I have long wondered about the cruel sense of humor of the Universe. After all, how different 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 that we must also account for them everyone their properties to truly understand why stars explode.
Stars generate energy in their nucleus and fuse lighter elements into heavier elements. This is how a star prevents its own gravity from causing it to collapse; the heat generated inflates the star and creates pressure that holds it.
The most massive stars take this energy production process to the extreme; while stars with a lower mass like the sun stop after melting helium into carbon and oxygen, massive stars continue to melt elements up to iron.
Once the core of a mighty star is iron, a series of events take place that actually remove energy from the core, causing gravity to dominate. The nucleus collapses and creates an enormous explosion of energy that is so large that it blows away the outer layers of the star and creates an explosion that we call a supernova.
An important part of this event is the generation of incredible numbers of neutrinos. These are subatomic particles, which, taken individually, are just as insignificant as the universe makes. They are so disgusted to communicate with normal material that they can go through large amounts of material without notice; to them the earth itself is completely transparent and they travel through it as if it is not there at all.
But when the iron core of a massive star collapses, neutrinos are created with such high energy and in such numbers that the shower material just outside the star core absorbs a large amount of it; it also helps that the material rushing down is extraordinarily dense and can trap so many.
The amount of energy that this soul-evaporating wave of neutrinos brings to the matter is enough to not only stop the collapse, but also reversed that, which still causes tons of stellar matter to explode outward at a significant fraction of the speed of light.
The energy of a supernova only in visible light is so great that it can compare the output of an entire galaxy. Yet it is only 1% of the total energy of the event; the vast majority of them are released as energetic neutrinos. This is how powerful a role they play.
Before it was understood, theoretical astronomers had a hard time getting the nuclear collapse to cause the explosion. Simple physics models showed that the explosion of the star would stop, and that a supernova would not occur. Over the years, as computers became more sophisticated, it was possible to make the comparisons’ input into the models more complex and to do a better job of matching reality. After neutrinos were added to the mixture, it became clear what important part they added.
The models are doing pretty well now, but there is always room for improvement. We know, for example, that neutrinos occur in three different species, called scents: rope, electron and muon neutrinos. We also know that the odors oscillate under certain conditions, which means that one type of neutrino can change into another type. All three have different properties and vary with matter in different ways. How does this affect supernovae?
A team of scientists investigated it. They created a very sophisticated computer model of the core of a star as it exploded, allowing neutrinos to not only change in taste but also communicate with each other. When this happens, the taste changes occur much faster, which they ‘call’ quick conversion.
What they found is that it changes the circumstances at the core of the collapsing star if we include all three of the scents and allow it to communicate and switch. Neutrinos, for example, cannot be emitted isotropically (in all directions), but rather have an angular distribution; they can preferably be radiated in some directions.
This can have a very different effect on the explosion than assuming is istropism. We know that some supernovae explosions are not symmetrical but do not occur in the center, or that the energy blows out more than the others in one direction. The amount of energy in the neutrino release is so large that even a slight asymmetry can give the nucleus a large kick, which can send the folded nucleus (now a neutron star or black hole) like a rocket.
The models used by the scientists are a first step in understanding this effect and how large it can be. They showed it is possible that it may be important to include all the neutrino properties, but what still happens in detail has yet to be determined.
This is exciting after all. When I went to elementary school in the physics of star interiors, the latest models were still having trouble making stars explode. And now we have models that not only work, but also begin to reveal the unknown aspects of these events. Not only that, but we can also turn it around, observe real supernovae in the air and see what their explosions can tell us about the neutrinos themselves.
This is funny: Supernova explosions create a fair amount 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 and rocks and almost everything. Neutrinos are crucial to this creation, and within a few moments it will give birth to so many that we must live. Once these particles are made, they ignore the matter and go through it carefully, and ghosts ignore the inhabitants as they move through walls from one place to the next.
Once made, matter is old news for neutrinos.
I anthropomorphize the Universe and think it has a sense of humor. But I think sometimes the universe gives the proof that I’m right.