Supernovae and their remnants
Supernovae are the most explosive events in normal galaxies and they occur quite regular; our own Galaxy is thought to have a supernova rate of 2-3 per century. Supernovae come in two main flavors: core collapse supernovae and thermonuclear (or Type Ia) supernovae.
Core collapse supernovae are the explosions of the most massive stars (> 8Msun on the main sequence). There explosion energy comes from the gravitational energy released when the stellar core can no longer support the gravitational pressure and collapses. Most of the implosion energy (10^53 erg/10^46 J) is released in the form of neutrinos, but about 1% is used for disrupting the remainder of the star. The imploded core remains as a neutron star, or in some cases as a stellar mass black hole. The ejected stellar material is rich in oxygen and other alpha elements, but also contains some iron-group elements.
Thermonuclear/Type Ia supernovae Ia are exploding carbon-oxygen white dwarfs, which explode because, as their mass approaches the Chandrasekhar limit, the pressure increases, and carbon-oxygen ignites. This results in runaway nuclear fusion reactions, disrupting the star. Although the source of energy is very different from that of core collapse supernovae, the total kinetic energy of thermonuclear supernovae is very similar: 1051 erg. It is still not clear why the central density pressure increases. Is it caused by accretion from a normal stellar companion, or is it caused by the merging of two white dwarfs? Type Ia supernovae result in the production of mainly iron-group elements, about 0.7 solar masses per explosion.
Both types of supernovae eject a large mass of recently synthesized elements into the interstellar/circumstellar medium, with a kinetic energy of 10^51 erg/10^44 J, and velocities up to 10,000-20,000 km/s. This results in a shock wave in the interstellar/circumstellar medium, which heats up the gas to temperatures of 10^7-10^8 Kelvin and pushes this hot plasma into a shell. In this process the supernova material also gets heated to similar temperatures. At these temperatures the hot plasma emits copious X-ray emission, with prominent X-ray line emission indicating the composition of the material ejected by the supernova. As more material is shock-heated the velocity of the shock decreases, and the
average temperature of the plasma declines. After 50,000-100,000 years the large supernova remnant shell hardly emits X-rays anymore, and slowly the shell disappears. In the early phases supernova remnants are of interest as they can be used to study the composition and energy of the supernova material.
The supernova remnants shocks are also of interest as these are sites of particle acceleration. It is generally thought that during the first 1000-10,000 yr particles can be accelerated by the shock to energies as high as 10^15 eV. These particles escape the shells and collectively are called cosmic rays. Cosmic rays penetrate the solar system and we detect them as they bombard the Earth atmosphere. While being accelerated by the supernova remnants shocks, the cosmic-rays interact with the local plasma and magnetic fields and emit radiation all across the electro-magnetic spectrum. Studying this radiation reveals where and how cosmic rays are produced.
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