Supernova 1993J in M81 (NGC 3031) was discovered by Spanish amateur astronomers on 28.86 March 1993 (Ripero & Garcia 1993). The first IUE spectra were taken on 30.2 March at VILSPA a few hours after notification of the discovery (Wamsteker et al. 1993) and the supernova was regularly observed by IUE over the next several weeks. This paper summarizes the principal results of the IUE observations (see Fransson & Sonneborn 1994 and Sonneborn et al. 1994 for observational details and more extensive discussion).

The first photographic detection of the supernova was on 28.30 March at magnitude 13.6 (Merlin & Neely 1993). Modelling of the supernova V light curve indicates that the explosion occurred on 27.8 March and that shock breakout should have occurred at ∼ 28.0 March (Shigeyama et al. 1994). Careful analysis of pre-outburst plates and images has identified the progenitor and shown that its colors are consistent with a late-type supergiant (cf. IAU Circular No. 5739) and the supernova's Type II classification.

In Fig. 1 we show the first UV spectra of SN 1993J from 30.2 March to 3.5 April, where the very rapid cooling of the exploding photosphere is readily apparent. On 30.2 and 31.2 March the temperature was ∼22,500K and ∼14,500K, respectively. Here we have assumed a Galactic extinction law and EB–V = 0.18, as determined from a fit of the 2200 Å dust feature on 30.2 March.