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Hot Crescent Rolls… A Bubble?

The Crescent Nebula by Dietmar Hager and Immo Gerber

The Crescent Nebula by Dietmar Hager and Immo Gerber

The Crescent Nebula, also known as NGC 6888, is a very well renown and most intriguing object located in the constellation Cygnus in the northern hemisphere. At an apparent size of about 18 by 13 arc-minutes it is a very pale nebula. Even in a moderate amateur telescope you can’t quite see this one unless you have absolute dark skies (or narrow band filters) and a decent “light bucket”. So how do we get a chance to study it? Photographically, of course…

Spanning some 25 by 18 light years, gazing at NGC 6888 means we are looking 4700 years into the past, a past that renders a nebula fueled and excited by the blue star at the center. And not just any blue star – but a high mass super-giant star – one that depleted its fuel at “full speed”. Not only was it a super giant, but hot… in the class of “Wolf Rayet” stars (HD 192163). Now, after only a couple of million years the “stellar gas” is almost used up and the star is standing right before a significant change: a supernova candidate. Behold a star that vents its outer layers into space at terrific speed!

“Images are used to constrain models of the ionization structure of nebular features.” says Brian D. Moore (et al) of the Department of Physics and Astronomy, Arizona State University, “From these models, we infer physical conditions within features and estimate elemental abundances within the nebula. The results of our analysis, together with the degree of small-scale inhomogeneity apparent in the images, call into question the assumptions underlying traditional methodologies for interpretation of nebular spectroscopy. The thermal pressure of photoionized clumps is higher than the inferred internal pressure of the shocked stellar wind, implying that the current physical conditions have changed significantly over less than a few thousand years.”

While the central star sustains severe loss of mass, the gas is holding lots of oxygen and hydrogen… just before the individual big “bang” of the WR-star creating a “hot bubble” whose struture can’t quite be explained yet. “A detailed analysis of the H I distribution at low positive velocities allowed us to identify two different structures very probably related to the star and the ring nebula. From inside to outside they are: (1) an elliptical shell, 11.8×6.3 pc in size, that embraces the ring nebula (labeled inner shell); and (2) a distorted H I ring, 28 pc in diameter, also detected in IR emission (outer shell). The borders of the inner shell strikingly follows the brightest regions of NGC 6888, showing the sites where the interaction between the nebula and the surrounding gas occurs. A third structure, the external feature, is a broken arc detected at slightly higher velocities than the former shells.” says Christina Cappa (et al), “We propose a scenario in which the strong stellar wind of HD 192163, expanding in an inhomogeneous interstellar medium, blew the outer shell during the main sequence phase of the star. Later, the material ejected by the star during the LBV (or RSG) and WR phases created NGC 6888. This material encountered the innermost wall of the outer shell originating the inner shell. The association of the external feature with the star and the nebula is not clear.”

For a look inside, view the full size image!

Many thanks to Dietmar Hager and Immo Gerber of TAO-Observatory for sharing this incredible image!


Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

Comments on this entry are closed.

  • Anaconda October 21, 2009, 10:11 AM

    What started as a post with a beautiful image and some innocuous comments turned into an exhibition in Saul Alinksy style tactics.

    But ultimately, it’s counter-productive because fair-minded people are repelled by this kind of harsh rhetoric and see through it and realize it says more about its practitioners than about its targets.

  • DrFlimmer October 21, 2009, 12:52 PM

    Well, Anaconda, the problem is your past. And this past makes your post everything but innocous. So the reaction you’re faced with is not that “innocous” post but thing you said and did in recent times.

    Of course, it could have been that you have changed, I would welcome that, but — I doubt that!

  • Hon. Salacious B. Crumb October 21, 2009, 2:48 PM

    Do be too smug. The fallout over this is likely over this story is going to be more far reaching.
    As for Tammy, pity she has no concept of the kind of rotten and dodgy practices applied by pseudo-science and nutters like yourself.
    Science employs serious discipline, mockery of science by individuals who haven’t a clue lies with the baboons. Really, having an “opinion” doesn’t mean you know what your talking about.


    As for the claim that “ring nebula” is; “…automatic link system of the website assigned it to M57”, still hasn’t be corrected in this article. It is not connected to the “Ring Nebula”

    Also, it is interesting that the material quoted in this article has not been properly attributed. The quotes are actually taken from the paper from; Cappa, Cristina Elisabet., et.al., “The Interaction of NGC 6888 and HD 192163 With the Surrounding Interstellar Medium.“., AJ., 112, 1104 (1996) Why haven’t you given this as the source in the formal article – especially when making use of extensively paraphrasing? Why is their no attribution to the article or the Astrophysical Journal?
    I just wonder how the Astrophysical Journal would feel about this, especially if information isn’t made for ‘profit’?

    As for saying; “sometimes i teach – and sometimes i learn – sometimes i get burnt… but it’s all good”

    There is one thing that you left-out though… you seem very happy, almost gleeful, to expunge your own personal moral standing on others, and more than willing to burn others. I suppose “but it’s all good”, too?