Blowing Up Stars: A 50 Year Old Question Goes Down?

 

by Adam Frank

NASA/JPL-Caltech/R. Gehrz (University of Minnesota)

  The Crab Nebula is the shattered remnant of a massive star that ended its life in a supernova explosion.

There are some critical, crossroads problems in science that just refuse to go away. They linger like ghosts haunting researchers for decades and eluding all attempts at resolution. Entire careers are spent searching for an answer and sometime entire lifetimes go by without that answer found. But every so often one of those vexing problems on which so much depends falls to human effort and ingenuity.
Today may be one of those days.
Supernova, the apocalyptic explosions of massive stars, are the brightest most energetic events in the Cosmos second only to the Big Bang which birthed the Universe itself. Visible from across the Universe Supernovae result from the self-immolation of stars. More than fireworks supernova matter acting as nuclear forges that create many of the heavy elements on which life depends. But living at the heart of these beasts is a mystery that 50 years and untold effort has been unable to solve. Until perhaps today.
When a massive star (more than eight times the sun mass) reaches the end of its life it runs out of nuclear fuel. With no visible means of support against its own titanic gravity, the star comes crashing down on itself squeezing tremendous amounts of mass into an ever smaller space. In less than seconds, a kind of nuclear alchemy occurs in the compressed center. A giant hyperdense core (a proto-neutron star) forms like a hard rubber ball that can only be squeezed so far. The dense core resists further compression. As the outer layers of the star that are still freefalling inwards slam into this nuclear brick wall a rebounding shock wave forms that blows the star apart.
At least that was the story.
The problem was the story never really worked. For 50 years, astronomers have been trying to find some variation on this theory –- the story –- that could make a massive dying star blow up. The added rotation, included the effects of ghostly particles called neutrinos, thought about jets and magnetic fields forming at the ultra dense core. Some of these ideas almost work, or work in some cases. But when explored in detail, they never produced a convincing, universal mechanism for creating the supernova that we know exist. It was a great and grand puzzle. Now, perhaps, the puzzle has been solved.
Jason Nordaus is a post-doctoral student working with Professor Adam Burrows at Princeton University. In a paper that has just appeared today, Nordhaus may have found the key to nature’s most extreme fireworks. Now you will have to excuse me for being a bit proud as Nordhaus got his PhD in our theory group at University of Rochester and we love our graduates going on to do great work with other scientists. If this result holds up it will be very important for astronomy. More importantly the answer Nordhaus found goes beyond science and directly touches our beloved Cosmos & Culture theme.
Using very high performance computers Nordaus ran simulations of exploding stars that focused on one key piece of physics. Squeezing matter produces lots of particles called neutrinos. The absorption of these neutrinos near the shock wave had already been suggested as a way to power it up and blow the star to bits.
What mattered for Nordaus was the ability to simulate the explosions — in detail — in 3-D. Lacking the computational power many previous studies were forced perform simulations in lower dimensions. That means imagining the star to be highly symmetric limiting the ability for neutrinos to deposit their energy in the gas and blow the star up. By tracking the full lumpy, bumpy, 3-D behavior of the collapsing star and the neutrinos Nordau’s found he could consistently get his model stars to explode.
The answer to this 50-year-old mystery turned out to be “nothing more” than our inability to explore detailed physics in its detailed 3-D behavior. That is where the link between Cosmos and Culture emerges.
Nordhuas’ result was simulation at the “Peta”scale frontier. PetaBytes means a million, billion bytes. PetaFlops means a million billion computations a second. That is the edge computational science is now crossing. With it comes our first real shot at virtual reality. Things are going change at Petascale domains in everything from video games to immersive computing (you don’t drive your car, your car drives you for example).
As we cross this frontier in machine power we can expect explosions in a lot more than model stars.