Type I supernovae are caused when a white dwarf star sucks material from another star until the white dwarf explodes. The light curve of the exploding star peaks quickly and then dies away. The luminosity of the supernova can reach that of 10 billion suns before it slowly fades.
The spectral lines of Type I supernovae also lack hydrogen lines, though subclasses of Type I supernovae can have silicone and helium lines. Spectral lines are seen when the radiation of the supernova is spread out into its component colors and wavelengths. Type I supernovae are seen more often in elliptical galaxies.