A nearly perfect circle in space

By Phil Plait | July 29, 2011 7:00 am

I don’t get a chance very often to combine two previous posts, but I was thinking recently about planetary nebulae — winds of gas blown off by dying stars — and remembered my very favorite one in the whole sky, Abell 39:

[Click to ennebulenate.]

Isn’t that awesome? It’s like it’s right out of Star Trek. I’ve written about the giant haloes surrounding some planetary nebulae before, and also about why some objects look like smoke rings. In a (nut)shell, as a star like the Sun begins its long, slow path to dying, it expands into a red giant and blows off a thick wind of matter. This material expands spherically in most cases, streaming off in all directions into space and forming what’s called a giant outer halo.

limbbrighteningIn most planetaries (like the famous Cat’s Eye nebula, and the less famous but also cool NGC 6826) the outer halo slams into material floating in interstellar space, causing it to get all clumpy or form a bright rim as the surrounding matter gets plowed up. But the rim around Abell 39 isn’t like that; while it’s bright, it’s actually an illusion! Near the edge, we are seeing through more material than we are through the center, and that would be true no matter from what direction we see the nebula. That makes the outer edge look brighter than the inner parts, giving the nebula the appearance of a vast, eerie smoke ring.

It’s pretty rare to get such a near-perfect circle of gas from a planetary nebula, and to be honest Abell 39 is one of only a very few I’m aware of. One reason it’s so perfect is that it’s located well above the plane of the galaxy. Down here, in that plane, there’s copious gas and dust. But Abell 39 is well away from all that, so its expanding red giant wind can retain its almost exactly spherical shape.

It’s also huge: 5 – 6 light years across, which is twice as large or more than most other planetaries, implying it’s old. Given the expansion velocity and size, it must be 20,000 years old or so. It’s also 7000 light years way, so it’s fairly faint, making it a rare target for amateur astronomers… though not unknown.

Readers with a keen eye might have noticed the star in the middle is a bit off-center. No one knows why. I’d love to see a telescope like Hubble or Gemini North get a deep view of this smoke ring. Why is the star misplaced? Why is the limb on the lower left brighter than on the right? Plus, the hint of detail in this image would be greatly enhanced in a deeper exposure, and that would really be incredible to see.

Image credit: WIYN/NOAO/NSF


Related posts:

The knotty Cat’s Eye halo
A delicately violent celestial shell game
A star on the edge of a weird, lovely death
The beginning of the end for a star

CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

Comments (34)

  1. I hope everyone clicks on the link to John Chumack’s photo (…amateur astronomers…though not unknown). That is one seriously gorgeous image.

  2. What beautiful images! Very cool.

  3. I like the galaxy seen through the nebula at about 2 o’clock.

  4. Bob

    So that is where I left my ball…

  5. Kurt L

    If the star is moving across our field of view from our perspective, would that cause it to appear off center?

  6. No wait, that’s not a galaxy, it’s a Romulan warbird!
    Cue Star Trek battle theme: Da da – da da – da da – da DUM!

  7. Jon Hanford

    In 2008 three amateurs discovered a large bubble-like planetary nebula close to the well known Crescent Nebula (NGC 6888) in Cygnus. Now informally known as the “Soap Bubble Nebula” (aka PN G75.5+1.7), a view of it through the 4-meter scope at Kitt Peak can be found here: http://www.noao.edu/image_gallery/html/im1059.html

    [The similarity to Abell39 is noted]

    A color image of the “SBN” taken by one of the co-discoverers can be found here: http://www.lostvalleyobservatory.com/imagelib/sitebuilder/misc/show_image.html?linkedwidth=actual&linkpath=http://www.lostvalleyobservatory.com/sitebuildercontent/sitebuilderpictures/und.bubble.cyg.kbq.jpg&target=tlx_new

    And a web page detailing all the work done to confirm this new discovery (and more great images too) can be found here: http://www.lostvalleyobservatory.com/review.nebula1/

    I like the way this planetary seems to “float” in the rich Milky Way star field.

  8. Jon Hanford

    Two more worthy bubble-like Abell planetaries:

    Abell 33: http://www.mistisoftware.com/astronomy/images/pk238_050412_2000.jpg

    Abell 34: http://www.mistisoftware.com/astronomy/images/pk248_0602_2000.jpg

    [note the two galaxies on the rim of Abell 34 at the 3 o’clock and 4 o’clock positions…..sweet!]

    Both of these beautiful images are by master astrophotographer Jim Misti.

  9. alfaniner

    Precisely what bugged me about the updates to the original Star Wars movies. Originally, Alderaan and the Death Star exploded in a spherical shape (oops – Spoiler!!!). The changes to make them explode into rings just didn’t make any sense. All because of the Praxis wave sequence in Star Trek VI, where it actually did make sense, as only a portion of the planet exploded.

