Another nearly perfect circle in space!

Bad Astronomy
The entire universe in blog form
Aug. 4 2011 7:03 AM

Another nearly perfect circle in space!

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:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


[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 itself is interesting. It's also located near to a vast complex of gas and dust, which is weird: I'd expect the surrounding material to mess up the nice, neat, spherical structure of this nebula. Most likely the Soap Bubble is actually between us and the complex, in relatively empty space.

Also nifty is that this object was discovered by an "amateur", Dave Jurasevich. I know Dave; he works at Mt. Wilson in California and helped us run the 100" Hooker telescope when we were filming Bad Universe. He's a good guy, and discovered the Soap Bubble in 2008 while photographing the sky in the constellation Cygnus. It's a good story, which he recounts on his site.

The picture above is from the 4 meter Mayall telescope at Kitt Peak, taken by Travis Rector (his name may be familiar; I've posted stuff by him before). He used two filters: one picks out light from warm hydrogen (colored orange in the picture) and another that selects oxygen (colored blue). Planetary nebulae strongly emit at that latter wavelength -- it's not that they have more oxygen, it's that oxygen is a very vigorous producer of light in thin gas -- so it looks blue in this photo.

In the comments of my Abell 39 post are links to even more pictures of nearly circular planetaries. If I had known about all these a few years back when I was working on Hubble, I would've applied for time to observe them. A spectrum of these guys might reveal a lot more about them, and give us a clue as to why they are so nearly perfect. It would tell us if they are expanding evenly in all directions, for example, and maybe the density of the material around them.

I wonder how many of these exist? I've never seen a catalog of them. It would be interesting to know more about them, like position in the sky, age, and so on. Someday, our own Sun will expand into a red giant. If it swallows Mercury and Venus, as we expect it will, it might be a planetary nebula too. I wonder what it will look like, 6+ billion years from now...?

Tip o' the [OIII] filter to Astron/JIVE. Credit: T. A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF.

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