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The Question

(Submitted February 27, 1998)

I just watched a movie on your site showing a Hubble view of a1987 supernova. My question is why the energy expanding around the supernova is in the form of a ring, and not a sphere? The only possible answer I can think of is that it has something to do with the rotation of the star.

The Answer

This is a very good question. This ring around SN1987A already existed before the supernova explosion; the light from the supernova event just lit it up. The actual supernova is the point-like source in the center of the picture. It has only recently begun to be resolved with the highest resolution telescopes.

The explanation of one article I read, and the one that makes the most sense to me, is more-or-less as follows. The very massive star that existed before the supernova explosion, blew off very strong stellar winds. Probably because this star was spinning a denser region formed in the equatorial plane around the star -- kind-of like a planetary disk. Then as the star changed from a red supergiant to a blue supergiant, the winds changed. The winds then became focused to the poles of the star, making an hour-glass shaped region of material. This material was not observed until the light from the supernova hit it.

What we see is like an echo. The initial blast of radiation from the supernova ionizes the material, so it emits light.

Something you should know, however, is that spherical shells usually look like rings on the sky. In the case of 1987A it is probably really a ring (at least the inner ring is), but often something is not. This is because on the edge of a shell lots of material is along our line on sight, while in the middle has less. So, we see a ring not a shell even when something really is a shell. This is often observed in stars and older supernova remnants.

Good luck,

Jonathan Keohane
for Ask an Astrophysicist

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