The initial optical rise of the supernova was very rapid, reaching maximum only about 3 days after the explosion date. The presence of hydrogen lines in the spectrum showed that SN1993J was a type II event. Unusually, the UBVRI light curves went through two maxima at 3 and 20 days after the explosion. To study the evolution of the supernova luminosity a bolometric (UV, optical and IR) light curve was derived using La Palma UBVRI photometry plus infrared observations from other observatories. It was shown that the supernova peak luminosity exceeded 2.5 x 10⁹ solar luminosities and the temperature peaked at over 17000K. The supernova cooled rapidly to 8000K, with the photospheric velocity declining from 13000 to 4000 kms^-1. The second rise was brought about by the decay energy released from ⁵⁶Ni created deep in the core of the explosion. The decay energy diffused (and mixed) upwards, eventually meeting the recombination wave which was moving inwards with respect to the ejecta.

The rapid initial decline of the light curve and the speed with which it went into the second rise implies that at the time of the explosion, the progenitor must have been a relatively low mass for a type II explosion. The hydrogen envelope was probably less than 0.5 solar masses and the helium mantle could not have been larger than about 4 solar masses. This, in turn, implies the progenitor mass was about 15 solar masses on the main sequence, including about 10 solar masses of hydrogen. Thus, as the progenitor evolved, it must have lost about 10 solar masses of its envelope. The most likely mechanism for this mass loss is via mass transfer to a close companion. Indeed the transfer may have become so rapid that a common envelope, extending as far as one light year, might have been generated. Thus the picture of the supernova progenitor which has developed is a K supergiant which, by the time it exploded, had been stripped of most of its hydrogen envelope due to mass loss onto a companion blue supergiant.

Among the evidence that SN1993J does indeed have an extended envelope released before the explosion is the observation of very narrow optical emission lines in the early spectra. These lines are caused by ionisation of a progenitor wind by the extreme ultraviolet flash from the supernova, followed by its recombination. In addition, near infrared photometry, when compared with ING optical photometry, reveals the development of an infrared excess since about day 50. The most plausible explanation is an infrared echo, ie the re-radiation of the supernova light released near maximum by dust in the progenitor wind lying some light months from the supernova

By day 50, the bolometric light curve was falling exponentially with a e-folding time of 54 days. There was no observed slowdown for a long period. This is quite different from the behaviour of SN1987A where there was a period of 100 days in which the bolometric light curve slowed to match the radioactive decay (with an e-folding time of 111 days). This also points to a low mass in SN1993J: as early as day 50 a significant and increasing fraction of the gamma rays from the ⁵⁶Ni must have been escaping.

About a month after the explosion, the SN1993J spectrum appeared less and less like that of a type II event. Helium, oxygen and calcium lines increased in prominence, suggesting that the supernova was changing into a type Ib event. Supernova of type Ib form a rather mysterious subclass thought to arise from massive progenitors which have lost their hydrogen envelopes. More ING spectra, obtained when the supernova was about 300 days old, confirm its metamorphosis into a type Ib. SN1993J may be a kind of missing link between type II and type Ib. It suggests that rather than there being discrete sub-types of core collapse supernovae, there is in fact a continuum of progenitors with a range of hydrogen envelope masses.

During the first few nights following the discovery of supernova 1993J, researchers observed the interstellar Ca II and Na I absorption lines in its spectrum at high spatial resolution (FWHM ~ 5-6 kms^-1) using the Utrecht Echelle Spectrograph on the WHT. They found a crowd of absorption components in the radial velocity range between -135 and +165 kms^-1. In order to understand the features, the group re-examined previous surveys of local interstellar gas, high velocity gas and intergalactic gas in the field. They also observed the line of sight to the supernova at 21cm using the 100m radio telescope at Effelsberg. The components between -85 and -165 kms^-1 originate in intergalactic gas within the M81 group of galaxies which has been tidally pulled from one of the galaxies in the group. Components at -135 and -119 kms^-1 originate within M81, indicating that the supernova is embedded in the inner region of the galaxy. The remaining components arise in our own galaxy, including the halo.