In 1987, a a giant star exploded right next to our own Milky Way galaxy. It was the brightest and closest supernova since the telescope was invented four centuries earlier, and almost every observatory has turned to take a look. Perhaps the most exciting, specialized observatories buried deep underground capture shy subatomic particles called neutrinos coming out of the explosion.
These particles were first proposed as the engine of supernovas in 1966, which made their detection a source of comfort to theorists who had tried to understand the inner workings of explosions. Yet over the decades, astrophysicists have constantly encountered what appeared to be a fatal flaw in their neutrino models.
Neutrinos are very distant particles, and questions remained as to exactly how neutrinos transfer their energy to ordinary star matter under the extreme conditions of a collapsing star. Whenever theorists tried to model these complex movements and interactions of particles in computer simulations, the shock wave from the supernova would jam and fall back on itself. The failures “have entrenched the idea that our main theory of how supernovas may explode doesn’t work,” said Sean Couch, a computer astrophysicist at Michigan State University.
Of course, the specifics of what goes on deep inside a supernova when it explodes have always been a mystery. It is a cauldron of extremes, a turbulent soup of transmuting matter, where particles and forces often overlooked in our everyday world become critical. To compound the problem, the explosive interior is largely hidden from view, enveloped in clouds of hot gas. Understanding the details of supernovas “has been a central unsolved problem in astrophysics,” said Adam Burrows, an astrophysicist at Princeton University who has studied supernovas for more than 35 years.
In recent years, however, theorists have been able to focus on the surprisingly complex mechanisms that make supernovas work. Exploding simulations have become the norm, rather than the exception, Burrows wrote in Nature this month. Rival research groups’ computer codes now agree on how supernova shockwaves evolve, as simulations have advanced so far that even the effects of Einstein’s notoriously complex general relativity are included. The role of neutrinos is finally understood.
“This is a watershed moment,” Couch said. What they find is that without turbulence, collapsing stars may never form supernovas at all.
A chaotic dance
For much of a star’s life, the inward pull of gravity is delicately balanced by the outward thrust of radiation from nuclear reactions inside the star’s core. As the star runs out of fuel, gravity sets in. The nucleus collapses on itself – plunging 150,000 kilometers per hour – causing temperatures to rise to 100 billion degrees Celsius and merging the nucleus into a solid ball of neutrons.
The outer layers of the star continue to fall inward, but when they hit this incompressible neutron nucleus, they bounce off it, creating a shock wave. In order for the shock wave to become an explosion, it must be pushed outward with enough energy to escape the star’s gravity pull. The shock wave must also fight against the inner spiral of the star’s outermost layers, which continue to fall on the core.
Until recently, the forces fueling the shock wave were understood only in the most vague terms. For decades, computers were only powerful enough to run simplified models of the collapsing kernel. The stars were treated as perfect spheres, with the shock wave emanating from the center equally in all directions. But as the shock wave travels outward in these one-dimensional models, it slows down, then weakens.
It is only in the last few years, with the growth of supercomputers, that theorists have had enough computing power to model massive stars with the complexity necessary to make explosions. The best models now incorporate details such as micro-level interactions between neutrinos and matter, disordered movements of fluids, and recent advances in many areas of physics – from nuclear physics to stellar evolution. In addition, theorists can now perform numerous simulations each year, allowing them to freely adjust models and try different starting conditions.