1987, a huge star exploded near our Milky Way galaxy. Since mankind invented the telescope, this is the brightest and the closest supernova explosion to the earth. Almost all observatories have focused their attention on this supernova explosion. Perhaps the most exciting thing is that some special detectors built deep underground also captured the mysterious subatoms emitted from this explosion: neutrinos.
In 1966, scientists proposed for the first time that these mysterious particles might be the driving force behind the explosion of supernovae. The discovery of neutrinos has comforted theoretical scientists who have been trying to understand the internal principles of supernova explosions for many years. However, for decades, astrophysicists have repeatedly encountered a dilemma. Their neutrino supernova model seems to have fatal flaws.
Neutrinos are notoriously aloof particles. Under the extreme conditions of a collapsing star, how exactly do neutrinos transfer their energy to the ordinary matter of the star remains unanswered. Whenever theoretical scientists try to model these complex particle motions and interactions in computer simulations, the shock wave of a supernova inevitably stops and then retreats. Sean Kutcher, a computational astrophysicist at Michigan State University, said that repeated failures “let us gradually begin to believe that our existing mainstream supernova explosion theory may not work.”
Of course, what is going on deep inside the supernova explosion has never been known. It is like a cauldron containing all kinds of extremes, and like a bowl of turbulent hot soup full of transmutation materials; here, the particles and forces that we usually ignore in our daily life become crucial. A more complicated problem is that the interior of supernova explosions is largely shrouded in hot gas clouds, making it difficult to see. Adam Burrows, an astrophysicist at Princeton University, has been studying supernovae for more than 35 years. He said that understanding the details of supernova explosions “has always been an unsolved core problem in astrophysics.”
However, in recent years, theoretical scientists have been able to pinpoint the extremely complex mechanisms that lead to supernova explosions. Burrows wrote in the journal Nature that supernova explosion simulations have become the norm, not the exception. The computer code of multiple competing research groups is now gradually reaching a consensus on the evolution of supernova shock waves. At the same time, simulation technology has made great progress so far, and it can even incorporate the effects of Einstein’s extremely complex general theory of relativity. At this point, people have finally been able to clear the fog and try to understand the role of neutrinos in supernova explosions.
Kutcher said: “This is a watershed.” They found that without chaos, a collapsing star may never become a supernova.
Chaos Dance
for most of the star’s life, the radiation produced by nuclear reactions inside stars brought out the stars thrust inward gravitational maintain a delicate balance. When the star’s fuel is exhausted, inward gravity begins to prevail. The core of the star itself began to collapse (suddenly collapse at a speed of 150,000 kilometers per hour), causing the temperature to surge to 100 billion degrees Celsius and fusing the core into a solid neutron ball.
The outer layer of the star will continue to collapse inward, but when the outer layer hits this incompressible neutron core, the outer layer will bounce back, generating shock waves. For the shock wave to become an explosion, it must have enough energy to push the shock wave outward to help it escape the gravitational force of the star. At the same time, the outermost layer of the star is still collapsing inward. Therefore, the shock wave must also break through the outermost inward vortex.
For a long time, scientists’ understanding of the forces that propel shock waves has been limited to the most vague terms. In the past few decades, our computers were not powerful enough to run simplified models of collapsed cores. Stars have always been regarded as perfect spheres, with shock waves spreading out in the same way from the center in every direction. However, under the one-dimensional model, when the shock wave moves outward, it will eventually slow down and then gradually weaken.
Only in recent years, with the development of supercomputers, have theoretical scientists have enough computing power to model massive stars and the complex conditions necessary to achieve supernova explosions. The best model at the moment integrates many details, such as the interaction between neutrinos and matter at the micro level, the disordered motion of fluids, and the latest developments in many different fields of physics (from nuclear physics to stellar evolution, etc.). In addition, theoretical scientists can now run multiple simulations per year, and they are free to adjust the model and try different initial conditions.
