3D supernova simulations reveal why they explode
Turbulent matter swirls around the center of the collapsing star. Turbulence gives an additional impetus to the supernova shock wave (blue), after which the dense core of the star located in the center forms a neutron star
In 1987, a giant star exploded near our Milky Way Galaxy. It was the brightest and closest supernova since the invention of the telescope (almost 400 years ago), and almost all observatories turned in this direction to take a closer look at the event. The most interesting result of the observation was that special observatories located deep underground were able to detect shy subatomic particles, neutrinos, the stream of which rushed from the center of the explosion.
The first suggestion that these particles are the driving force of supernova explosions was made in 1966. Finding these particles has become a source of comfort for theorists trying to understand how explosions work. However, in the decades that followed, astrophysicists constantly stumbled upon one seemingly fatal lack of neutrino-based models.
Neutrinos are known to be indifferent, and how exactly neutrinos transfer energy to ordinary star matter under extreme collapse conditions remained unclear. In simulations of the movement and interaction of particles on a computer, theorists have always worked out so that a supernova blast wave stops and falls back onto the star. Because of all these failures, "the idea is ingrained that our leading theory of supernova explosions doesn't work," said Sean Couch , a computational astrophysicist at Michigan State University.
Of course, the specific processes taking place in the depths of the supernova during the explosion have always remained a mystery. It is a cauldron of extreme conditions, a turbulent soup of transforming matter. The particles and forces that we usually ignore in our daily life become critical. To make matters worse, the interior of the explosion is largely hidden from view by clouds of hot gas. Understanding the details of how supernovae work “has been a central unsolved problem for astrophysics,” said Adam Burroughs , an astrophysicist at Princeton University who has studied supernovae for over 35 years.
However, in recent years, theorists have been able to gain a deeper understanding of the surprisingly complex processes of supernovae. Exploding simulations have become the norm, not the exception, as wrote Burroughs in the journal Nature in January 2021. The computer programs of rival research teams agree on how shock waves evolve in a supernova explosion. Simulations have gone so far as to include even details of Einstein's extremely complex general theory of relativity. The role of neutrinos is finally beginning to be understood.
“This is a watershed moment,” said Couch. Physicists have discovered that without turbulence, collapsing stars could not form supernovae at all.
Chaos dance
For most of the life of a star, the gravitational attraction acting towards the center is in unstable equilibrium with the outward pressure of radiation from nuclear reactions occurring in the star's core. When a star runs out of fuel, gravity wins. The star collapses at a speed of 150,000 km / h, which abruptly raises the temperature to 100 billion ° C and melts the star's core, turning it into a solid ball of neutrons.
The outer layers of the star continue to fall inward, however, when colliding with this incompressible neutron core, they bounce off it, creating a shock wave. For a shock wave to become an explosion, it must be accelerated outward with enough energy to overcome the gravitational pull of the star. Also, the shock wave has to fight against the outer layers of the star falling inward, onto the core.
Until recently, little was known about the forces driving the blast wave. For decades, computers were not strong enough to work only with simplified models of a collapsing kernel. The stars were considered ideal spheres, and the shock wave spread out from the center symmetrically in all directions. But in these one-dimensional models, the blast waves slow down as they move, after which they subside.
Only in the last few years, with the increase in the power of supercomputers, have theorists had enough computer power to build sufficiently complex models of massive stars capable of producing explosions. The best models to date take into account interactions between neutrinos and matter, the disordered movement of fluids, and recent advances in science from nuclear physics to stellar evolution. Moreover, theorists can run several simulations a year , tweak the model settings, and experience different initial conditions.
One of the turning points happened in 2015 when Couch and his colleagues launched a 3D computer model of the last minutes.collapse of a massive star. Although the simulation only covered 160 seconds of the star's life, it clearly revealed the role of an underestimated force in helping to transform decelerating blast waves into full-blown explosions.
