An enigmatic blue bubble in space

By Phil Plait | March 13, 2012 7:00 am

There is something aesthetically pleasing about symmetry, whether it’s on Earth or as it is in the heavens. That may be one of the reasons I love planetary nebulae, the eerie and beautiful structures created when dying stars cast off their outer layers. They come in many strange shapes, and oddly it’s quite rare to find one that appears perfectly circular.

But they do exist, and one of the best examples I have seen yet is Abell 33, the glowing winds from a star located something like 1500 light years away:

How gorgeous is that? [Click to embiggen.] This image was made by BABlog regular Adam Block, using the 0.8 meter (32") Schulman Telescope at the Mount Lemmon SkyCenter. He combined images through four filters for a total observation of 630 minutes, over ten hours, to make this dream-like shot.

A few things jumped out at me. First, that bright star on the edge of it giving it a diamond-ring look is almost certainly just a chance alignment. It’s probably much closer or farther away than the nebula, and coincidentally lined up. But it’s very pretty!

Second, it looks like the central star may be a binary, two stars orbiting each other. I measured their separation on the image, and assuming the distance to the nebula really is 1500 light years (though that’s fairly uncertain; I was surprised that there aren’t many papers in the literature about this object), those two stars are no closer than about 0.03 light years from each other — about 2000 times the distance of the Earth from the Sun! That’s a long way, though not unheard of in binaries. Still, I can’t rule out a coincidental alignment.

Also, it’s obvious the central star(s) is off-center! That does happen, usually if the star is moving rapidly through space. The wind it expels gets "blown back" by gas in between the stars, like steam coming from an old-time train’s spout as it rolls down the track. If that were the case, though, I’d expect the nebula itself to be compressed along one side, which it isn’t. That’s a bit of a mystery.

Third, of course, is how circular it is. The outer rim is nearly a perfect circle, which is really uncommon! You might think that in space spheres are common (which would then look like circles as seen from the outside; see here for why), but in fact that’s rarely the case for planetary nebulae. Most are oval, or barrel-shaped, or something even weirder. It has to do with the way the dying star blows off winds, streams of subatomic particles from its surface. If a star is just sitting there, or rotating very slowly, then sure, the wind might expand as a sphere.

Apparently, though, for most stars something happens to increase their spin before they become planetary nebulae. One thing that could do it would be planets: big planets orbiting the star close in would get swallowed up as the star expands into a red giant (the first step in becoming a planetary nebula). The planet orbits the star faster than the star itself spins, so the planet acts like an egg beater or a whisk, stirring up the star’s insides, and spinning it up. We know planets are common — it’s possible most stars have them — so this explanation makes a lot of sense.

But if you look more closely at the picture of Abell 33, things get weirder. The rim is circular, sure, but the fog on the inside is not. It has two oval holes in it, slightly off center. Moreover, the two ovals are parallel, slanted a bit at the same angle. I’m pretty sure this is a sign that the nebula itself may be more barrel shaped, and the two dark ovals are the open ends of the barrel structure. It makes me think it’s a lot like NGC 1514 (inset here; click to embiggen), which has a similar structure (and also a binary central star, hmmm). In the infrared, NGC 1514 looks like a space station! I’d dearly love to see a high-res infrared shot of Abell 33. Would it have those weird rings too?

All in all, this is a pretty interesting nebula, and I’m fairly surprised it hasn’t been studied more. There are some unusual things going on here, which would make for a pretty good project for someone, I’d bet. Until then, though, I guess we’ll have to be satisfied with just soaking in the beauty of this enigmatic object.

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Related Posts:

Warm, dusty rings glow around a weird binary star
A dying star with the wind in its hair
A nearly perfect circle in space
Another nearly perfect circle in space!

CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

Comments (20)

  1. Timmy

    Now I’m picturing the universe as a vast cosmic bubble party with stars floating through the ether until they pop out of existence. I hope God doesn’t get soap in His eye.

  2. Jeffersonian

    Freaking weird. So blue.

  3. Ray Bellis

    Just wondering, but what leads you to believe that the bright binary near the center is actually the source of the nebula? There’s a much fainter star which is almost bang in the middle.

  4. Chris

    Actually I think there are quite a few references to Abell 33. A quick search on SIMBAD come up with 117 references. Granted quite a few are just catalog references, but still a bunch referencing this object. For others who are interested, don’t enter “Abell 33” it goes to the wrong place. Put in
    PN G238.0+34.8

    Also I believe that big bright star is HD 83535. It has a parallax of 4.43 milliarcseconds which puts it at about 730 light years, so much closer than the nebula.

  5. Peter Davey

    “And now a bubble burst; and now a world.” – Alexander Pope.

  6. Doug A

    Another analogous object is the Owl Nebula, sharing in common those two low surface brightness ovals. So are we looking down the cylinder, in projection?