  10. Cynthia Moreno

    Thats wicked. Period. =)

  11. Would it be possible that something has moved into the region that is able to affect the star more strongly than the gas through gravity? Perhaps it only looks spherical from our angle and from one 90 degrees off-set it would look more like a balloon.

  12. @Kurt L – I had a similar thought, but if the star’s ejecta was spherically symmetric, it should be travelling along with the same radial velocity as the star. I suspect the key lies with the brightened left limb. That brightening could indicate that more material was ejected from one side of the star than the other. That might have acted like a thruster rocket, giving the star a delta-v relative to the ejecta, and eventually causing it to move off center.

  13. Bemopolis

    The central star could be off center because of a slight density gradient in the interstellar medium (cf. the more extreme case of NGC 7635). In that case, though, I would expect the brighter limb to be closer to the central star rather than further, as appears to be the case in that image.

  14. @ alfaniner:

    Yeah, and Solo shot first!

    //geek

  15. katieM

    Maybe that star is actually behind the nebula. Other images of bubble type nebulae don’t seem to have a bright star in their centers. Its a beautiful nebula.

  16. @Bemopolis – Perhaps it’s my eyes, but the star appears to offset to the right, away from the brighter limb. As indicated in the article, there is much less medium for the expanding gas to interact with, being out of the galactic plane. I would think that if there was enough medium to cause a velocity shift, that would also cause the bubble to be aspherical and would cause excitation of the medium, resulting in additional spectral lines that should be easily identified.

  17. Sam H

    @13 Paul Knight: while my knowledge of technical astrophysics is very limited, just by looking at the image could I say: “easy, the star’s just throwing off a bit more material to the right.” But how often can this happen? I don’t really know anything about angular momentum and the related kinds of physical symmetry, but something tells me that those laws would favour a symmetrical shape. So since I dropped Math early in last year (now screwed because I feel like I haven’t done any Math in a lifetime :() and took Chem instead of Physics, how incorrect am I? :)

    @15 kuhnigget: being younger than most here I grew up with the ’97 edition so I favour it more, but that is still not true: I can’t find a link but apparently it was originally intended he shoot second. And the later edit of him and Greedo shooting almost at the same time just makes it better for me :)

  18. Bemopolis

    @Paul Knight: I agree — that’s why I noted it in the last sentence. As to asphericity, a linear density gradient will preserve the spherical shape of the wind-blown nebula while offsetting the star from the center by approximately r*2/2h, where r is the radius of the bubble and h is the scale height (i.e., the slope) of the density gradient.

    That’s based on a calculation in my thesis, but it references work done by V. Icke in 1973 (Astronomy & Astrophysics, 26, 45). (Finally, a justification for keeping my thesis at work.)

  19. @ Sam H – There are actually a number of mechanisms that could cause an asymmetrical distribution of ejected material. How that material interacts with the star’s magnetic field probably being one of the most influential, but the shape of the star might also play a part. What if the star had a high rotation about it’s axis and was significanly flattened as a result?

    We are also assuming that the bubble is, in fact, spherical. We are essentially looking at a cross-section, but as we look toward the edges we are looking through more material, causing the “smoke ring” effect. What if the bubble isn’t quite spherical, but is, say, an oblate spheroid, and we just happen to be looking at it’s spherical cross-section?

  20. @Bemopolis – If that is the case, then shouldn’t there also be indications of H I or H2 interaction? Are there signs of molecular absorption or perhaps excitation of neutral hydrogen along the wave front?

  21. Bemopolis

    @Paul Knight — First, I should clear up some sloppy writing in my first comment. The mass of the shell arises from *circumstellar* material, rather than *interstellar* material. That is, the primary component is atmospheric material shed during the star’s earlier AGB phase. Also, the nebula is significantly above the Galactic disk (a kiloparsed or so), so the density of the ISM is also fairly low. That being said, a gradient in the density of the ISM (or, more exactly, its pressure — which under isothermal conditions is the same thing) can shift the star from the center of the AGB wind in the same manner, so my earlier back-of-the-envelope calculation still stands.

  22. amphiox

    Precisely what bugged me about the updates to the original Star Wars movies. Originally, Alderaan and the Death Star exploded in a spherical shape (oops – Spoiler!!!). The changes to make them explode into rings just didn’t make any sense. All because of the Praxis wave sequence in Star Trek VI, where it actually did make sense, as only a portion of the planet exploded.

    Well, perhaps we could rescue those special effects by positing that first, whatever mechanism the Death Star laser uses to blow up a planet isn’t simply just a big explosion to blow it up, but sets off some kind of chain reaction in a planet’s core that involves a rapidly spinning disc/ring or material, and then initiates the explosion. And second, for the Death Star itself, we can propose that the space station’s fusion reactor consists of a rapidly rotating ring of concentrated plasma, in which the reactor does its thing….

