A stunning image of the supernova remnant Cas A combines visible light images from Hubble with X-ray observations by Chandra. The different colors represent different energies of the light.
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A stunning image of the supernova remnant Cas A combines visible light images from Hubble with X-ray observations by Chandra. The different colors represent different energies of the light, which tend to come from different elements like oxygen, calcium, and iron. Credit: X-ray: NASA/CXC/RIKEN/T. Sato et al.; Optical: NASA/STScI

Tasting neutrinos: Flavor changing in the cores of exploding stars

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Jan 12, 2021, 9:00 AM EST (Updated)

I have long wondered about the Universe's wry sense of humor. After all, how else 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 not only do neutrinos play an important role in supernova explosions, but we need to account for all their characteristics to truly understand why stars explode.

Stars generate energy in their cores, fusing lighter elements into heavier ones. This is how a star prevents its own gravity from making it collapse; the heat generated inflates the star, creating pressure that holds it up.

The most massive stars take this energy production process to the extreme; while lower mass stars like the Sun stop after fusing helium into carbon and oxygen, massive stars continue on, fusing elements all the way up to iron.

However, once a mighty star's core is iron, a series of events takes place that actually removes energy from the core, allowing gravity to dominate. The core collapses, setting up a huge blast of energy that is so immense it blows away the outer layers of the star, creating an explosion we call a supernova.

M 1, the Crab Nebula supernova remnant, observed by Hubble Space Telescope. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

A crucial part of this event is the generation of staggering numbers of neutrinos. These are subatomic particles, that, taken individually, are as insubstantive a thing as the Universe makes. They are so loathe to interact with normal matter that they can pass through vast amounts of material without notice; to them, the Earth itself is wholly transparent and they travel through it as if it weren't there at all.

But when the iron core of a massive star collapse, neutrinos of such high energy and in such numbers are created that the infalling material just outside the star's core actually absorbs vast numbers of them; it helps too that the material rushing downward is extraordinarily dense and able to capture so many.

The amount of energy this soul-vaporizing wave of neutrinos imparts on the matter is enough to not only stop the collapse but also reverse it, sending octillions of tons of stellar matter exploding outward at an appreciable fraction of the speed of light.

The energy of a supernova just in visible light is so huge it can equal the output of an entire galaxy. Yet this is only 1% of the total energy of the event; the vast majority of it is released as energetic neutrinos. That's how powerful a role they play.

Before this was understood, theoretical astronomers had a difficult time getting the core collapse to actually create the explosion. Simple models of the physics showed the star's explosion would stall, and a supernova wouldn't occur. Over the years, as computers got more sophisticated, it was possible to make the equations input into the models more complicated, doing a better job matching reality. Once neutrinos were added to the mix it became clear what a key part they added.

The models do quite well now, but there is always room for improvement. For example, we know that neutrinos come in three different kinds, called flavors: tau, electron, and muon neutrinos. We also know that under certain conditions the flavors oscillate, meaning one kind of neutrino can change into another kind. All three have different characteristics and interact with matter differently. How does this affect supernovae?

A team of scientists looked into this. They created a very sophisticated computer model of the core of a star as it explodes, allowing the neutrinos to not only change flavor, but also to interact with each other. When this happens the flavor changes happen much more rapidly, what they call a fast conversion.

What they found is that including all three flavors and allowing them to interact and convert does potentially change the conditions inside the collapsing star's core. For example, neutrinos may not be emitted isotropically (in all directions) but instead have an angular distribution; they can be emitted in some directions preferentially.

This can have a very different affect on the explosion than assuming istropism. We know that some supernovae explosions are not symmetric, occurring off-center in the core or with the energy blasting out in one direction more than another. The amount of energy in the neutrino release is so huge that even a slight asymmetry can give the core a huge kick, sending the collapsed core (now a neutron star or black hole) off like a rocket.

The models the scientists used are a first step in understanding this effect and how big it might be. They've shown it's possible that including all the neutrino characteristics may be important, but what happens in detail is still to be determined.

A stunning image of the supernova remnant Cas A combines visible light images from Hubble with X-ray observations by Chandra. The different colors represent different energies of the light, which tend to come from different elements like oxygen, calcium, and iron. Credit: X-ray: NASA/CXC/RIKEN/T. Sato et al.; Optical: NASA/STScI

Still, this is exciting. When I was in grad school taking classes in the physics of stellar interiors the state-of-the-art models were still having trouble getting stars to explode. And now we have models that not only work but are starting 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 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 nearly everything. Neutrinos are crucial for this creation, in a few moments giving birth to so much that we need to live. Yet, once made, these particles ignore that matter, passing through it without a care, ghosts ignoring the residents as they move through walls from one place to the next.

Once made, matter is old news to neutrinos.

I anthropomorphize the Universe, thinking it has a sense of humor. But I think sometimes the Universe provides the evidence that I'm right.

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