Collapsing stars, supernovae, and gamma-ray bursts

by Hans-Thomas Janka

When massive stars die, they don't just fade away. Their lives end in the most spectacular and most luminous explosions that we know. For weeks they can become nearly as bright as a whole galaxy. The stellar debris is expelled with velocities up to ten percent of the speed of light and an energy of motion that equals the radiation of the Sun during its whole life. In rare cases this amount of energy can even be released in an enormously intense flash of gamma radiation. Such a gamma-ray burst outshines all stars of the universe for a period of seconds to many minutes and can be accompanied by a stellar explosion ten or even fifty times more energetic than usual.

X-ray images of Cassiopeia A, taken by the Chandra satellite. Cas A is the gaseous remnant of  a supernova, which exploded around 1680 A.D. The compact remnant, which is probably a neutron star but no pulsar, can be seen as bright spot near the geometrical center of the expanding gas cloud.

These truely gigantic events have fascinated astronomers ever since they had discovered that the night sky is not as unchangeable as it might seem at first glance. The "new star" whose appearance Chinese astronomers had reported in 1054 has left a glorious gaseous remnant now known as the Crab nebula. The term "supernova" for these outbursts was coined by the Swiss astronomer Fritz Zwicky in 1933, and it was Zwicky and his colleague Walter Baade who first speculated that supernovae are the explosions of stars. They suggested that the core of the star collapses to a neutron star, an ultralarge atomic nucleus which has the mass of the Sun and the diameter of a big city and consists mostly of neutrons. The release of gravitational energy during its formation would be sufficient to power the ejection of the outer stellar layers in a supernova explosion. It took more than 30 years before the discovery of a pulsar in the Crab nebula and its interpretation as a rapidly spinning neutron star confirmed the existence of such exotic objects and established their role as relics of supernova explosions.

Supernovae are of pivotal importance for many questions in astrophysics. Not only does their extreme energy release heat the interstellar medium and thus has important influence on the gas dynamics and history of galaxies. Setting the end point of the life of massive stars, they disseminate the chemical elements which were assembled during the stars' millions of years of evolution. The extreme temperatures, densities, and neutron-richness during the explosion enable the formation of nuclei heavier than iron and in particular of radioactive material. Elements that make up the crust and atmosphere of the Earth and form the basis of life,  e.g. carbon, oxygen, silicon, and iron, but also rare, very heavy elements like gold and uranium, are thus produced and mixed into the interstellar gas.

The brilliant display of the explosion that is visible by radiation in different wavelengths, is actually only a weak aftermath of the violent processes that occur in the interior of a dying star. The core of a supernova provides the most extreme conditions in the universe after the big bang. Supernovae are therefore laboratories of great interest for nuclear and particle physics. Reactions of electrons and positrons with neutrons and protons, the constituents of atomic nuclei, produce a huge number of neutrinos. These neutral elementary particles interact only relatively weakly with the stellar matter. Because they escape fastest from the dense center, they carry away about 99 percent of the gravitational binding energy that is set free during the neutron star formation. The tiny rest only drives the mass ejection.

Hubble Space Telescope image of the Crab Nebula. High-energy particles accelerated by the Crab pulsar, a neutron star spinning with a period of 33 ms at the center of the remnant, cause the bluish glow of the interior. The outer filaments are the tattered remains of the star and consist mostly of hydrogen.

When massive stars evolve quietly for millions of years, nuclear reactions build up successively heavier elements inside the stellar core. In a sequence of accelerating stages hydrogen burns to helium, helium to carbon and oxygen, these further to neon and magnesium, then silicon, sulfur and calcium, until finally iron and nickel are formed. Reaching there, further energy release by nuclear fusion is not possible, because the nuclei of the iron-group elements have the highest binding energies per nucleon. The stellar core of iron resists the pull of its own gravity mostly by the quantum mechanical pressure of a gas of degenerate electrons and to a smaller degree by the thermal pressure of the matter. But when the core grows and contracts, its density and temperature becomes so large that electrons are squeezed into the atomic nuclei and high-energy photons start breaking up iron to alpha particles (i.e. helium nuclei) in a process called photodisintegration. Both reduces the pressure support in the core, leading to further contraction and accelerated electron captures and photodisintegration. A catastrophic implosion is thus unavoidable. Within fractions of a  second the stellar core collapses to form a neutron star.

Only when the density reaches that of nuclear matter, strong repulsive forces between neutrons and protons, the constituents of nuclear matter, prevent further compression and the infall is abruptly stopped. A powerful shock wave is launched as the outer layers of the stellar core crash with supersonic speed onto the rebounding central part. For a while it was thought that this shock waves races directly through the star and causes its disruption in the supernova explosion. Detailed computer simulations, however, showed that this does not happen. The shock expansion is damped by energy losses mainly in photodisintegration reactions until it finally comes to a halt before the shock is able to leave the stellar core.

