20 years ago on Friday, Supernova 1987A exploded; this remains the closest supernova to be seen since the invention of the telescope. The above picture, taken by the Hubble Space Telescope 3 years ago, shows about what the supernova looks like today. The bright ring is material tossed off of the star long before it exploded; the Hubble press release site contains a video (taken over several years) where you can see the ring light up as the shock wave from the supernova plows into the material. In the middle of the ring, the tenuous object you see is the expanding nebula, the remains of the star expanding outward at almost a thousand miles each second.
So, what have we learned from this supernova? First, we were surprised to learn that the progenitor star was a blue star, not a red supergiant. Astronomers had thought that a star had to swell up into a red supergiant star before the explosion. We now know that this need not be the case. As time goes on, we have seen other supernovae in other nearby galaxies, and we can identify the star they came from. Most of these are indeed red supergiants. So why was the star that made Supernova 1987A blue? This is still a bit of a mystery. Maybe the star that exploded had a fainter, nearby companion star. That companion star could have stripped away gas every time the dying star tried to puff up, keeping the hotter, bluer inner parts of the star exposed. Or perhaps the companion star and the dying star had spiraled together and merged into a single star recently, which would also make a red giant bluer. Or perhaps the material we see as rings around the star may have been blown off of a red supergiant in a near-explosion eons ago, essentially getting rid of the outer layers of the star. It's hard, now that the star is gone, to study these possibilities in too much detail.
Second, we learned that stellar death is quite messy. Supernova 1987A is surrounded by rings and shells of gas and dust, all shed by the parent star long before it exploded. This wasn't THAT surprising -- it had been suspected -- but the supernova lit up all of that invisible gas and dust so we could study its structure. This gives us clues as to when and how the star lost that material.
Finally, we've collected a ton of information about the process of a supernova itself -- details about the elements made in the explosion, about the production of various types of light from radio through X-rays. And some mysteries remain. Supernovae caused by dying stars exploding leave behind a remnanat, either a neutron star or a black hole. So far, we haven't seen anything at the center of this supernova. Did it make a black hole? Or is the expanding cloud of dust and gas still too thick to allow the neutron star to show through? We just don't know, and until the next nearby supernova goes off -- maybe tomorrow, maybe in a few hundred years -- Supernova 1987A will continue to be intensely studied from Earth.
To end, I'd like to shout out a happy anniversary to Oscar Duhalde, one of the co-discoverers of SN 1987A. I've worked with Oscar during several observing runs in Chile, and I was there last year to see him get several pats on the back on the 19th anniversary of being the first human since Johannes Kepler to see an exploding star with his bare eyes.