According to current theory, two different mechanisms produce supernovae: First, stars considerably more massive than our Sun can most probably not evolve quietly into an end state as a white dwarf. When coming to age, these massive stars explode in a most violent detonation which flashes up at a luminosity of up to 10 billion times that of the sun, called supernova (of type II, or Ib or Ic), and ejecting the very greatest part of the stellar matter in a violently expanding shell. These explosions are thought to leave a compact remnant, such as a neutron star. Aternatively, infalling matter on a white dwarf star can cause it to explode as a supernova of type Ia; these events do probably not leave a stellar remnant.
The classification of supernovae in types was introduced by Rudolph Minkowski (Minkowski 1941) on the grounds of their spectra: Type I supernovae show no hydrogen lines in their spectra, whereas these lines are present in those of Type II. Later these types were subdivided, again based on their spectra; for a summary see e.g. Turatto (2003). For Type I, subtype Ia shows no Helium in spectrum, characteristic absorption features, and in its later phase, emission lines from elements in the iron group. Type Ib shows helium lines, type Ic no helium but in its later phase, oxygen and calcium lines. Supernovae of types Ib and Ic are thought to originate from massive progenitor stars which have been stripped off their outer layers by companion stars, and thus lost their hydrogen and for Ic, also their helium. Supernovae of type II may be further distinguished both for their light curves and for their spectra: Subtype IIP ("Plateau"), sometimes referred to as "normal" SN II, shows a plateau in its light curve when the luminosity decline after maximum stops for about 2-3 months. SN IIL ("linear"), on the other hand, exhibit a linear decline of brightness after maximum. SN IIn eventually show "narrow" emission lines in their spectra.
The Crab Nebula M1 is the only Messier SNR, the remnant of the supernova of 1054 (very probably of Type II), one of few historical supernovae observed in our Milky Way galaxy. However, other supernovae have appeared in Messier galaxies (see our table), and produced SNRs. These special kind of nebulae can be observed in some cases, e.g. the remnant of the Supernova 1993J in M81.
With only one supernova remnant in Messier's catalog, the larger NGC and IC contain only two and one further object, respectively, in the Milky Way Galaxy: The Veil Nebula or Cygnus Loop (with different NGC and IC numbers for its parts), the Pencil Nebula NGC 2736 as part of the Vela SNR, and IC 443. Moreover, NGC contains two supernova remnants in the Large Magellanic Cloud: NGC 1918 and NGC 2060. As of March 2009, the total number of SNRs known in the Milky Way has been given as 274, and to May 2014, as 294, given by D.A. Green's Catalogue of Galactic Supernova Remnants (Green 2009, 2014).
The knowledge of the nature of the supernova phenomenon, and the name "supernova", goes back to Walter Baade and Fritz Zwicky, who studied Novae in the early 1930s at Mt. Wilson Observatory. They were especially interested, and successful, in finding extremely bright "novae" in other galaxies, comparable to the one which had been observed in the Andromeda Galaxy M31 in 1885 (S Andromedae). They coined the term "Super-Novae" in 1934 (Baade, Zwicky 1934).
Last Modification: August 15, 2019