Similar to types lb and lc, type II supernovae are core-collapse resulting from the collapse of the core of large stars at least 8 or 9 solar masses. They differ from types lb and lc by having hydrogen in their spectra. These massive stars do not stop the fusion process after helium fusion in the same way as stars like the Sun, and continue to produce heavier elements. In the last stages, nickel-56 is produced which decays in a very short time, half life about 6 days, into cobalt-56 which itself decays, with a half life of 77.3 days, into iron-56. At this point, because nickel and iron have the highest binding energy of all the elements, energy must be input to fuel any further fusion. Thus, the star builds an iron core. The massive gravity compresses the iron until it becomes electron degenerate, and further collapse is prevented by the Pauli Exclusion Principle. Once the mass of the core reaches the Chandrasekhar limit, the core collapses, forcing protons and electrons together to produce neutrons and electron neutrinos. The neutrinos escape, as they interact so feebly with other matter, and take energy away from the core, which accelerates the collapse. During the collapse, the outer parts of the core are moving inwards at up to 23% of the speed of light, so the process takes only a few milliseconds. The collapse is halted by neutron degeneracy pressure. The in falling layers bounce off the core, throwing off all the outer material, producing the supernova.
At this stage, the temperature at the center of the core is about 100 billion degrees kelvin, so thermal neutrinos are produced as neutrino/anti-neutrino pairs of all three flavors; electron, muon and tau neutrinos. This neutrino burst lasts about 10 seconds, and produces a stupendous blast of energy. The time from when the star starts to burn its helium until its death is about 10% of the time the star spends on the main sequence, burning hydrogen.
If the star started with more than about 20 solar masses, it is likely to continue to collapse, overcoming the neutron degeneracy pressure, to form a black hole. This will accrete some of the matter that bounced of the neutron core over a period of time. If the star started with more than about 40 or 50 solar masses, the "bounce" off the neutron core is skipped, avoiding the supernova stage, and the star collapses immediately into a black hole; see the next section on Other Supernovae.
Type II supernovae occur in three broad types.
The type llL supernova has a light curve that decreases over time in a linear fashion after reaching maximum luminosity. SN 1979C, in the galaxy M100, is an example that is interesting as, though it has faded predictably in visible light, it has maintained its output in X-Rays. It is the brightest X-Ray source in its galaxy, outside the galaxy center. The progenitor star had a mass of about 18 solar masses, and had a very strong stellar wind. This image shows the supernova, which is labeled, and the containing galaxy M100. It is a composite image with the infrared image from Spitzer in red, Chandra’s X-ray image in gold, and optical imagery in red, green, and blue. Image Credit
The light curve for the type llP decays more slowly, then flattens for a while creating a plateau region before continuing to decrease. An example is SN 1987A, though some observers suggest this was a quark nova. The first image shows it in the Tarantula Nebula in the Large Magellanic Cloud. The second image shows a series of pictures of the ejecta from the explosion over a period of about twelve years. Interestingly, as at January 2017, no visible evidence of a neutron star remnant has been found.
This is very similar to a Type Ib supernova except that some hydrogen remains in the outer layers of the star, and is seen in the star's spectrum, which is not the case for the type Ib. Cassiopeia A is the remnant of a type IIb supernova that occurred approximately 11,000 light-years away. The expanding debris cloud of material left over from the supernova is now approximately 10 light-years across, and is expanding at between 4,000 and 6,000 km/second.
Astronomy & Cosmology -
Stars - Life & Death of Stars