Introductory Astronomy: Type I Supernovae

Note:Type I supernovae have nothing to do with the normal sequence of star formation. In other words, the normal evolution of a star will never produce a Type I supernova. They are NOT a step in the sequence, but are caused by another, completely different process (described below). We include this page here only for completeness.

Type I supernovae are less well understood than Type II supernovae. Type I supernovae occur when the mass of a white dwarf (which is part of a binary system) exceeds the Chandrasekhar limit and begins to burn carbon degenerately, causing an explosion that can destroy the white dwarf star. Scientists believe that the mass of the white dwarf increases past the Chandrasekhar limit because it is accreting matter from its companion star (in other words, matter from the companion star is flowing onto the white dwarf, causing the mass of the white dwarf to increase).

All Type I supernovae have some characteristics in common. They are classified by the fact that there are no hydrogen lines in their spectrum. In addition, the lightcurves of type I supernovae are all very similar. They all have a maximum absolute magnitude of -19 (or about 4 billion times brighter than the Sun) and a similar decreasing light pattern after the explosion. Because of the similarity of their lightcurves, type I supernovae can be used as standard candles. A standard candle is an object of known absolute magnitude (M). Because M is known and m (the stars apparent magnitude) can be observed, it is simple to determine the distance to the object using the distance modulus formula (m - M = 5 log(d) - 5).