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Hank Green: Astronomers have recently discovered the zombies of the star world, objects that survive even after they're supposed to have been obliterated in a supernova explosion.  Zombie stars.  During a supernova, a star releases as much energy as all of the other hundreds of billions of stars in its galaxy combined.  This fantastic outburst can happen in a few different ways, and zombie stars appear to be the result of a new type of self-destruction that astronomers didn't know about until recently.  

One of the most stunning types of supernova is called the Type 1A.  These explosions are thought to originate in binary star systems, where at least one of the stars is a white dwarf, a remnant of a dying star that is almost inconceivably dense.  Imagine the mass of the sun compressed to the size of the Earth.  According to one prominent theory, the enormous mass of this white dwarf draws in matter from a close orbiting companion star, and as the mass piles up, the temperature inside the white dwarf increases.  It increases so much so that the relatively heavy elements already in its core, like carbon and oxygen, fuse to even heavier elements, like nickel and iron.  When that starts to happen, it is only a matter of seconds before the entire star is consumed in a runaway cascade of fusion reactions releasing enough energy to completely rip it apart.  So, you know, if you happen to be near one, you should sunscreen SPF 60 for sure.  By the time it's all over, all of the white dwarf's matter has been flung out into space at thousands of kilometers per second, leaving nothing behind.

But there's another major theory out there that suggests that Type 1A is formed by the merging of two white dwarfs, and that the collision of these two mega-dense stars is what blows them apart.  Astronomers think they actually witnessed just such an explosion in 2011, when two white dwarfs in the galaxy M101 merged and exploded pretty much right before our eyes.  But no matter how they form, type 1A supernovae are incredibly useful for astronomers, because they all have the same intrinsic brightness.  This makes them one of the most important tools scientists have for measuring distances in space.  And this is where the zombies come in.

Between 2008 and 2012, astronomers at Harvard identified several dozen supernova explosions, scattered throughout a bunch of nearby galaxies that kinda looked like Type 1As.  But there were a few perplexing differences, starting with the fact that they were too faint.  The astronomers were also able to measure how much material these explosions had thrown out into the space around them, and found that the mass that they ejected just didn't add up.  Type 1As usually release the equivalent of about 1.4 times the mass of the sun, or 1.4 solar masses.  But these weird new ones were kicking out only about half of a solar mass, sometimes even less, and they were releasing it a lot more slowly than a Type 1A.  So it seemed that in these new explosions, given the name 1AX, that only part of the white dwarf exploded, and the rest stuck around in a sort of undead state after it was supposed to have been destroyed.  

So, how's that possible?  Well, a big breakthrough in understanding 1AXs came when the Hubble Space Telescope found one of these zombies in the process of forming.  In 2012, a star exploded in the galaxy NGC 1309, which Hubble happened to have been observing off and on since 2005.  And when a team of astronomers went back and looked at Hubble's earlier observations, they found the binary star system that exploded several years before it actually went supernova.  So, by luck, we had both the before picture and the after picture of the explosion, which turned out to be incredibly useful.  The before picture showed a relatively bright blue star paired up with a white dwarf, which was already in the process of pulling away much of the hydrogen from the blue star.  And the after picture of the supernova itself, looked a lot like the weird ones that the Harvard astronomers had found.  It was too faint and releasing its material too slowly and hadn't cast off enough mass to be a regular type 1A.  

So Hubble not only confirmed that there's a new type of supernova out there, it also seems to have shown us how they form.  Essentially, the pictures suggest that these weaker, weirder supernova are really the result of one white dwarf interacting with a normal companion star.  Then, for reasons we don't understand yet, the fusion reactions that trigger the supernova suddenly stop, releasing less energy than normal and leaving part of the white dwarf half-dead, but intact.  And some astronomers think that if these new supernovae are produced by a white dwarf and another kind of star, then it may well be that the original Type 1As, like the ones that we happened to spot in 2011, occur when two white dwarfs smash into each other.  So basically, we're rethinking everything at this point.

For now, astronomers will keep studying these new explosions and their zombie-like remains to help us understand how stars that go supernova form, live, and die, and maybe even become undead.

Thanks for joining me for this episode of SciShow Space.  If you want to learn how you can help us keep exploring the universe for the world and with the world, you can go to Subbable.com/SciShow and don't forget to go to YouTube.com/SciShowSpace and subscribe.

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