its spectrum showed prominent h and he emission lines with narrow peaks and broad wings, superimposed on a blue continuum. these narrow lines indicate the presence of a dense circumstellar medium (csm) in the vicinity of the supernova. the high luminosity of sn1998s is due to the interaction of fast material (ejecta) with previously-expelled slowly-expanding material (csm), which can more effectively convert kinetic energy of ejecta into radiation energy.