Supernovae
A supernova is an explosive state that massive stars reach at the end of their lifetime. During the supernova phase, the luminosity increases up to a factor 108 to 1010 and exceeds, for a short time interval, the overall luminosity of the host galaxy of the star. The huge amount of energy released greatly affects the surrounding area of the star and leaves such an imprint in various wavebands that the remains of a supernova (the so-called supernova remnant) can be detected for several thousands of years after the explosion. The picture on the right shows a false color image, created from optical and X-ray observations, of the Crab Nebula, which is a supernova remnant of a supernova explosion that happened in 1054 AD and was witnessed by Chinese astronomers as a new and ephemera star, four times brighter than Venus, in the constellation Taurus.
There are two mechanisms which can lead to a supernova explosion: one is that when the fuel of a massive star, with a mass of more than eight solar masses, runs out its central part collapses into a neutron star, a pulsar. The potential energy released is partially absorbed in the star's outer layers, which are explosively repelled and cross the interstellar medium as a shock wave at about 1% of the speed of light. The second mechanism that leads to a supernova happens in binary systems composed of a star and a white dwarf that is accreting mass from its partner. When the white dwarf reaches a critical mass of 1.4 solar masses (called the Chandrasekhar limit), the carbon fusion explosively ignites which tears the star apart in a supernova explosion.
Supernovae are one of the most important mechanisms of production of elements heavier than oxygen, and the only known process where elements heavier than lead can be generated. By nuclear fusion the star generates energy and produces heavy elements up to the most stable element, iron. Heavier elements are produced by complicated processes during the supernova explosion. Supernovae are also extremely strong neutrino sources. About 99% of the energy released by a supernova explosion is carried away by neutrinos.
During the last 10 years the observation of some supernova remnants in the high energy gamma-ray band (E>100 GeV) by Imaging Atmospheric Cherenkov Telescopes has proved that young supernova remnants are able to accelerate electrons to high energies (1014 eV). Current research focuses on the longstanding and crucial question: Besides electrons, do supernova remnants also accelerate protons and therefore constitute the source of galactic cosmic rays? A measurement of the flux of highly energetic neutrinos from supernova remnants by a neutrino telescope would be direct evidence for the acceleration of protons. The study of the spectral features of supernova remnants with Cherenkov Telescopes can provide additional evidence to answer this question.
Cosmos | Supernovae | Pulsars | Galaxies | Cosmic Rays | γ-Astronomy