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Why do supernova remnants look like rings rather than spheres?

When I see supernova explosions depicted on tv i.e. Discovery, tlc etc, the supernovae expansions are shown as a ring. Shouldn't they be in the form of a sphere? Doesn't the result of an explosion go outwards in all directions?

It is spherical, but it looks like a ring. Spherical shells tend to look like rings when projected on the sky. This effect is called "limb brightening". The idea is that if you're looking towards the center of spherical shell, you're not seeing much material along your line of sight, so it's transparent. But along the edges of the shell, you're looking through a lot of material, so it's opaque.

November 2002, Christopher Springob (more by Christopher Springob) (Like this Answer)

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