On the death of stars

Before everything else, there was hydrogen.

Hydrogen, the simplest element, was also the first existing element. Even today, hydrogen makes up three-quarters of the universe. It makes sense then that stars use this abundance of hydrogen to fuel their existence. By fusing hydrogen atoms into helium, stars are able to produce light and heat and keep from collapsing.

When average-sized stars (like our sun) run out of hydrogen, they swell up into red giants, but are able to survive a little longer by fusing helium instead. It doesn’t last long though because fusing helium takes more energy than fusing hydrogen, and eventually the star dies by quietly fading out.

But not all stars are made equal, and some stars —more massive stars — instead go out with a bang. As extremely massive stars run out of hydrogen, they too are forced to start fusing heavier elements together in order to survive. Unlike smaller stars, however, they’re massive enough to fuse past helium. They start fusing heavier and heavier elements, which takes more and more energy, until they reach iron. It costs more energy to fuse iron atoms than it produces, so the star can no longer produce energy by fusion.

By this point, the star is so huge that without fusion energy to support it, gravity takes over and forces it to collapse on itself. The in-falling material compresses the core until it reaches nuclear density — the density at which the core will no longer compress. Then the material bounces off the core and rebounds into space, causing what we call a core-collapse supernova explosion.

Why does any of this matter? Well, sometimes these core-collapse supernovas leave behind artifacts like super-dense neutron stars, or even black holes. Both of those deep-sky objects are absolutely fascinating to astronomers and physicists — especially black holes, since many of the established laws of physics break down near them.

But even more importantly, supernovas scatter the heavy elements that the now-dead star produced, and the energy in a supernova can even fuel the production of even heavier elements. And we wouldn’t exist without those heavy elements! As Carl Sagan once famously said, “We’re made of starstuff.” It’s literally true! We have elements like iron and calcium in our bodies, elements that wouldn’t exist if millions of stars hadn’t exploded long ago.

So supernovas are definitely important. Yet somehow we still don’t know how they really work — for instance, how does a star actually explode? When we first tried to simulate supernovas using computer models, we couldn’t make them explode at all. The stellar material always ended up in a standing spherical shock wave — it would bounce off the core for a certain distance, but then it would just stay there.

Then we found that if something perturbed the shock wave, turbulence built up like a ripple effect and made the shock wave move around. We’re still not sure what causes that initial perturbation (we think it could be a sound wave) but this ripple effect is a well-accepted phenomenon called the Spherical Accretion Shock Instability, or SASI. The turbulence eventually grows to the point where it distorts the shock wave into a non-spherical shape, and only then will the supernova actually explode.

Now we know that turbulence causes stars to explode, but we still don’t know a whole lot about it. We all have an intuitive idea of what turbulence is. On airplane flights, pilots ask people to sit down during bumpy periods of wind turbulence; river rapids are great examples of water turbulence. To put it simply, then, turbulence is when fluids (like air or water) don’t move smoothly. Unfortunately, it turns out that not-smooth motion is difficult to describe mathematically.

The best model we have for turbulence so far involves vortices, or whirlpools. Turbulent fluid tends to move in whirlpool-like shapes, and as time goes on, those whirlpools tend to either merge together into larger whirls, or split apart into smaller whirls. We can then model turbulent energy by describing the average size of those whirlpool shapes.

That’s where my research comes in. We run supernova simulations on supercomputers in order to extract these data about the turbulent energy that drives the SASI process. We’re currently comparing two- and three-dimensional simulations — it turns out that the number of dimensions affects the way the turbulence works, and we’re trying to figure out what that difference is.

If we can do that, we would better understand the turbulence that drives supernovas. This would hopefully help us develop more accurate supernova simulations; since we can’t very well see inside an exploding star, accurate computer models are important to help us understand how supernovas work.

Supernova turbulence might sound like a completely obscure and esoteric topic, but by trying to understand the life and death of stars, we’re really learning about our own origins. We’re star-stuff studying star-stuff — as Carl Sagan so elegantly put it, “a way for the cosmos to know itself.”

And that’s pretty amazing.

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Mia de los Reyes is a freshman and Park Scholar at North Carolina State University majoring in physics and mathematics. Her research (mentored by Dr. John Blondin, Physics Department head, NC State University) will hopefully be published sometime soon. E-mail her at [email protected].