What could have caused the nebular ring around Orion's head?

Question

A picture of the mysterious nebular ring in the constellation Orion, taken in infrared, is viewable in the link that follows.

http://macrocosm-magbook.blogspot.com/2007/07/akari-shows-hidden-side-of-universe.html

Answer 1

I've read that it's likely a supernova remnant, like Barnard's Loop (also in Orion). It's nice to finally see a good question in Yahoo Answers...

Answer 2

Stellar explosions are usually spherical in shape (since most stars are spherical) and when a sphere in space is viewed from Earth it appears as a ring (the material is thin, so we only see the material at the thickest part which is the edge of the ring.
There are nebulas that are very obviously bubbles, and there are others that appear as rings even though observations at different wavelengths indicate the ring is just part of a ball (such as the Ring Nebula and Helix Nebula, both planetary nebula created by stars puffing off their outer atmospheres).

Since the ring is visible in infrared but not visible light, it would likely have been caused by the shockwave of a spherical stellar explosion (nova, very strong flare, etc.) that has not cooled down enough yet to be detectable in visible light.

Answer 3

Ring structures are fairly common in nebulae, as the result of symmetrical stellar explosions. These propagate outward as spheres, appearing circular in the sky. This one could be the shock wave from a supernova expanding through the gas and dust. Additional density in the already radiating gas, and/or heating due to compression cause the shock front to be visible.

Answer 4

Man I love astronomy, but my knowledge is very limited about it. Nice to know though.