I have long wondered about the cruel sense of humor in the Universe. After all, how 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 out there?
New research indicates that neutrinos not only play an important role in supernova explosions, but we must also take them into account. all its characteristics to really understand why the stars explode.
Stars generate energy in their nuclei, fusing lighter elements with heavier ones. This is how a star prevents its own gravity from collapsing it; the heat generated inflates the star, creating a pressure that keeps it.
The most massive stars take this process of energy production to the extreme; while lower-mass stars like the Sun stop after fusing helium with carbon and oxygen, continue to ignite massive stars, which fuse elements down to iron.
However, once the core of a powerful star is iron, a series of events occur that actually remove energy from the core and allow gravity to dominate. The nucleus collapses, creating a huge explosion of energy that is so immense that it makes the outer layers of the star disappear, creating an explosion we call a supernova.
A crucial part of this event is the generation of an astonishing number of neutrinos. These are subatomic particles that, taken individually, are as insubstantial as the Universe does. They are so disgusting to interact with normal matter that they can pass through large amounts of material without warning; for them, the Earth itself is completely transparent and they travel through it as if it were not there.
But when the iron core of a massive star collapses, neutrinos of such high energy are created and in such large numbers that the material that falls just outside the star’s core absorbs a large amount of it; it also helps the material running downwards to be extraordinarily dense and capable of capturing so many.
The amount of energy that this wave of neutrinos vaporizes the soul is enough to not only stop the collapse, but also other way round sending octilions of tons of stellar matter that explode outward at an appreciable fraction of the speed of light.
The energy of a supernova with visible light is so great that it can match the output of an entire galaxy. However, this is only 1% of the total energy of the event; the vast majority are released as energy neutrinos. So powerful they have a role.
Before understanding this, theoretical astronomers had difficulty getting the nucleus to collapse to actually create the explosion. Simple physics models showed that the star’s explosion would stop and that a supernova would not occur. Over the years, as computers became more sophisticated, it became possible to complicate the equations introduced into the models, making them a better reality. Once neutrinos were added to the mixture, it became clear which key part they added.
The models work pretty well now, but there is always room for improvement. For example, we know that neutrinos come in three different types, called flavors: tau neutrinos, electrons and muons. We also know that under certain conditions the flavors oscillate, that is, that one type of neutrino can change to another. All three have different characteristics and interact with matter differently. How does this affect supernovae?
A team of scientists studied this. They created a very sophisticated computer model of the core of a star when it explodes, allowing neutrinos to not only change flavor, but also interact with each other. When this happens, flavor changes occur much more quickly, what they call a fast conversion.
What they found is that including the three flavors and allowing them to interact and convert can change conditions within the core of the collapsed star. For example, neutrinos may not be emitted isotropically (in all directions), but have an angular distribution; they can be emitted preferably in some directions.
This can have a very different effect on the explosion than assuming hissing. We know that some supernova explosions are not symmetrical, they occur off-center in the core or with energy exploding in one direction rather than another. The amount of energy in neutrino release is so great that even a slight asymmetry can give a big kick to the nucleus, 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 how big it could be. They have proved it possible that including all the characteristics of neutrinos may be important, but what happens in detail is yet to be determined.
Still, that’s exciting. When I was in undergraduate school, taking stellar interior physics classes, the latest generation models still had trouble getting the 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 upside down, observe real supernovae in the sky and see what their explosions about neutrinos themselves can tell us.
It’s funny: supernova explosions create a good deal 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 everything. Neutrinos are crucial to this creation, in a few moments they give birth so much that we need to live. Yet, once done, these particles ignore that matter, passing it carelessly, and ghosts ignore residents as they move through walls from place to place.
Once done, matter is old news for neutrinos.
I anthropomorphize the Universe, thinking it has a sense of humor. But I think sometimes the Universe provides the evidence that I am right.