What makes a supernova go boom?

March 11, 2018

   To be fair, many of my colleagues are working day and night trying to figure out exactly how supernovae explode, so I won’t claim it’s a well-understood process. But the general idea has been known for decades so I’ll try to explain it here briefly (and simply).

 

   Let’s say, somewhere in the Universe, in the middle of nowhere, a significant amount of stellar dust is collected. That’s the starting point for the formation of a new star. Let’s also say that the majority of that stardust is hydrogen, which is generally true, even if there are small amounts of other elements mixed in. Physicists always start with some generally simple scenario (like “let’s assume a spherical cow”) and increase the complexity later on. So in our simplistic scenario we have a hydrogen cloud that is pulled inward by its own gravity. In this process, our cloud gets more and more dense and also more and more hot.

 

   The only way to stop the gravitational pull is by having some other source of energy at the center of the star. This is where nuclear reactions come to the rescue. When the center of the star is hot enough, nuclear reactions with hydrogen start to be possible, and these reactions release a lot of energy. At this point there is a nice balance between the hydrogen nuclear reactions and the gravitational force, so the star can live happily at this state for billions of years, converting hydrogen into helium. This is exactly the current state of our Sun.

 

   The outer layer of the star will always have mainly hydrogen because the temperature never went high enough for hydrogen to fuse into helium. The center however will burn most of its hydrogen and the nice balance between gravity and hydrogen fusion comes to an end. At this point our star has two layers, an outer hydrogen layer and a central helium layer. At these temperatures nuclear reactions on helium are not yet possible so there is nothing to stop further gravitational collapse.

 

   And the story starts again. Star collapses due to its own gravity. Temperature increases at the center. Nuclear reactions with helium can happen and release energy. Balance between gravitational and nuclear energy until we run out of helium. And the cycle continues until the star forms something like an onion-layer structure, with hydrogen on the outside, then helium, carbon, oxygen, and so on until it forms an iron core. I talked about how special iron is in a previous post. The bottom line is that nuclear reactions on iron do not release energy so at this point there is nothing to go against the gravitational forces of the star. As a result, the star undergoes a catastrophic collapse towards the center, followed by a bounce-back or a shockwave that propagates through the onion layers, finally ejecting material into space. BOOM!

 

   This is called “core-collapse supernova” or supernova type II. The most famous supernova, which was discovered in 1987(SN1987A), was a supernova type II. There are other types of supernovae that explode differently but I’ll come back to them in a future post.

Supernovae are pretty cool astrophysical phenomena that release huge amounts of energy, about 100 million trillion times more energy than our Sun. They also produce a lot of heavy elements and shoot them into space. Most likely supernova cannot create the heaviest elements all the way to uranium, we need neutron-star mergers for that, but they are definitely responsible for a lot of the elements we see in out Universe today.

 

Image credit: NASA/CXC/SAO/JPL-Caltech

 

 

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