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Poor astronomy | Neutrinos play a major role in exploding stars



I have long wondered about the Universe’s crooked sense of humor. After all, how else can it be that one of the most ethereal and ghostly particles in the cosmos is fundamentally responsible for some of the most colossal and violent explosions in it?

New research indicates that neutrinos not only play an important role in supernova explosions, but we need to take that into account all their properties to really understand why stars explode.

Stars generate energy in their nuclei and melt lighter elements into heavier ones. This is how a star prevents its own gravity from causing it to collapse; the generated heat inflates the star and creates pressure that holds it up.

The most massive stars take this energy production process to the extreme; while stars with lower masses like the sun stop after melting helium into carbon and oxygen, massive stars continue to melt elements together all the way up to iron.

However, once the core of a mighty star is iron, a series of events take place that actually remove energy from the core so that gravity can dominate. The nucleus collapses and creates a huge energy explosion that is so huge that it blows the outer layer of the star away, creating an explosion we call a supernova.

A crucial part of this event is the formation of a staggering number of neutrinos. These are subatomic particles that, taken individually, are as insignificant things as the universe produces. They are so disgusting to interact with normal matter that they can pass large amounts of material without warning; to them the earth itself is completely transparent, and they travel through it as if it were not there at all.

But when the iron core of a massive star collapses, neutrinos are formed with such high energy and in such numbers that the falling material just outside the core of the star actually absorbs a large number of them; it also helps that the material crashing down is extraordinarily dense and capable of catching so many.

The amount of energy that this self-evaporating wave of neutrinos adds to the case is enough to not only stop the collapse, but also backwards it emits more and more tons of stellar material that explodes outward at a noticeable fraction of the speed of light.

The energy of a supernova in visible light is so enormous that it can correspond to the production of an entire galaxy. Yet this is only 1% of the total energy of the event; the vast majority of it is released as energetic neutrinos. That’s how strong a role they play.

Before this was understood, theoretical astronomers had a hard time getting the nuclear collapse to actually create the explosion. Simple physical models showed that the star’s explosion would stop and a supernova would not occur. Over the years, as computers became more sophisticated, it was possible to make the equations input to the models more complicated and do a better job that matches reality. When neutrinos were added to the mixture, it became clear what important part they were adding.

The models are doing pretty well now, but there is always room for improvement. For example, we know that neutrinos come in three different kinds, called taste: rope, electron and muonne neutrinos. We also know that the flavors fluctuate under certain conditions, which means that one kind of neutrino can change to another kind. All three have different characteristics and interact differently with matter. How does this affect supernovae?

A research group investigated this. They created a very sophisticated computer model of the core of a star when it explodes, allowing neutrinos to not only change taste but also interact with each other. When this happens, the taste changes happen much faster, what they call one fast conversion.

What they found is that it includes all three taste experiences and allows them to interact and convert potentially changing conditions inside the core of the collapsed star. Eg. Neutrins may not be emitted isotropically (in all directions), but instead have an angular distribution; they can preferably be emitted in some directions.

This can have a very different impact on the explosion than assuming istropism. We know that some explosions of supernovae are not symmetrical, but occur outside the center of the nucleus or with energy bursting out in one direction more than another. The amount of energy in the neutrino release is so enormous that even a slight asymmetry can give the nucleus a huge kick and send the collapsed nucleus (now a neutron star or a black hole) out like a rocket.

The models that the researchers used are a first step in understanding this effect and how large it can be. They have shown it possible that it may be important to include all neutrino characteristics, but what happens in detail still needs to be determined.

It’s still exciting. When I went to elementary school and took classes in the inner physics of the stars, the latest models still had trouble getting stars to explode. And now we have models that not only work, but are beginning to reveal previously unknown aspects of these events. Not only that, but we can turn it around, observe real supernovae in the sky and see what their explosions can tell us about the neutrinos themselves.

It’s funny: Supernova explosions create a whole lot of the thing you see around you: Calcium in your bones, iron in your blood, the elements that make up life and air and rocks and just about everything. Neutrinos are crucial to this creation in the few moments that give birth so much that we need to live. Yet, once made, these particles ignore the matter, passing through it carelessly, and ghosts ignore the occupants as they move through walls from one place to the next.

Once made, matter is old news to neutrinos.

I anthropomorphize the universe and think it has a sense of humor. But I sometimes think the universe provides proof that I’m right.


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