The Sciences

One Ring to Rule Them All

Bad AstronomyBy Phil PlaitJun 6, 2005 7:56 PM


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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 in our Hubble images. My job was to analyze the data and figure out what was going on with it.

I was able to find out quite a bit about it: how big it was, how much gas was in it, how dense it was, and by really pushing the data to its limit I was able to get an educated guess at the cross-sectional shape of it (like slicing a ball in half shows it has a cross-section of a circle, or cutting a piece of wood shows it has a rectangular cross-section; bizarrely, the ring appeared to have the cross section of a crescent, so it was shaped like the metal rim of a bicycle wheel). What we couldn't figure out, to be blunt, was what it was doing there. It didn't make sense that such a dense doughnut of gas would encircle a star like that. We had ideas, and we knew they were the right track, but the devil is in the details, and the details are what kept us up late at night. Worse, the star had two other rings around it, offset from it, like the two bulbs in an hourglass. What the heck were they? We'd seen them around other stars, but again, the specifics of this case made it hard to understand what they were doing there.

Things got weirder. In the middle of the ring, the exploding star was expanding (you can see that debris as the elongated nearly-vertical stuff in the middle of the ring in the images above) . Eventually, after more than 10 years, that debris started hitting that inner ring. We expected to see the ring light up again starting in the lower part then moving around in both directions, like a circular fuse lit at one spot. That didn't happen; we got a single bright blob for a while, with no other action. Finally, months later, we started seeing more blobs. And yet, after all that, this weird structure had (at least!) one more surprise waiting for us. When a star like this detonates, only the outer layers explode outward. The inner part, the core, collapses down, and you get either a black hole or a weird dense object called a neutron star. Everyone expects this event to have formed a neutron star, but no one can find it. There were early reports it had been seen in the late 1980s, but they turned out to be false alarms. But now, 18 years after the event, we really should be seeing it. But recent observations show it just ain't there, or, if it is, it's much, much fainter than expected. The team tried everything they could to find it, and came up blank. Generally, a young neutron star is bright because gas and junk left over from the explosion fall onto it. So maybe there is some reason that the junk was cleared away by the blast. Or maybe there is more dust blocking the view than expected. Or maybe it formed a black hole, or maybe the magnetic field of the neutron star is too weak, or maybe maybe maybe. There are lots of maybes to go around. But no matter how you slice it, Supernova 1987a is a weird object, and probably always will be. There may be other objects similar to it out there, but 87a was the first we ever discovered, and is the best studied. And yet, for all we know about it, there are still mysteries to be found. Personally, I like things that way. I enjoy finding things out. If we knew everything, what fun would that be?

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