The core-collapse process is believed to be the cause of the explosions of massive stars at the end of their lives, and SN 1987A provided strong experimental confirmation of this idea. Unfortunately, the Hubble Space Telescope was not yet in operation when the supernova exploded, since it was not launced until April 1990. From these the Heritage team constructed the color image which are released this month in honor of the 12th anniversary of the explosion. Since 1990, Hubble has kept an attentive eye on SN 1987A, obtaining both imaging and spectrographic observations at least once a year. The origin and the nature of the beautiful circumstellar rings are still a mystery.
Spectroscopic observations show that the rings are enriched in the element nitrogen.
Both the slow speeds and the unusual composition show that the rings were expelled from the progenitor star when it was a red supergiant, more than 20,000 years before that star exploded as a supernova. However, one would have expected such a star to eject material in a more regular fashion, steadily expelling material in all directions, rather than puffing rings like a pipe smoker. Another puzzle is that the observations of the star just prior to the explosion show that it was a blue supergiant. This was a puzzle in 1987, because up to that time theorists had believed that only red supergiants could explode as a supernova. Apparently the star was, until relatively recently, indeed a red supergiant, but over the millennia before the explosion, it shrank in size and its surface heated up gradually.