Supernova

A supernova is a gigantic explosion of a star shining as bright as a whole galaxy for several weeks. Two fundamental types are disciminated observationally, dependent on whether hydrogen lines can be seen in the spectrum (type II) or not (type I). Theory distinguishes also between two different events that lead to supernova explosions. In so-called "core-collapse'' supernovae the core of a massive star collapses at the end of the star's life to give birth to a neutron star or sometimes a black hole. In contrast, when a white dwarf explodes as a supernova, no compact remnant is left and all of the stellar material ends up in a diffuse cloud of gas that is ejected into interstellar space at a tenth of the speed of light. Supernovae play a key role in breeding heavy elements by nuclear reactions and are sources of the cosmic radiation. They act as an engine for reprocessing and heating the gas in galaxies.

A neutron star is an extremely dense star with about the mass of the Sun but a diameter of only 20 kilometers. Neutron stars are born as compact remnants of supernova explosions which occur when the iron core of a massive star collapses at the end of the star's life. Neutron star matter consists mostly of neutrons which are compressed to a density higher than that in atomic nuclei.

or shock fronts are discontinuities in gas or fluid flows where the density, pressure and velocity jump abruptly. Shock fronts can be created by objects that move supersonically through a medium, for example in the earth atmosphere by a plane flying faster than the speed of sound. The bow shocks of astrophysical jets propagate with up to 10 000 times the speed of sound and with up to 99,99 percent of the speed of light through interstellar and intergalactic space.

Neutrinos are elementary particles whose existence was postulated by W. Pauli in 1930 to explain energy conservation in nuclear reactions. It took more than 30 years to experimentally confirm their existence. Neutrinos exist in three different "flavors'', do not have charge, probably have more than hundred thousand times smaller masses than electrons, and interact with matter extremely weakly. In astrophysical objects they are produced when particle or nuclear reactions take place at extreme densities and temperatures, for example in the early Universe, in stellar cores and supernova explosions.

The production of heavier elements from lighter ones by nuclear fusion is called nuclear burning. At the very high temperatures present in stellar interiors, atomic nuclei have sufficiently large thermal energies to overcome the repulsive electrostatic forces of their positive charges. Nuclear fusion reactions become possible, producing heavier elements from lighter ones and setting free large amounts of energy. Nuclear burning generates the energy for the stars to shine for billions of years and also yields the pressure to balance gravity for that time. In the center of the Sun and all other stars, hydrogen is first burnt to helium. Only in the later and much shorter evolutionary stages of massive stars, the central temperature becomes higher and higher and nuclear fusion builds up successively heavier elements until iron, the most tightly bound atomic nucleus, is formed.

On February 23, 1987, a bright "new'' star lit up in the Large Magellanic Cloud, the largest companion galaxy of the Milky Way, 167 thousand light-years away. The enormous brightness immediately suggested that it was a supernova. Named Supernova 1987A, it was the closest such event that was visible to the naked eye since Kepler's supernova of 1604. It offered modern astronomy a unique possibility to study the death of the blue supergiant star Sanduleak - 69-202. Although the glittering light of the explosion has faded away long ago, Supernova 1987A still presents a spectacular display. It is surrounded by a thick ring of glowing gas in the equatorial plane. Two larger but fainter rings can be seen above the poles. The origin of these rings is not fully understood. They are probablycomposed of material which has been lost by the pre-supernova star about 20000 years before the explosion. The series of four panels shows the evolution of the supernova debris. A decade after the explosion, the matter ejected from the stellar interior has expanded enough to be resolved with the Hubble Space Telescope. Ahead of the stellar debris, the supernova shock tears into the circumstellar medium. The initial flash of light from the supernova explosion causes the ring to glow. Debris hurl into space, the fastest moving at one tenth the speed of light. The supernova's shock wave causes the ring to glow again. The closer the pieces of the ring are to the shock wave, the sooner they light up. Eventually, the whole ring lights up. Hubble observations in 1997 showed a brightening knot at the inner edge of the equatorial ring. This was the first sign of the dramatic and powerful collision between the outward moving blastwave and the ring material. This collision will continue over the next few years. It will rejuvenate Supernova 1987A as a powerful source of radiation. Supernova 1987A marked the spectacular end of the life of a star with 20 times more mass than the Sun. At the pre-supernova stage, 10 million years after its creation, the blue giant star was 40 times larger than the Sun. A nuclear fire supplied the star with the pressure necessary to withstand the enormous forces of gravity. The stellar interior has a so-called "onion shell'' structure. A thick hydrogen envelope surrounds successive layers of helium, carbon, oxygen, and silicon. These are the ashes of a sequence of nuclear burning stages. At the center a core of iron has formed with about the size of the Moon but more massive than the Sun. No more energy release by nuclear reactions is possible. Therefore the core collapses within less than a second until the density of atomic nuclei is reached. Now a neutron star begins to form at the center. At the same time a shock wave is launched and starts traveling outward. At the very center of the explosion, in a region only 50 kilometers in diameter, the nascent neutron star is stirred up by extremely rapid overturn motions. These transport energy from the interior to the neutron star surface. From there it is radiated away by neutrinos.These neutrinos deposit the energy for the explosion behind the shock front. The heated gas around the neutron star begins to rise in bubbles and pushes the shock farther out. Two tenths of a second after the shock was launched, it has reached a radius of more than 1500 kilometers, well inside the silicon shell. It is on its way out to disrupt the star in a violent supernova explosion.