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Planetary Nebula
Magnitude of central star: 18.7 Distance: 4300 ly
Minimum requirements to detect: 4-inch telescope under suburban skies |
NGC
7139 in a very overlooked planetary nebula in Cepheus. Although it
has been detected in a 4-inch telescope, most observers will require at least a
6-inch and dark skies. And while this planetary isn't faint enough to be
considered a challenge object, the surface brightness isn't all that high
(probably somewhat fainter than the 22.2 mag/arc-secē
quoted above).
In smaller apertures (8 inches or less) this will probably be an averted vision object, appearing as a faint round hazy patch of sky, just a bit brighter than the surroundings. A UHC or OIII filter will help bring it out, particularly at low magnification (less than 100x) or with less than dark skies.
In my 18-inch f/4.5 it appeared as a "faint round smudge" at 94x without a filter. I was able to recognize the nebula without having to identify the field stars but it is faint enough that it could have been easily overlooked. A better view came at 270x, where it was readily seen, but frustratingly faint as far as seeing any detail. At this magnetization it appeared as a soft round glow with hints of some sort of central structure. No central star was observed. An OIII filter helped, even at this magnification. Averted vision brought out tantalizing hints of internal structure that I simply could not pin down.
Having never seen an image of this planetary I wrote, "I bet the image is cool." Looking at the image above, I was probably seeing hints of the faint stars that lie within the nebula rather than real detail. Regardless, it was an interesting view. Like most planetary nebulae this one has a personality all its own. Be sure to try a variety of magnifications!
The field in an 6-inch f/8 at 60x. North is down and east is to the right.
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