supernova

supernova A violent explosion in which certain stars end their lives; given the variable-star type designation SN. In a supernova explosion, the star may become over a billion times brighter than the Sun, and for weeks may outshine the entire galaxy in which it lies. However, the optical luminosity represents only 0.01% of the energy released in the explosion. Most of the energy emerges in the form of neutrinos, and 1% goes into the kinetic energy of the gas expelled. The last supernova seen in our Galaxy was in 1604 (Kepler's Star), although Supernova 1987A in the Large Magellanic Cloud reached naked-eye brightness. However, two or three supernovae are expected to occur every century in a typical spiral galaxy like ours, which suggests that many have been missed due to absorption of light by dust in the galactic plane.

 Supernovae are classified into Types I and II, the latter type showing hydrogen in its spectrum, whereas the spectrum of a Type I supernova shows no hydrogen. There is further subclassification into Types Ia, Ib, and Ic according to other details of the spectrum. Type Ia supernovae reach a maximum magnitude of about −19, while Types Ib and Ic are about 1.5 magnitudes fainter. Type II have a wide range of peak magnitudes, but on average are similar to Types Ib and Ic. Types II, Ib, and Ic occur in young stars of Population I, and thus are concentrated in the disks of spiral galaxies. Type Ia supernovae occur among old stars of Population II, as are found in elliptical galaxies and the halos of spirals.

 Type Ia supernovae are believed to be due to the explosion of a white dwarf in a binary as a result of matter falling on to it from its companion star. When the mass of the white dwarf eventually exceeds the Chandrasekhar limit, it undergoes runaway carbon burning and explodes, ejecting about 1 solar mass.

 Type Ib and Ic supernovae are thought to result from the collapse of the cores of massive stars which have lost their hydrogen envelopes, either through a stellar wind or by transfer of matter to a companion in a binary. Types Ib and Ic show minor differences in spectra, indicating different compositions of the progenitor stars, which are probably Wolf–Rayet stars stripped of different amounts of their outer layers. Despite their classification, Type Ib and Ic supernovae are more closely related to Type II than to Type Ia.

 Type II supernovae arise from the explosion of stars of more than 8 solar masses. Nuclear reactions cease once the star's core consists of iron and heavier elements, because these elements cannot be burnt to produce energy. The stars then collapse under their own gravity, reaching densities so high that protons and electrons combine to form neutrons, producing a neutron star or even a black hole. The formation of a neutron star causes the overlying layers of material to rebound violently. In this process much of the envelope of the original star, amounting to many solar masses, is ejected at speeds of 2000–20 000 km/s.

 Type II supernovae can be subdivided into II-P, II-L, and IIb. Type II-P (for plateau) remain at near-constant brightness for 2–3 months after the outburst before fading. The rarer II-L (for linear) type fades more rapidly from an initial peak. Type IIb show a double maximum in brightness. Type IIb are thought to result from massive stars that have lost most, but not all, of their hydrogen envelope before exploding. The second peak, a few weeks after the initial outburst, is caused by the decay of radioactive nickel and cobalt in the supernova debris. See also supernova remnant.