Supernova 1987A

In 1987 a Type II supernova named SN 1987A was observed in a nearby galaxy called the Large Magellanic Cloud. By looking back at older images of the same part of the sky, the star that went supernova (the progenitor) was identified. It is shown in the adjacent image before (right) and after (left) the explosion.

An Unusual Progenitor

Sanduleak -69^o202 (the catalog name of the progenitor) was something of a surprise since it was a blue supergiant of about 18 solar masses. That it was blue was the surprise, since it was thought that supernova explosions were likely to occur in the red supergiant phase. In addition, the light curve was not the usual Type II light curve, as illustrated in the figure below left. It did not rise directly to maximum brightness, as is normal, and was a factor of about 10 less bright than a normal Type II curve.

Earlier Red Giant Phase

One plausible theoretical explanation for the unusual light curve of SN 1987A is that the star had already gone through a red supergiant phase where it had lost some mass. Because it was poor in metals (see the right panel), it then contracted and heated to a blue supergiant (specifically, a spectral class B3I star of apparent magnitude 12). Thus the size of the outer envelope was much smaller than expected in a Type II explosion.