A supernova is an astronomical event that occurs during the last stellar evolutionary stages of a massive star’s life, whose dramatic and catastrophic destruction causes one final, extremely bright ‘explosion’ in the night sky. Type II supernovae are particularly interesting due to their potential for use as standard candles, as well as providing insight into the physics of stars and nucleosynthesis.
Type II supernovae are generally categorized based on their spectra at maximum light. This means that they exhibit strong hydrogen lines in their spectrum, with Type II-L having especially weak or even absent hydrogen lines. The prototype for this class is SN 1987A in the Large Magellanic Cloud. There are also Type II-P supernovae, which show evidence of a plateau phase in their light curve where the luminosity remains approximately constant for around 100 days after explosion. An example of this class is SN 1999em in NGC 1637. Finally, there are Type IIb supernovae which appear similar to Type Ib (see below), but with weaker helium lines and often with some residual hydrogen near maximum light; an example is 1993J in M81. It is thought that all three subclasses arise from progenitors with different degrees of mass loss prior to explosion.
Type Ia and Ib/csupernovae are both thought to arise from white dwarf stars in binary systems accreting matter from a non-degenerate companion star. In the case of Type Ia supernovae, it is thought that when the white dwarf reaches the Chandrasekhar mass limit (~1.4 solar masses) through accretion or mergers with other white dwarfs, it becomes unstable and undergoes runaway nuclear fusion reactions, leading to complete disruption (a so-called ‘thermonuclear explosion’). For Type Ib/c supernovae, it is thought that accretion onto a white dwarf can lead to ignition of carbon and oxygen burning under conditions where no explosive nuclear burning can occur; again this leads to complete disruption of the star (‘core collapse’). While there has been much debate about what types of star can serve as viable companions for producing these types of events (especially for Type Ia), recent studies seem to suggest that most if not all companions must be evolved stars on or near the red giant branch (RGB).
It should be noted that while many core collapse events may appear similar spectroscopicallyto thermonuclear explosions (e.g., 2002ap), there are some key differences between the two classes which allow them to be distinguished: 1) Core collapse events tend to have broader spectral features than thermonuclear explosions; 2) Core collapse events tend to have higher expansion velocities (~10000 km/s compared to ~2000 km/s for thermonuclear); 3) The light curves of core collapse events typically rise more quickly than those for thermonuclear explosions; 4) Core collapse events often display strong emission lines due to interaction between fast moving ejecta and circumstellar material surrounding the progenitor star prior to explosion; 5) Polarimetry measurements indicate that core collapse events tend to be more highly polarized than thermonuclear explosions; 6) X-ray observations indicate that shock breakout from core collapse events tends t ooccur at softer energies than for thermonuclear breakouts (~0