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The Bubble Nebula (NGC 7635)
Shell Nebula & HII region
NGC 7635, LBN 549
Integrated Visual Magnitude: 11
Apparent Diameter: 3'
Distance: 11,000 ly
Actual Diameter: 10 ly

Minimum requirements to view: 6-inch scope and dark skies


Lying near the open cluster M52, this faint nebula is a challenge for users of large and small telescopes alike. Just glimpsing any of it in six inch or smaller instruments is a bit of a feat, and larger telescopes promise to reveal intricate details. Sky & Telescope described, "a bright uniform disc 2' across surrounding a 9th magnitude star," as seen in a six inch telescope.

For me, this one was tough in my six inch, even from the dark skies of the Sacramento Mountains of New Mexico. I've shaded in the area in the chart below to match my observation, which represents the field as seen in my scope at 50x. North is down and east is to the right. You may need to adjust your monitor to see the shaded region.

Using averted vision I was able to briefly glimpse a very faint hazy bar between stars 1 and 2. I also had the impression of some haze to the right of the bar. I made this observation not having read any descriptions of the nebula. I failed to notice the bright disk described above, which would surround my star 1, but I tend to dismiss haze around bright stars as the result of humidity. I only recently realized that I was seeing nebulosity around stars in the Pleiades, for instance, but that's another story... 
The image below was generated by combining red and blue second generation DSS images. It shows the central area above (star 1 is the bright blue star to the left, 2 is just off the image on the bottom left).  It is clear from the image that I saw the surrounding nebulosity in the 6-inch rather than the Bubble itself. 

In larger instruments a round glow can be seen about my star 1. At 150x or higher this glow may appear asymmetrical, splitting into distinctly separate arcs.

The Bubble appears to be the consequence of the massive, hot 06.5III star SAO 20575 (centered in the image above). Not only is this star exciting the surrounding gas to glow via its prodigious UV radiation, it apparently ejected a shell of gas and dust out into those surrounding clouds some 300,000 years ago.  Both the star and the ejected material were initially moving rapidly through this region. As the ejected material interacted with the nearby molecular cloud it created the arcs of the bubble--a dusty blister on the edge of a molecular cloud. In the process it was also slowed. The "central" star continued along, moving away from the bubble center in the intervening years to where we see it now. 

The Bubble bears a remarkable similarity to Thor's Helmet.
 

Millennium Star Atlas Vol III Chart 1070
Sky Atlas 2000 Chart 3
Uranometria 2000 Vol I Chart 34