I have long wondered about the universe’s sincere sense of humor. After all, how else can it turn out that one of the most ethereal and ghostly particles in space is radically responsible for some of the most colossal and powerful explosions in it?
New research shows that not only do mornings play an important role in supernova explosions, but we must take into account everything their characteristics to truly understand why stars explode.
Stars generate energy in their nuclei, merging the lighter elements into heavier ones. This is how the 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 an extreme; while lower-mass stars such as the Sun stop after fusing helium into carbon and oxygen, massive stars continue to fuse elements all the way to iron.
However, since the core of a powerful star is iron, a number of events occur that actually remove the energy from the core, allowing gravity to dominate. The nucleus collapses, creating a huge burst of energy that is so huge that it blows the outer layers of the star, creating an explosion we call a supernova.
A crucial part of this event is the generation of a stunning number of neutrinos. These are subatomic particles that, taken individually, are as important things as the universe does. They are so hated to interact with normal matter that they can pass through vast amounts of material without notice; for them the Earth itself is completely transparent and they travel through it as if it does not exist 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 huge number of them; it also helps that the downward material is unusually dense and able to capture so much.
The amount of energy that this soul-evaporating wave of neutrinos gives to matter is enough to stop not only the collapse, but also reverse it, sending octillion tons of stellar matter exploding outside at a considerable rate of light.
The energy of a supernova only in visible light is so enormous that it can be equal to the output of an entire galaxy. And yet this is only 1% of the total energy of the event; most of it is released as energy neutrinos. Here is how powerful a role they play.
Before that could be understood, theoretical astronomers had a hard time dealing with the collapse of the nucleus to create the explosion. Simple models of physics show that the explosion of the star will stop and there will be no supernova. Over the years, as computers improved, it was possible to make the equations introduced in the models more complex by doing a better job in line with reality. After neutrinos were added to the mixture, it became clear what key part they added.
The models are doing pretty well now, but there is always room for improvement. For example, we know that neutrinos come in three different types, called aromas: tau, electron and muon neutrinos. We also know that under certain conditions the aromas fluctuate, which means that one type of neutrino can change into another. All three have different characteristics and interact differently with matter. How does this affect supernovae?
A team of scientists examined this. They created a very sophisticated computer model of a star’s nucleus as it exploded, allowing neutrinos not only to change their taste but also to interact with each other. When this happens, taste changes happen much faster, what they call a fast conversion.
What they found was that incorporating all three flavors and allowing them to interact and convert potentially changed the conditions inside the core of the collapsing star. For example, neutrinos may not emit isotropically (in all directions) but instead have an angular distribution; they can be broadcast in some directions preferentially.
This can have a much different effect on the explosion than the assumption of isropism. We know that some supernova explosions are not symmetrical, occur outside the center of the nucleus or with the burst of energy in one direction more than another. The amount of energy in neutrino release is so enormous that even a slight asymmetry can give the core a huge blow, sending the collapsed nucleus (now a neutron star or black hole) like a rocket.
The models used by scientists are the first step in understanding this effect and how large it can be. They showed that it was so possible that the inclusion of all neutrino characteristics may be important, but what will happen in detail remains to be determined.
And yet this is exciting. When I was in the city school, taking physics classes on star interiors, modern models still had problems with star bursts. And now we have models that not only work, but begin to reveal hitherto unknown aspects of these events. Not only that, but we can reverse this, observe true supernovae in the sky, and see what their explosions can tell us about the neutrinos themselves.
It’s funny: supernova explosions create a lot of the matter you see around you: the calcium in your bones, the iron in your blood, the elements that make up life and air, the rocks, and just about anything. Neutrinos are crucial for this creation, after a few moments they give birth to so many that we have to live. And yet, once made, these particles ignore this matter, passing through it carelessly, ghosts that ignore the inhabitants as they move through walls from one place to another.
Once made, matter is old neutrino news.
I anthropomorphize the universe, thinking it has a sense of humor. But I think sometimes the universe provides evidence that I’m right.