  7. The outer rim is also very well defined, almost as if someone drew the outside edge. If you look at it for a while, you can almost sense a 3D structure, though given the perspective, it is difficult to imagine how that structure actually appears from differing angles.

  8. R.G.Daniel

    Some days the most fun I have is when I change my desktop wallpaper. This could be one of those days. Thanks for this!

  9. Katherine

    I strongly suspect interstellar warfare, and the other side just won.

  10. @ Ray Bellis:

    That was my question, too. It does seem to be the central star, however. This, from The Astronomical Journal, July 1999.

    Abell 33.The companion to Abell 33 was first detected by Cudworth (1973); the separation and position angle of the pair has not changed significantly since then. The low probability of a chance superposition and the reasonable implied parameters of the system suggest that the stars are physically associated. Additional evidence for this conclusion comes from the approximate agreement between our distance of 1.2 kpc and the statistical distance estimates of 0.7 kpc (CKS), 1.6 kpc (Maciel 1984), and 2.9 kpc (Zhang 1995).

  11. Crux Australis

    If the source system was a binary, wouldn’t the nebula be less symmetrical? As in, wouldn’t the gas be released at different speeds as the two stars orbited each other, resulting in some more pronounced structure within the nebula?

  12. fedra

    There must be a misunderstanding. Isn’t Abell 33 a cluster of galaxies? This object is named – together with the other labels – PN G238.0+34.8.
    However, very cool object indeed!

  13. JR

    @4 SIMBAD FTW. The most recent reference (Weidman & Gamen) points to (among many other things) Ciardullo et al. 1999, which calls the central star a probable binary:….118..488C

    The HST data they use for that paper can be seen at the Hubble Legacy Archive:


  14. chief

    Abell 33 reminds me of the PanAm sequence in 2001. Cue the music.

    Don’t see enough blue stuff, really catches your eye.

  15. JR (13): aHA! I didn’t use SIMBAD for this (ironically, since I was just using it for another unusual object I have yet to write about). Interesting. The distance they get is more than twice the old estimate, but is likely to be better. I measured the binary separation off my screen, so I’m not surprised I was off a bit there (though in a coincidence they get the same physical separation of 2000 AU).

  16. Snif! I pointed out that Hubble paper first! ./.

    mumble mumble mumble… (cue Talking Heads’ Psycho Killer)

  17. Brad

    “The wind it expels gets “blown back” by gas in between the stars… I’d expect the nebula itself to be compressed along one side, which it isn’t”

    “Most are oval, or barrel-shaped, or something even weirder.”

    OK, I’m probably really grasping at improbability straws here, but… Is there any tiny chance that it once was more oval-shaped and then got “blown back” from one side into a spherical shape? And maybe we just happen to be at the perfect angle and perfect time where it’s spherical from out POV.

  18. Dragonchild

    A wise man once said, “If you think that looks like a vagina. . . you need to get on the Internet more.”

  19. Mike Torr

    This is beautiful!

    When I saw “bubble” in the title, I thought you might be going to talk about that “spherical object eating plasma from our sun” video. Has that been brought to your attention yet Phil?

  20. Great Planetary nebula and image – cheers! :-)

    @18. Dragonchild : or perhaps “off” the internet instead? 😉

    @17. Brad :

    OK, I’m probably really grasping at improbability straws here, but… Is there any tiny chance that it once was more oval-shaped and then got “blown back” from one side into a spherical shape? And maybe we just happen to be at the perfect angle and perfect time where it’s spherical from out POV.

    That second suggestion sounds most plausible to me. I think the appearance of a lot of Planetaries depends fairly heavily on which angle we’re seeing them from so yeah.

    OTOH, the interstellar medium (ISM) is denser in some parts that others and that could play a role in shaping the morphology of this planetary too so, not out of the question in my view. Seems less likely to result in such a symmetrical bubble and that ISM density loops of gas / dust / whatever are so conveniently placed across such a relatively small area however.

    Disclaimer though – I’m not an expert on Planetaries – just educated speculation. Maybe someone who knows more can enlighten us?

    @12. fedra :

    There must be a misunderstanding. Isn’t Abell 33 a cluster of galaxies? This object is named – together with the other labels – PN G238.0+34.8.

    Well, doing a google images search using ‘Abell 33’ turns up firstly and mostly this Planetary neb so that seems to be a correct name for it. It is quite possible for this planetary to have both those designations and more as well. Indeed it is very common practice – if confusing – for any given astronomical object to have multiple ctalaogue designations for instance, Rho-1 Cancris is also 55 Cancris and no doubt has other designations as well. UV Ceti is also Luyten’s Flare Star, Luyten 726-8, Gliese 65 & BL Ceti.

    Wikipedia notes the existence of the Abell galaxy cluster catalaogue – click on my name here for link – but also acknowledges an Abell Planetary nebula catalogue exists as well although the link to that is currently dead. :-(


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