The supernova is a well-publicized and frightening phenomenon. There's also a phenomenon known as an "unnova." You don't want it in your backyard any more than you want a supernova.

But to properly understand what an unnova is, you need to know what a star is like at the end of its life. For one thing, its heart is failing. A relatively young star, like our sun, heats hydrogen atoms so much that they smash together and form helium atoms. Once the star is running low on hydrogen to fuse, it will start fusing helium.

It will fuse helium because of the process going on in its outer layers. They act as a sort of bellows, heating the star whenever it runs low on energy. As a star cools its outer layers drop inwards, exerting pressure; more pressure means more energy means more heat, and the start heats up again, pushing the layers outwards again. This process goes on throughout its life.

But remember, the star is also spinning in space. Anyone who's seen a wet dog do the twist knows what happens to loose matter when it's spun around β€” as the sun's layers puff outwards and spin, some extra material can fly outwards and get lost. It all depends on the relative speed of the spin, the gravity exerted by the star, and how much and how violently the outer layers are moving back and forward.

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When the star stops being able to make enough energy, the outer layers collapse inwards. They hit the dimming core of the star with incredible force, enough that the star will go supernova, blowing off its outer layers. If the star is relatively small, its remains will become a neutron star β€” a cooling, slowing bit of matter. Larger stars collapse inwards after supernovae and become black holes. The inner layers of the star are left spinning around the black hole. They swirl around the black hole, as if they were debris in a whirlpool; because they are spinning, they have kinetic energy, of course. They also have gravitational energy β€” the energy they give off falling towards the black hole. This energy can shoot out jets of matter which astronomers can observe.

Maybe. Or maybe something else happens.

The "unnova" process begins right at the end of the star's life, while the star is running out of fuel, the outer layers are on the brink of collapse, and the inner layers are caught between. Except this star isn't spinning very fast, and much of this star's outer layers aren't about to fall inwards. When this star collapses, its outer layers are left behind. Its inner layers aren't spinning fast enough to swirl around the newly-formed black hole and simply fall down into the black hole. No boom. No jet of matter. No gamma rays. The star simply disappears, and leaves behind a black hole.

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How common is this? It's tough to say. The problem is, most of the time, supernovae don't leave behind black holes, they leave behind neutron stars. While some scientists believe that the supernova is the standard path to a black hole, some think that most black holes might be created by an unnova. So while a supernova blows up the neighborhood, the unnova may be "quiet one" that makes everything around them disappear.

Top Image: ESO/M. Kornmesser. Second Image: NASA.

[Via The Case of the Disappearing Star, The Observable Signature of Black Hole Formation]

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