A white dwarf supernovae is caused by carbon fusion starting, the carbon fuses all at once causing an explosion. A white dwarf supernovae happens when an object with very little hydrogen in its outer layer collapses so, there is little or no hydrogen absorption in the spectrum.
A massive star supernovae typically has a fair amount of hydrogen left in its envelope, or at least in its immediate surroundings.
White dwarf supernovae is used as a standard candle. By comparing the known luminosity of the standard candles to its observed brightness, the distance to the object can be computed using universe-square law.