In 2015, Kutcher and his collaborators ushered in a turning point. At the time, they were running a three-dimensional computer model. The model simulates the situation of a massive star in the last few minutes of collapse. Although the simulation only depicts a 160-second view of the life of a star, it reveals the role of a previously unrecognized substance in this process. This substance turns the stagnant shock wave into a real explosion.
The particles hidden in the belly of the beast moved in disorder and chaos. Kutcher said: “It’s like boiling water in a furnace. There is also such a violent tumbling inside the star, at a speed of several thousand kilometers per second.”
This chaos creates additional pressure behind the shock wave, pushing the shock wave further away from the star’s core. The farther you are from the star’s core, the weaker the inward gravitational force and the less inwardly collapsing matter that hinders the shock wave. The chaotic matter behind the shock wave also has more time to absorb neutrinos. The energy produced by neutrinos can heat up chaotic matter and continue to turn shock waves into explosions.
For many years, researchers have not realized the importance of chaotic matter, because only a three-dimensional model can reveal its full impact. Burrows said: “What nature can do without any effort, but we need decades of time, from one-dimensional to two-dimensional to three-dimensional, to achieve.”
These simulations also show that chaos can also lead to explosions. The asymmetry of the star makes the star look a bit like an hourglass. As the explosion spreads in one direction, the matter continues to collapse toward the core in the other direction, further replenishing energy for the star’s explosion.
These new simulations allow researchers to better understand how supernovae shape the universe we see today. Burrows said: “We can get the right range of the explosion energy, and we can also get the mass of the neutron star left after the star explodes.” Supernova explosions created most of the heavy elements in the universe, such as oxygen and iron. Theoretical scientists are also beginning to use simulations to accurately predict how many of these heavy elements are. Ohio State University theoretical and computational astrophysicist Ta Geer multi-Sack Bird said: “Now we are working to solve the problem in the past could not have imagined.”
Explosion under
despite the exponential growth of computing power, but Simulations of supernova explosions are still much less than those observed in space. Harvard University astronomer Edo Berger said: “20 years ago, we could find about 100 supernovae every year. Now, we can find 10,000 or 20,000 supernovae every year.” Because we now have new telescopes, we can Scan the entire night sky quickly and repeatedly. In contrast, our theoretical scientists can only perform computer simulations about 30 times a year. A simulation that took several months can only reproduce the collapse of the star in a few minutes. Kutcher said: “You go to check the model every day, and then you find that it is only a millisecond.”
The wide accuracy of the new simulation makes astrophysicists very excited about the next close-up supernova explosion. Eileen Tambora, a theoretical astrophysicist at the University of Copenhagen, said: “While waiting for the next supernova explosion in the Milky Way, we still have a lot of work to do. We need to improve theoretical modeling to understand what we can detect. Features. This is a rare opportunity that you can’t miss. ”
Most supernovae explode because they are so far away from the Earth that the detectors cannot detect the neutrinos in them. Supernova explosions near the Milky Way, such as the 1987A supernova, occur only once every half a century.
However, if such a supernova explosion does appear, Berg said, astronomers will be able to “see directly inside the explosion” by observing its gravitational waves. He said: “Different research groups focus on different processes, which are very important to the actual explosion of stars. In addition, these different processes also have different characteristics of gravitational waves and neutrinos.”
Although theoretical scientists have reached a broad consensus on some of the most important factors in the formation of supernovae, challenges remain. In particular, Sackbird said, the result of the explosion “strongly depends” on the core structure of the star before it collapsed. The collapse of chaos will magnify small differences and lead to various results. Therefore, theoretical scientists must also accurately model the evolution of the star before it collapses.
Other issues include the role of strong magnetic fields in the rotation of the star’s core. Burrows said: “It is very likely that you will see the mixing mechanism of magnetic fields and neutrinos. The way neutrinos change from one type to another, and how this change affects supernova explosions, etc., these issues All have to be resolved.”
Tambora said: “There are still many factors that need to be added to our simulation. If there will be a supernova explosion tomorrow, and it also meets our theoretical predictions, then this may indicate that we are not currently considering The factors mentioned can be safely ignored. But if this is not the case, we need to figure out why.”
