It's a mystery. I don't like mysteries! They give me a bellyache, and I've got a beauty right now. -Captain James T Kirk, "Man Trap"
Starship captains may not like mysteries, but scientists do. Back in the 1990s I had my teeth sunk into a big one. I was studying a ring of gas a light year across, centered on a star that had exploded in 1987 [note: that link will take you to a series of essays I wrote on this star and the ring, and is a good preliminary read to get more details on what I'm talking about here]. I was a graduate student working on a team led by Bob Kirshner, and we had many observations of this object using Hubble Space Telescope. The exploded star, called Supernova 1987a, had been studied for years by people all over the globe, but the ring was seen best ...
