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I have long wondered about the universe’s ironic sense of humor. After all, how come one of the most ethereal, ghostly particles in the cosmos is fundamentally responsible for some of the most colossal and violent explosions?
New research indicates neutrinos not only play an important role in supernova explosions, but we need to take this into account all their characteristics to really understand why the stars explode.
Stars generate energy in their nucleus, fusing lighter elements into heavier ones. This is how a star prevents its own gravity from causing it to collapse; the heat generated inflates the star, creating a pressure that holds it back.
The most massive stars take this process of energy production to the extreme; while lower mass stars like the Sun stop after fusing helium into carbon and oxygen, massive stars continue, merging elements to iron.
However, once the core of a powerful star is made of iron, a series of events occur that remove energy from the core, allowing gravity to dominate. The nucleus collapses, creating a huge explosion of energy that is so immense that it washes away the outer layers of the star, creating an explosion that we call a supernova.
A crucial element of this event is the generation of an impressive number of neutrinos. They are subatomic particles which, taken individually, are as insubstantial a thing as what the Universe does. They are so reluctant to interact with normal matter that they can pass through large amounts of matter without warning; for them, the Earth itself is completely transparent and they pass through it as if it was not there at all.
But when the iron core of a massive star collapses, neutrinos of such high energy and in such numbers are created that the infallible material just outside the star’s core actually absorbs one. large number; it also helps that the material rushing down is extraordinarily dense and able to capture so much.
The amount of energy that this soul vaporizing neutrino wave imparts to matter is enough not only to stop the collapse, but also reverse it, sending octillions of tons of stellar matter exploding outward at a sizable fraction of the speed of light.
The energy of a supernova just in visible light is so huge that it can equal the output of an entire galaxy. Yet this is only 1% of the total energy of the event; the vast majority are released as energetic neutrinos. This is how powerful they play a role.
Before it was understood, theoretical astronomers struggled to collapse the core to actually create the explosion. Simple models of physics have shown that the star’s explosion will stall and a supernova will not occur. Over the years, as computers got more sophisticated, it was possible to make the equations entered into the models more complicated, which made it possible to better match reality. Once the neutrinos were added to the mix, it became clear what key element they added.
The models are working pretty well now, but there is always room for improvement. For example, we know that neutrinos are of three different types, called the flavours: tau, electron and muonic neutrinos. We also know that under certain conditions the flavors oscillate, which means that one type of neutrino can transform into another type. All three have different characteristics and interact with matter in different ways. How does this affect supernovae?
A team of scientists has looked into this question. They created a very sophisticated computer model of a star’s core when it exploded, allowing neutrinos not only to change flavor, but also to interact with each other. When this happens, the flavor changes occur much faster, which they call a fast conversion.
What they found was that including all three flavors and allowing them to interact and convert potentially alters conditions inside the collapsing star core. For example, neutrinos may not be emitted isotropically (in all directions) but instead have an angular distribution; they can be emitted preferentially in certain directions.
This can have a very different effect on the explosion than the hypothesis of an istropism. We know that some supernova explosions are not symmetrical, occur off-center in the nucleus or with energy exploding in one direction more than another. The amount of energy in the release of neutrinos is so huge that even a slight asymmetry can give the nucleus a huge kick, sending the collapsed nucleus (now a neutron star or black hole) like a rocket.
The models used by scientists are a first step in understanding this effect and its magnitude. They showed that it is possible that the inclusion of all the characteristics of the neutrinos may be important, but what happens in detail remains to be determined.
Yet it is exciting. When I was in high school and taking a course in stellar interior physics, cutting-edge models still had a hard time blowing up stars. And now we have models that not only work but are starting to reveal hitherto unknown aspects of these events. Not only that, but we can turn the tide, observe real supernovae in the sky, and see what their explosions can tell us about the neutrinos themselves.
It’s funny: Supernova’s explosions create a lot of the material you see around you: calcium in your bones, iron in your blood, the elements that make up life, air and rocks and almost all. Neutrinos are crucial to this creation, giving birth in a few moments to so many things that we need to live. Yet, once done, these particles ignore this matter, moving through it carefree, the ghosts ignoring the residents as they move through the walls from place to place.
Once made, matter is old news for neutrinos.
I anthropomorphize the Universe, thinking it has a sense of humor. But I think sometimes the Universe provides proof that I’m right.
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