A delicately violent celestial shell game

One of my favorite types of objects in space are the thin, ethereal shells of gas stars create when they die. So I was thrilled* to see this new image of one taken in exquisite detail by the Hubble Space Telescope:

[Click to supernovenate.]
I studied weird soap bubbles like this for quite some time for both my post-graduate degrees, and they still fascinate me. This one, called SNR 0509, is actually a very interesting example. There are lots of ring-shaped objects in the sky — the Helix nebula (seen below) may be the most canonical — but usually the ring itself is thick, the width of the band being a large fraction of the object diameter itself. Why does SNR 0509 have such a thin ring?

I’m glad you asked! OK, so I asked, but if I were you I would’ve asked that. And I’m me and I did.
First off, we need to understand what happened here to form the object in the first place. When a relatively low-mass star like the Sun dies, it blows off a steady wind of subatomic particles, like the solar wind but much thicker. The physics is a bit complex, but ...