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Now and then, stars called novas give off huge bursts of light that make them up to millions of times brighter in the night sky. They're not as violent or bright as supernovas, the more famous exploding stars (which can outshine an entire galaxy) but novas also cause a temporary spike in a star's brightness.

The mechanism is totally different, though, and for decades astronomers have thought that novas got most of their brightness from material burning off their surface…. But some scientists started to question that model in recent years. And earlier this week, astronomers had to lay that old idea to rest as new observational evidence showed what's really lighting up these stars.

Unlike supernovas, which explode at the end of their lives, novas aren't dying. They happen in situations where an already-dead star is slurping up gas from its companion star. To get a nova, you need a pretty specific circumstance: you have to have two stars orbiting each other in a binary system.

One can be just an ordinary star—really, all you need there is something gaseous. But it needs to be really close to its companion, which must be a white dwarf. White dwarfs form when a mid-sized star dies— or stops going through nuclear fusion.

They're not as ridiculously dense as other stellar remnants, like neutron stars, but they've still got a lot of mass packed into a small space. And when you have a dense little package orbiting close to a star that's more gaseous and puffy, it can siphon off that star's outer layer of gas. As the white dwarf accumulates mass, it doesn't grow bigger, though:.

It just gets denser and denser until that new gas starts fusing and triggers a thermonuclear explosion—basically just a quick bout of nuclear fusion in the outer layers of the star. For a long time, astronomers thought that was how novas happened —that this brief explosion created a brief burst of light. But researchers started to question that model once they realized that an explosion like this would also send shockwaves traveling through all that accumulated gas —and that should also cause a change in brightness.

See, the different shockwaves don't all travel at the exact same speed, so some of them should collide with each other, and that would cause the material they're passing through to heat up and emit light. So recently, the big debate among astronomers studying novas has been, did the initial explosion produce most of the light? Or was it the resulting shockwaves?

And according to a paper published in Nature Astronomy on Monday, it is the shockwaves. The authors looked at the nova V906 Carinae, and they used. NASA's Fermi Gamma-ray Space Telescope to look for a telltale sign of shockwaves: gamma rays.

Gamma rays get emitted as the shockwaves accelerate the particles they pass through to near the speed of light. And they found that this star had the brightest gamma-ray emissions of any recorded nova! This told the authors that there were some really strong shockwaves running through it.

And when they compared their gamma-ray observations to observations of the star's brightness, they found that gamma-ray peaks happened right at the same time as the visible-light peaks. Which was a pretty good sign that the shockwaves were producing both. This was the first direct evidence that these shockwaves are the main source of light in novas.

But knowing how significant shockwaves can be in producing light could also help researchers understand other astronomical events that produce lots of light, like stellar collisions and supernovas. Speaking of exploding stars, on Monday scientists also announced in Nature Astronomy that they'd discovered the brightest supernova anyone has ever recorded. It's twice as bright as the previous record-holder, and between 50 and 100 times as massive as the Sun.

And this incredible supernova might just be an example of an explosion mechanism that scientists thought existed for decades but so far have never seen: pulsational pair-instability. It's a hypothetical process that scientists came up with to explain how super-giant stars die. According to the hypothesis, when these stars reach the end of their lives, extreme heat in their cores starts producing pairs of electrons and positrons—which have the same mass but opposite charges.

Since so much energy is going into producing these pairs of particles, the core no longer has enough pressure to hold up the rest of the star, so it contracts. As it gets denser, atoms begin to fuse, sending a pulse of energy out through the star. That energy is enough to shake off the star's outer shell, like a snake shedding its skin - except it's a big ball of hot gas in space.

And this process doesn't just happen once – it repeats. The star will eject matter, re-stabilize, eject matter, and so on, until it's built up a dense envelope of gas around the remaining little nugget of a star. That's pulsational pair-instability.

At least, that's the theory. And, according to the theory, eventually, that entire nugget of a star collapses in on itself and ignites into a supernova. A star going supernova by this mechanism would be super bright, because matter from that final explosion would collide with the dense outer envelope of gas, which would burn up and release a ton of energy.

And because the supernova this team observed was so bright, that may be what's going on here. At least, it's the best candidate yet for this hypothesis of pulsational pair-instability. While this explosion has already faded, explosions like it will be great candidates for the James Webb Space Telescope to study more closely when it gets into space.

It's not often that we see something in space that changes before our eyes, but exploding stars are some of the rare exceptions. They show us how dynamic space really is and they give astronomers a way to test their theories about how these incredible stars live and die. Thank you for watching this episode of SciShow Space!

And while you're here, I wanted to tell you about SciShow's. Space Pin of the Month. It's of Vostok 1, which was the first spacecraft to carry a crew to space—and it could be yours!

To place an order, check out the link in the description. [♪ OUTRO].