Imagine that on your way home from work you suddenly see some fireworks. For sure you would stop and enjoy the spectacle. Maybe afterwards you would get curious who or what was actually celebrated there. In the starry sky, one can also observe spectacular and beautiful explosions: supernovae. These stellar explosions can be so bright that they can outshine an entire galaxy. Since their first observations in historic times, astronomers and physics are curious about the nature of these impressive events.
There are several different astronomical sub-types of supernovae. One prominent example are “type Ia”, in which a special kind of compact star, a so-called white dwarf, is exploding. The mechanism behind these explosions is comparable to that working in the fireworks we have on earth: some fuel is burned in an explosive manner. In terrestrial fireworks the fuel is charcoal which is oxidized in a chemical reaction, whereas in supernova Ia it is atomic nuclei in the white dwarf which undergo a nuclear fusion reaction. Therefore these supernovae are also called thermonuclear supernovae. The resulting explosion is so powerful that the entire white dwarf gets disrupted.
This article deals with another class of supernovae, so-called core-collapse supernovae, which have a different explosion mechanism that will be explained below. The astronomical supernova types Ib, Ic, and II (denoting different characteristic types of spectra) belong to this class. They represent the explosion of ordinary stars with masses above approximately ten times the mass of our sun. A famous example is the core-collapse supernova which happened in the year 1054. It was so bright that it was visible even at the day sky with the naked eye! Using a simple telescope, the leftovers of this explosion can still be observed today. It is the famous and very beautiful Crab Nebula. The figure below shows an image of the Crab Nebula obtained with the Hubble Space Telescope.
Interestingly, in 1968 a neutron star (the Crab Pulsar) was discovered in the center of the Crab Nebula. To get some clue what is happening in this kind of cosmic fireworks, and why there is a neutron star as a remnant in the center, one needs some basic understanding of the processes inside massive stars at the end of their life time. Stars are powered by continuous and steady nuclear fusion. The ultimate ashes of these nuclear burning processes is iron, which accumulates in the core of the massive star. If the iron core reaches a certain critical mass, called the Chandrasekhar-mass with a value about 1.4 times the mass of our sun, gravity becomes so strong that the core will collapse under its own gravitational weight. During this collapse a gigantic amount of gravitational energy is released.
The collapse ends when densities are reached in the center that are comparable to those in atomic nuclei. At such exceedingly high densities, the iron nuclei which made up the iron core have transformed into a new state of matter, consisting mostly of neutrons. The interactions among these neutrons at high densities are so strongly repulsive that they are able to withstand and eventually stop the collapse.
At this point the entire iron core has been compressed to the size of about only hundred kilometers. What has happened is the birth of a neutron star! Core-collapse supernova are therefore not only spectacular explosions marking the death of massive stars, but also the birth places of neutron stars. Core-collapse supernova are also interesting for another reason: during the explosion, new elements are synthesized and then ejected into the interstellar space. At least parts of the heavy elements which we find here on earth and which humans are made from were originally produced in supernovae.
The amount of gravitational energy that is released during the collapse is so huge, that it is about hundred times more than what would actually be needed to explode the star. However, it is a non-trivial question how to turn the initially collapsing, inward motion of the iron core, which can be considered as an implosion, into an explosion which is traveling outwards. Even after decades of research, this question is not completely answered yet, and turned out to be much more complex than initially thought.
At first one considered a simple bounce mechanism, where after the formation of the neutron star its outer layers are bouncing off the almost incompressible neutron matter in the center. However, detailed numerical simulations have shown that this does not work, because the bounce, which is happening, is not strong enough. The bounce generates a shock wave which initially is traveling outwards, but then stalls. No explosion takes place, and instead the outer layers of the star are collapsing further and continuously falling onto the stalled shock and the neutron star. Eventually the neutron star becomes so heavy that it will collapse to a black hole.
The most advanced simulations have revealed a possible solution to this longstanding puzzle, where neutrinos are found to play a crucial role. At first this might appear quite surprising, because neutrinos only barely interact with other particles. While you are reading this sentence, about a trillion of neutrinos have passed through your body without doing anything. How can one explode a star with such weakly interacting particles?
When the iron nuclei in the collapsing core are transformed into neutrons, a huge amount of neutrinos is produced. In fact, almost all of the released gravitational energy is initially stored in these neutrinos. The most advanced numerical simulations of core-collapse supernovae have shown that a small percentage of this energy is absorbed by the in-falling matter, which is called neutrino heating. Because there are so many neutrinos emitted from the neutron star, and because they carry so much energy, this neutrino heating can be sufficient to generate an explosion. This explosion mechanism of core-collapse supernovae is called the neutrino-driven mechanism. In some sense it resembles more an exploding pressure cooker than the fuel-burning in fire crackers.
While the neutrino-driven mechanism was shown to work in principle, there are still open questions regarding its details. For example it is known that it works only in multi-dimensional simulations of the core-collapse supernova. Using the simplifying assumption of spherical symmetry instead, explosions cannot be obtained, because the neutrino heating is not strong enough. However, for a full three-dimensional simulation with high resolution, an accurate description of neutrinos, and all the other physics involved in the supernova, even the fastest super-computers of the world are not yet fast enough! All of the current simulations therefore have to rely on approximations, which introduces uncertainties in addition to the uncertainties coming from unknown physics. The picture below shows a snapshot from the central part of a three-dimensional core-collapse supernova simulation of the astrophysics group at the University of Basel.
Researchers working in this field are eagerly waiting for the next close-by core-collapse supernova to come. The expectation is that in our own galaxy a few of them occur per century. The last close-by one happened in 1987. With the present-day, more advanced telescopes and neutrino detectors it will be possible to pin-down the details of the explosion mechanism of core-collapse supernovae and to get answers to the remaining open questions. The modern observations will provide something like a real-time video of the supernova, the processes in its interior, and the birth of the neutron star. Core-collapse supernova also emit gravitational waves. While being weaker than for example from neutron-star mergers, nevertheless the gravitational waves of a core-collapse supernova can be measured if it happens close enough.
The next galactic core-collapse supernova will not only be some fascinating and beautiful firework, but also lead to a fireworks of new discoveries. It will advance our understanding of several aspects of astrophysics and provide answers to some long-standing open questions. So watch out for the next galactic core-collapse supernova!
Authos: Matthias Hempel, Oliver Heinimann, Kevin Ebinger, Matthias Liebendörfer