Timeline for Supernova 1987A

Phil Plait
Feb 19, 1998

From a posting to sci.astro.amateur

Okay, here is a brief synopsis of what we think is the history of SN87A, starting some 10,000's of years before it blew:

  1. For a long time, it was a simple main sequence (but very massive and hot) star.

  2. It became a red supergiant (RSG), and started to blow a slow, dense wind. This wind was not spherical, but had some degree of axial symmetry (in other words, it may have not been exactly spherical, but squashed like an oblate spheroid). This was most likely *not* caused by simple rotation of the star, since RSG's rotate slowly. Perhaps it had a companion, like a close binary, or (my pet theory) a closeby superjovian which spun up the star when the RSG expanded around it. Anyway, this wind was more dense along the equator, and less towards the poles. This is important later.

  3. Sometime later the star turned into a blue supergiant (BSG). The wind became much faster and less dense. This wind catches up with the older RSG wind. Since the old wind is less dense in the polar direction, the BSG wind moves faster that way. It is slower in the equatorial direction, so you get a "bipolar outflow"; a funny shape like an hourglass. Two lobes form up and down, and it gets pinched in the middle. The middle pinch forms the inner ring we see. The gas gets compressed by the shock of the BSG slamming into it, so the hourglass shape has some finite thickness. It is still pretty thin, compared to its overall size. Its length is about 2 parsecs.

  4. Think about each lobe now. It is very roughly spherical, but squashed a bit. Somehow (the exact mechanism is unknown right now) each lobe gets a slight overdense region around *its* middle. These form the so-called "outer rings". The stage is now set, but note that this is still some 10,000 years before the star explodes.

  5. 1987: KABOOM! Saduleak -69 202 transforms itself into SN1987A. 10^57 or so UV photons scream out from the inferno. Some small fraction hit the ring system, causing it to glow. The gas in the hourglass is not terribly dense, nor is it thick, so it glows feebly. The rings, however, are denser, and the glow is more pronounced. 50,000 parsecs away, we see three rings glowing with almost no trace of the hourglass nebula itself.

  6. 1987-1990: For a short period of time (about two years), light travel time effects complicate matters here on Earth. Those effects are complicated and it would be difficult to explain them here. Luckily for this discussion they are not too important.

  7. Present: The rings fade as the gas recombines. This fading has helped us quite a bit in finding the temperature, density and even the structure of the gas.

  8. Now + a short time, maybe three to five years: The expanding debris from the explosion, moving outwards at perhaps 10% the speed of light, slam into the inner ring. The ring starts to glow as the debris heats it up. These fireworks may be bright enough to light up the the outer rings as well. Some hundreds of years later the debris hits the outer rings as well. [Note: this part is now superceded by the WFPC2 and STIS observations!]

  9. The future: Eventually, we get something that may look quite a bit like the Crab Nebula. There is some evidence that the Crab has a ring of emission in the middle, which you might expect to be the leftover ashes of the inner ring as it shaped the expanding debris. Incidentally, we are talking hundreds, or even a thousand, years in the future. Don't hold your breath!

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Phil Plait                    badastro@badastronomy.com
The Bad Astronomy Web Page: http://www.badastronomy.com

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