There are three types of supernovae: two subtypes SNIa and lb; and SNII. Late type galaxies produce all types of SN, whereas early types (E, SO, and non-Magellanic irregulars 10) have hosted only SNIa. The recently identified SNIb, like SNII, have massive stars as their progenitors.

Reviving Oemler & Tinsley’s (1979) suggestion that SNIa also come from short-lived stars, we have asserted that they need not occur in all early-type galaxies. SNIa occur only in those galaxies that have access to gas and can form stars in their main body. (SN in nuclear regions are a different matter altogether). In this model, SNIa are not associated with typical stellar population of E/SOs but with regions of localized star formation. Note that data on SNIa from spirals is already consistent with this model (see Kochhar 1989 for references and details).