Furious Finale

Despite its name, a supernova is not a new star; it is an explosion caused by a sun that has already been in existence for a long time. Astrophysicists differentiate between several different types of supernova according to the development of their brightness over time and their spectral properties. The mechanisms of the explosions also differ, but are not yet understood in detail.

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Death of a star in the sky: A supernova exploded in Cassiopeia in 1680. The image shows the expanding gas envelope.
Death of a star in the sky: A supernova exploded in Cassiopeia in 1680. The image shows the expanding gas envelope.

At the Max Planck Institute for Astrophysics in Garching, the group headed by Director Wolfgang Hillebrandt, Hans-Thomas Janka and Friedrich Röpke is seeking explanations for the background to such cosmic catastrophes. Broadly speaking, the scenarios can be described as follows:

In the case of Type Ia supernovae, a pair of stars orbit each other closely: a white dwarf, the corpse of an old star, greedily draws matter from its partner in a binary system. Ingestion of this “power nutrition” brings the white dwarf back to life. If it is overfed, it reaches critical mass (the Chandrasekhar limit). At this point, it becomes unstable and begins to contract. This, in turn, releases gravitational energy, which causes the star to heat up. As a result, carbon and oxygen in its core ignite, and silicon and nickel are produced in nuclear burning reactions. Finally, a burning wave in the form of a detonation or deflagration front passes through the gas sphere: the star “explodes.”

The other types of supernova, of which Type II is the most common, are the result of the explosion of a single star of at least eight solar masses. Once this star has consumed its main supply of hydrogen and helium at the end of its life, carbon – the ash of the helium fire – ignites in its core. The temperatures rise to a billion degrees Celsius. Neutrinos are produced in large numbers. Finally, over a period of just a few years, the star produces elements of increasing atomic weight: neon, oxygen, silicon, and finally iron. Iron is the last of these, since iron atoms cannot undergo further fusion. The reactor is extinguished.

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Death of a star in the computer: In a Type Ia supernova, a thermonuclear flame ignites in the white dwarf, tearing it apart.
Death of a star in the computer: In a Type Ia supernova, a thermonuclear flame ignites in the white dwarf, tearing it apart.

By this time, the core of the star possesses a mass close to the Chandrasekhar limit, and its density has risen to 10,000 tons per cubic centimeter. Electrons are squashed into the protons and form neutrons. This causes the pressure within the core to drop; the core then collapses within a fraction of a second to form an object that is 10,000 times as dense: a neutron star. The matter in the center of the neutron star presents great resistance to further compression. The stellar matter, which continues to fall from further outside, slams into this hard neutron star at ultrasonic velocity. Before long, the inevitable happens: a shock wave runs from the inside outward, tearing the gas envelope with it. The star bursts, suddenly shining billions of times more brightly than before.

In the matter that is projected outward, atomic nuclear reactions produce large quantities of radioactive material (primarily nickel), as well as isotopes of cobalt and titanium. Extreme conditions also prevail in the supernova explosion; under these conditions, heavy elements such as gold, lead and uranium are produced from atomic nuclei of the iron group as a result of successive capture of alpha particles (helium nuclei) and free neutrons and protons. Owing to the extremely rapid attachment of nucleons to existing atomic nuclei, these forms of nucleosynthesis are termed the r-process (r for rapid) in the case of neutron entrapment, and rp-process in that of proton entrapment.

Deep inside a supernova is the neutron star, a compact object with a diameter of just 20 to 30 kilometers and a mass one and a half times that of our Sun. Since the angular momentum is conserved when the rotating stellar core collapses, the neutron star rotates extremely quickly. Particles are continually emitted from its surface and accelerated in its strong magnetic field. In the process, they emit “synchrotron radiation” in two cones. If this cosmic lighthouse beam crosses the Earth, the star flashes at an interval ranging from milliseconds to seconds – and astronomers observe a pulsar.

The death of a star with over 30 solar masses leaves behind it an even more extreme object. The mass of the stellar core is so great that its collapse can no longer be prevented: the burned-out neutron sphere cannot be stabilized, and it collapses under its own gravity. The gravitation of such a structure is so immense that even light is incapable of leaving it: a black hole has emerged.

In space, the explosion debris of supernovae form luminous, in some cases bizarre gas clouds, enriched with heavy elements. Over time, they become mixed with the interstellar matter, from which new stars may again be born, and the cycle of the elements begins once more.

Helmut Hornung

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