This page shows the spectra of the three bright supernovae of 1998, SN1998S, SN1998aq and SN1998bu. SN1998S is an example of a Type II supernova. The spectra of Type II supernovae are characterized by broad emission lines of hydrogen, usually stronger than can be seen in this particular spectrum of SN1998S (earlier spectra, taken soon after discovery, showed much stronger lines of hydrogen). The progenitors of Type II supernovae are thought to be massive stars which have developed iron cores. The catastrophic collapse of this iron core expells the hydrogen-rich envelope, yielding a spectrum dominated by hydrogen. SN1998aq and SN1998bu were both Type I supernovae. The spectra of Type I supernovae are distinguished by broad absorption lines due to ionized metals, and the complete lack of any indication of hydrogen. As a consequence, the progenitors of these supernovae are thought to be binary systems with a carbon-oxygen white dwarf near to the Chandrasekhar limit as one of the components. Material from the other star falls onto the carbon-oxygen white dwarf. When the Chandrasekhar limit is exceeded, the white dwarf undergoes a carbon detonation, producing the supernova explosion. Type I supernovae are more luminous at their maxima than Type II supernovae, have easily recognizable spectra, and thus make excellent standard candles. The metal ions responsible for the broad absorption troughs in the spectrum of SN1998aq have been indicated below the spectrum.
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