geckzilla wrote:Interesting nebula highlighting the difficulty in measuring the distance to such objects. It's either a very close planetary nebula or a rather distant outflow without any particular name. I can't say it makes much sense to me either way.
My complete amateur impression is that Ou4 is not a planetary nebula. The reason is that the nebula is centered exactly on a hot, massive star, HR 8119. According to my software Guide, which quotes Bright Star Catalog, HR 8119 is the exciting star of the red Ha emission nebula, SH2-129. I find it too much of a coincidence that the blue-green OIII nebula, which is exactly centered on HR 8119, could be a nebula centered on a white dwarf exactly along the line of sight of HR 8119.
HR 8119 is undoubtedly a powerful star. According to Guide, which cites the most recent version of the Hipparcos Catalog, this bright sixth magnitude star (V = 5.728 ± 0.010) has a parallax of 0.76 ± 0.42 milliarcseconds, which suggests that its distance maybe about 4,000 light-years and its (reddened) V luminosity may be equal to about 8,000 Suns. Its proper motion is small too, reinforcing the impression that HR 8119 is a far-away, bright star. Guide classifies HR 8119 (or HD 202214) as a star of spectral class B0V. If this is correct, the star is still on the main sequence and bright for its spectral class. I have spent quite some time checking the color and the Hipparcos distance to hot blue stars, and my impression is that stars of spectral class B0V typically have a V luminosity between 1,000 and 2,000 Suns. For comparison, well-known Alnitak, the star "on the Betelgeuse side" of Orion's Belt, is a supergiant star of spectral class O9.5Ib, but it is about equally bright in V light as HD 202214, or about 8,500 times the Sun. Stars of the same spectral class may vary considerably in brightness, and HD 202214 is most likely a B0V whopper.
HR 8119 or HD 202214 is not a variable star, according to Guide. Or rather, its only variability has to do with the fact that it is a binary star, probably an eclipsing binary. But that strange green nebula certainly speaks of some kind of outburst in the past.