the differences between the spectra of type ia and type ib become more obvious as the supernova ages. in particular, the helium features in type ib spectra strengthen, and at late times the spectra show mostly intermediate mass elements like oxygen. this is a clear contrast to the type ia supernovae whose late-time spectra are dominated by iron and cobalt lines. this dominance of intermediate mass elements at late times indicated that type ib have a similar origin to the type ii supernovae, being massive stars that undergo core-collapse. the key difference between these three types of core-collapse supernovae is that snii are still surrounded by their shell of hydrogen gas. while type ib have lost this, only having their helium shell left. on the other hand, type ic have lost both their hydrogen and helium shells. this is reflected in the presence of hydrogen in the snii spectra, the absence of hydrogen but the presence of helium in type ib spectra, and the absence of both hydrogen and helium in the spectra of type ic. however, further evidence that the progenitors of type ib are massive stars, comes from the discovery of a limited number of supernovae which had normal type ii spectra at maximum but the spectra of type ib supernovae as late times. this transition from snii to snib is possible strongly suggests a common origin for both of these types of supernova.