I’ve long wondered about the universe’s wry sense of humor. After all, how else could 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 also need to take that into account all of them their characteristics to really understand why stars explode.
Stars generate energy in their cores and fuse lighter elements into heavier ones. This is how a star prevents its own gravity from collapsing it; the generated heat inflates the star, creating pressure that holds it back.
The heaviest stars take this energy production process to the limit; while lower-mass stars like the sun stop after fusing helium into carbon and oxygen, massive stars continue and fuse elements into iron.
But once the core of a mighty star is iron, a series of events take place that essentially remove energy from the core, allowing gravity to dominate. The core collapses, creating a massive burst of energy so immense that it blows away the star’s outer layers, creating an explosion called a supernova.
A crucial part of this event is the generation of staggering numbers of neutrinos. These are subatomic particles that, taken in isolation, are just as immaterial as the universe. They hate interacting with normal matter so much that they can pass through vast 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 of such high energy and numbers are created that the incident material just outside the star’s core actually absorbs large numbers of them; it also helps that the material rushing down is extremely dense and can catch so much.
The amount of energy that this soul-evaporating wave of neutrinos gives to matter is enough to not only stop the collapse, but also reverse it, in which octillions explode tons of stellar matter outward at a significant fraction of the speed of light.
The energy of a supernova in visible light alone is so great that it can match the output of an entire galaxy. Yet this is only 1% of the total energy of the event; the vast majority of them are released as energetic neutrinos. That’s how powerful they play a role.
Before this was understood, theoretical astronomers had a hard time getting the core to collapse to actually cause the explosion. Simple models of physics showed that the star’s explosion would stop and that no supernova would occur. Over the years, as computers became more advanced, it was possible to make the equations entered into the models more complicated, making them more consistent with reality. Once neutrinos were added to the mix, it became clear what an important part they were adding.
The models are doing quite well now, but there is always room for improvement. For example, we know that neutrinos come in three different types called flavors: tau, electron and muon neutrinos. We also know that the flavors oscillate under certain conditions, which means that one kind of neutrino can change into another kind. All three have different characteristics and deal with matter differently. How does this affect supernovae?
A team of scientists has investigated this. They created a highly sophisticated computer model of a star’s core as it explodes, allowing the neutrinos to not only change their taste, but also communicate with each other. When this happens, the changes in taste happen much more quickly, which is a lot fast conversion.
What they found is that taking in all three flavors and allowing them to communicate and convert may change conditions at the core of the collapsing star. For example, neutrinos are not emitted isotropically (in all directions), but instead have an angular distribution; they can preferably be broadcast in some directions.
This can have a very different effect on the explosion than assuming istropism. We know that some supernova explosions are not symmetrical, do not occur in the center of the nucleus, or where the energy shoots out more in one direction than the other. The amount of energy in the neutrino release is so great that even a small asymmetry can give the core a huge kick, causing the collapsed core (now a neutron star or black hole) to fly away like a rocket.
The models the scientists used are a first step to understand this effect and how large it can be. They have shown that it is possible that recording all neutrino characteristics can be important, but what happens in detail remains to be determined.
Still, this is exciting. When I was in high school taking classes in the physics of star interiors, the ultra-modern models still struggled to make stars explode. And now we have models that don’t work alone, but begin to reveal previously unknown aspects of these events. Not only that, but we can turn this 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 fair amount of the matter that you see around you: the calcium in your bones, the iron in your blood, the elements that make up life and the air and rocks, and almost everything else. Neutrinos are crucial to this creation, in a few moments they give birth to so much that we have to live. But once created, these particles ignore that matter, pass through it carelessly, ghosts ignore the inhabitants as they move through walls from one place to another.
Once created, matter is old news to neutrinos.
I anthropomorphize the universe, thinking it has a sense of humor. But I think sometimes the universe proves that I am right.