It now appears almost certain that the precursor of SN 1987a was the brighter of the components of Sk-69 202, a blue supergiant, with a precursor mass of perhaps 12-16 solar masses. Prior to the explosion the precursor had a core mass of order six solar masses, and 0.1 to 0.2 solar masses of residual hydrogen envelope. The compact nature of this star can account for many of the odd features of the subsequent light curve and spectral evolution.
An analysis of the light curve and colour evolution shows four distinct epochs, which probably relate to the initial expansion of the fireball and the escape of shock-deposited thermal energy, the hydrogen-rich layers becoming optically thin, the exposure of the helium core, and the increasing transparency of the helium core.
The supernova appeared to be at its maximum on May 10, but is dimmer than a normal Type II because its light is apparently derived from recombinations and the radioactive decay of 56Ni to 56Co to 56Fe rather than by the thermal energy deposited by the passage of the shock.