But how is the explosion initiated? How can the collapse of the stellar core be inverted and infalling matter be lifted out of the increasingly deeper gravitational well of the forming neutron star? The answer is not finally clear. Different possibilities are currently studied with computer models. Since neutrinos carry away such a huge energy, a widely favored theory considers them as the driving force of the explosion. Indeed, not all of the high-energy neutrinos that are radiated from the nascent neutron star can escape from the stellar interior. Some of them are absorbed in reactions with neutrons and protons and deposit their energy behind the stalled supernova shock. If this heating is sufficiently strong, the shock can be "revived" and the explosion of the star is initiated.

Highly anisotropic supernova explosions of a 15 solar mass star in two different computer simulations showing the color coded the gas velocities at one second after the start of the neutrino-driven explosion. Hydrodynamic instabilities that grow in the supernova core lead to spontaneous symmetry breaking and asymmetric mass ejection. In response to this the neutron star at the center of the explosion can obtain a recoil velocity of several hundred kilometers per second and in some cases even more than 1000 km/s, explaining the observed space velocities of young pulsars.

For stars between about 8 and 11 solar masses, which are at the low-mass end of those exploding as supernovae, this so-called neutrino-driven mechanism indeed seems to work in the computer models. More massive stars, however, have bigger iron cores and the ram pressure of the gas which continues to fall onto the stalled shock is much larger. This damps the expansion of the shock and makes explosions more difficult. Like water in a pot on a stove, neutrino heating stirs up the fluid behind the stalled shock in a process known as convection. The violent boiling of the hot matter causes large deformation of the shock. If the energy input by neutrinos is strong enough, the gas motions become so powerful that the star blows up in a highly asymmetric way.
Computer simulations show that the chemical composition of the exploding star gets mixed between the inner and outer layers. This might explain the anisotropies and clumpiness observed in many supernovae and supernova remnants. It might also explain why many young pulsars are measured with velocities of several hundred or even more than 1000 kilometers per second. While matter is thrown out by the explosion more strongly to one side, the neutron star receives a kick in the opposite direction.

It is, however, by no means clear that neutrinos are really the driving agents of the explosion. Computer models in three dimensions with a sufficiently accurate description of the complex neutrino transport and interactions require more power than current supercomputers can provide. Theorists are therefore not yet able to give definitive answers. But they are inventive and have suggested alternative ways to blow up massive stars. Magnetic fields might do it. Being amplified during the collapse of the stellar core and by dynamo motions and rapid differential rotation, the fields could become so strong that magnetic pressure or magnetic heating could reaccelerate the stalled shock. Also acoustic waves from a vibrating neutron star were recently proposed as such a backup possibility for getting explosions. More studies and better models are needed to clarify the situation.

The puzzle what really happens at the heart of a supernova, however, may ultimately be unravelled only through observations. Direct evidence is carried by the intense burst of neutrinos and by gravitational waves. Such propagating perturbations of spacetime are prediced by Einstein's theory of general relativity and should be created by the rapid nonradial mass motions that shatter the supernova core. Experiments are available and under construction which are likely to reveal some of the dark secrets of a future Galactic event.

Onset of a neutrino-driven explosion in a three-dimensional computer simulation. Neutrino absorption heats the surroundings of the newly formed neutron star (visible as dark grey surface) and leads to buoyant, rising bubbles of hot matter which pushes
the shock front of the explosion outward. The four snapshots show the violent boiling of the neutrino-heated gas (color coded is the entropy) at 0.1, 0.3, 0.4, and 0.5 seconds after the start of the explosion, which reaches a radius of roughly 2000 kilometers during this time.

Another long standing mystery has found its solution only recently. Observations were able to establish the connection of at least some of the cosmic gamma-ray bursts with extraordinarily energetic explosions of massive stars in distant galaxies. These "hypernovae" eject narrowly collimated flows of gas, so-called jets, which expand nearly at the speed of light and are the sources of the intense flashes of gamma radiation. The enormous energy output in these jets and in the stellar explosion may require an even more violent event than the birth of a neutron star: the collapse of a star to a stellar-mass black hole. This points to stars more massive than about 25 solar masses with cores that are too big to remain stable as neutron stars. If the dying star spins very rapidly, the infalling stellar gas can not be swallowed by the black hole directly. Instead, it swirls around the black hole, heats up to very high temperatures, and releases the huge energy that drives two jets along the rotation axis and a highly asymmetric, extremely powerful explosion. Gamma-ray bursts are rare events and thus seem to require a very special combination of conditions. A key ingredient may be very fast rotation at the onset of collapse.

Despite remarkable progress in the past years by theory and breakthroughs of observations, our understanding of these fascinating cosmic events is still very incomplete. Many aspects remain uncertain and controversial and demand further exploration. The field of supernova research can be expected to develop rapidly also in the coming years and promises exciting new discoveries and insights.