  23. Without sounding grumpy, that is not even “nearly perfect circle”.

    If it was a perfect circle, the ISM would HAVE to have a constant density in the immediate region around the planetary nebulae progenitor star for a few parsecs. It would also assume that the expansion of the shell is perfect and uniform in all directions: that will not happen due to various reasons. Additionally, irregularities in the surface brightness are seen across the face of the shell (which would suggest the shock wave is hitting a denser region of ISM than others). Anyways, I read that this was all just about covered, so yeh, excellent read!

    Sorry to be grumpy. Lovely picture none the less! :P

  24. @ Sam H (#18):

    kuhnigget: being younger than most here I grew up with the ’97 edition so I favour it more, but that is still not true: I can’t find a link but apparently it was originally intended he shoot second. And the later edit of him and Greedo shooting almost at the same time just makes it better for me

    Listen here, you scruffy young nerf herder, this old geezer saw that movie 21 times the first summer it came out! Solo shot first! ./.

    From the original script:

    GREEDO: That’s the idea. I’ve been looking forward to killing you for

    a long time.

    HAN: Yes, I’ll bet you have.

    Suddenly the slimy alien disappears in a blinding flash of

    light. Han pulls his smoking gun from beneath the table as the

    other patron look on in bemused amazement. Han gets up and

    starts out of the cantina, flipping the bartender some coins

    as he leaves.

    HAN: Sorry about the mess.

    None o’ that revisionist Lucas alternate alternate history stuff here, boyo.

    //geek

  25. Sam H

    @25: kuhnigget: I don’t herd nerfs, but I stand corrected :P

  26. MadScientist

    Meh – the sun’s a far more nearly perfect circle (and even then it’s been known for over a hundred years that it isn’t quite a circle).

  27. andy

    I have to wonder about the assumption that the red giant wind should be spherically symmetric. I was under the impression that usually the polar wind and equatorial wind from a star end up having different velocities for various reasons. Presumably red giants rotate slowly due to their increased size (unless they merge with a binary companion which dumps a lot of angular momentum into the star) – is this sufficient to remove the polar/equatorial wind distinction?

  28. @kuhnigget – careful, only a nerf herder can call another nerf herder nerf herder.

  29. @ ^ So .. Solo was a nerf herder then? Plus Princess Leia? ;-)

    (Han didn’t deny it – & he allowed Leia to call him one. Click my name for the scene.)

    @27. MadScientist :

    ..the sun’s a far more nearly perfect circle (and even then it’s been known for over a hundred years that it isn’t quite a circle).

    It’ll get more perfectly circular when it becomes a white dwarf right? ;-)

    (Gravitational compaction and all that – although will it rotate faster and thus be more distorted?)

    @28. andy :

    Presumably red giants rotate slowly due to their increased size (unless they merge with a binary companion which dumps a lot of angular momentum into the star) – is this sufficient to remove the polar/equatorial wind distinction?

    Good question. I don’t know the answer to it, alas.

    As a rule of thumb I’d expect the larger diameter (radius) stars to spin more slowly on the “spinning ice-skater” principle but then younger stars spin faster than older ones (more time for magnetic /accretion disk braking to occur) and most of the very largest supergiants and giants are short-lived and thus don’t have long enough time to spin down. So original stellar mass and thus lifespan and evolution could likely be a factor.

    I suspect then that it may well vary considerably from individual giant to giant – perhaps helping explain why planetary nebula shapes vary so much among themselves. Stellar and exoplanetary companions almost certainly are a big variable ( ;-) ) here as well.

  30. I have a question: are we sure the center star really is in the middle of the nebula, rather than it being either in front or behind it and coincidentally lining up?

    If it actually is in the nebula, perhaps its one member of a former binary system, the other star being the one that exploded. When that happened, the stellar remnant wasn’t massive enough to keep the other star in orbit, and go was apparently ejected. As for where the other star is, maybe the explosion collapsed it into a neutron star or a black hole, which would make it very difficult to detect.

  31. @Bemopolis – I’m still thinking that if there is sufficient ISM to produce a pressure gradient capable of shifting the nebula mass off center, there should be some observable evidence of its interaction with the outflowing ejected mass. No? After all, this is an emission nebulae, so there should be a some amount of energy tied up in the outflow that would either get translated to the ISM, or absorbed by it.

  32. @Arik Rice – This is a planetary nebula, which results from an aging star sloughing off part of its outer atmospheric shell. This is not the same thing as a nova remnant as the star has not exploded. The progenitor is not part of a binary as this would cause the resulting nebula to be much less spherical and probably would contain a spiral component.

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