In the womb of the monster, the particles spun and dashed chaotically. “It's like water boiling in a saucepan. A liquid is rotating in a star, moving at a speed of thousands of kilometers per second, ”said Couch.
Turbulence creates additional pressure in the blast wave, pushing it away from the center of the star. And the farther from the center, the weaker the gravitational attraction, and the less often the density of the matter falling towards the center, capable of pacifying the blast wave. Also, turbulent matter moving under the cover of a shock wave has more time to absorb neutrinos. Then this energy from the neutrino heats up the matter and accelerates the blast wave to the explosion of the star.
Researchers have underestimated the importance of turbulence for many years, since it only shows itself fully in 3D simulations. “It took us decades of work to do what nature can do without difficulty. We gradually moved from one dimension to two and then to three, ”Burroughs said.
In the first half second after the collapse of the star's core, swirling matter surrounds it. In this simulation, the colors of matter are assigned depending on the entropy, the measure of disorder [or rather, the measure of information about the system / approx. per.] (the closer to red, the greater the entropy). Due to turbulence, the explosion is asymmetric.
It also became clear from simulations that turbulence leads to asymmetric explosions, in which the star looks a bit like an hourglass. The explosion creates pressure in one direction, and matter continues to fall on the star's core in the other, further fueling the explosion.
The new simulations are giving researchers a better understanding of how supernovae have shaped the universe today. “We can get the right range of explosive energies, and the masses of neutron stars left behind,” Burroughs said. Supernovae are mainly responsible for fueling the budget of the heavy elements of the universe like oxygen and iron, and theorists are starting to use simulations that predict specific proportions of heavy elements in space. “We're starting to tackle problems that were never imagined to be solved in the past,” said Tuguldur Sukhbold , a computational theoretical astrophysicist at Ohio State University.
Next explosion
Despite the exponential growth of computing power, supernova simulations are carried out much less frequently than observations of them. “20 years ago, we found about 100 supernovae every year,” said Ido Berger , an astronomer at Harvard University. "Today, we open 10,000 to 20,000 units annually." The increase in the number of observations is due to new telescopes that quickly and regularly survey the night sky. Theorists carry out about 30 computer simulations a year. One simulation, which recreates a few minutes from the process of nuclear collapse, takes several months. “You check every day and it’s only one millisecond past,” Couch said. "It's like watching molasses flow in the cold."
The accuracy of the new simulations makes astrophysicists look forward to the next explosion, which would be near us. “While we wait for the next supernova in our galaxy, we still have a lot to do. We need to improve theoretical models to understand what process features we might detect, ”said Irene Tamborra , a theoretical astrophysicist at the University of Copenhagen. "The opportunity should not be missed, because it is such a rare event."
Most supernovae are ignited too far from Earth for ground-based observatories to be able to detect their neutrinos. Supernovae in the immediate vicinity of the Milky Way - such as SN 1987A - occur on average about once every half century....
But if a supernova occurs, astronomers can "look directly into the center of the explosion," Berger said. This will be possible thanks to the observation of gravitational waves. “Different groups consider the different processes that take place during the explosion to be important. And for all these processes gravitational waves and neutrino fluxes look different ”.
And while today theorists have practically agreed on the most important factors underlying supernovae, difficulties still remain. In particular, the outcome of the explosion "highly depends" on the structure of the star's core before the explosion itself, Sukhbold said. Small differences increase, leading to different results of chaotic collapse. Therefore, the evolution of the star, which preceded the collapse, must also be carefully modeled....
Other questions include the role of the strong magnetic fields that arise in the rotating core of a star. “It is possible that there could be a hybrid mechanism of magnetic fields and neutrinos at work,” Burroughs said. It is also unclear how exactly neutrinos change their type - "sort" - from one to another, and how this affects the explosion.
“There are still many ingredients to add to the simulations,” Tamborra said. - If a supernova explodes tomorrow, and it coincides with our theoretical predictions, then all the other ingredients that we lack today can be ignored. But if that doesn't happen, we'll need to figure out why. "