—begins its supernova death throes. The reason scientists are so interested in companion stars is not just to study their fate, but also because these survivors tend to influence the evolution of their binary system.

A binary star system in which neither star is exploding...yet. Credit: NASA

Sometimes, star stuff is transferred from one binary star to another. This is also known as a mass transfer, and such a system is known as a mass transfer binary. What ends up becoming the dominant star can develop blasting winds. The larger of the two may also eventually puff up into a supergiant, depending on the stars involved and the amount of mass exchanged between them. Stars that live that fast at that size are going to crash and burn.

Even white dwarves, the cores of dead stars that were not huge enough to either go supernova or collapse in on themselves and form a black hole, can be blown up again. Supernovas that occur in binary systems with one white dwarf are also known as type Ia supernovas, though mass transfer can happen in any type. Transfer from another star can mean that the outer layer of hydrogen they had burned up is rebuilt to the point of rupture. In a way, it’s as if they are zombified only to die again. Their companions will inflate like blisters when the inevitable happens.

When a star is on its last breaths, some of the hydrogen it has left may go back to its companion before it finally erupts. Hirai and his team ran hundreds of computer simulations to look further into exactly how these swollen companions react. Though they appear highly luminous and (surprisingly) cool, never mind that they have just been slammed with supernova debris. They saw that the mass of a companion determines how bright it gets and actually has nothing to do with how powerful an explosion it faces.

“We applied our results to a supernova called SN2006jc, which has a companion star with a low-temperature,” Hirai said in the same press release. “If this is in fact an inflated star as we believe, we expect it should rapidly shrink in the next few years.”

More companion stars are coming out of the darkness as scientists search for them. While it is rare that anyone finds a new one to observe, they can tell us more about the evolution of enormous stars and what happens in a binary system right before a supernova, as well as in the aftermath.

Horror sequel material? Almost.