Hard on the heels of my post on Abell 39 last week comes another nebula that forms perhaps an even more perfect circle: PN G75.5+1.7, aka the Soap Bubble Nebula:

[Click to ennebulenate.]

That’s really cool. As I pointed out in the earlier post, these are called planetary nebulae, and are the results of the dying stars blowing off winds of gas. They are very rarely circular, instead coming in all kinds of fantastic shapes. It’s thought that you might not get a PN unless the star is binary or swells up to eat its planets as it dies; when that happens the star can get spun up and eject the gas more easily.

It’s not really a circle, of course: it’s a sphere, or more properly a spherical shell. It really is like a soap bubble! The bright edge is due to an effect called limb brightening, which I explained in that earlier post.

This isn’t really well understood, but to get one this symmetric the star must be a loner, and spherical ones are pretty rare. The Soap Bubble is extremely round, maybe even more than Abell 39, so that in ...