NGC 7635 -- Emission Nebula (Bubble Nebula) in Cassiopeia
A Hidden Bubble
In photographs the Bubble Nebula is an impressive sight.  Visually, it is very difficult to
see.  Even under a dark sky, a nebula filter, and my 18-inch Dobsonian, the Bubble is
elusive and challenging.  With the much smaller 110mm Refractor at a local Park, seeing
the outline of the Bubble is impossible. But the much larger emission nebula surrounding
it, NGC 7635, is possible but still difficult.

The Bubble Nebula is the smaller and brightest of three bubbles formed from ferocious
solar winds from the star SAO 20575 (also known as BD +60 2522) slamming into the
surrounding gas/dust cloud.  This star also provides the radiation that fuels the
surrounding H II region (NGC 7635).  SAO 20575 is an O6.V star that shines at
magnitude 8.7 and has a mass of about 40 times that of our sun.

The brightest part of the huge emission nebula, NGC 7635, rests north of the star.  The
brightest section of the Bubble’s rim is the northern rim near the star itself.  Note that the
reason the star is not at the center of the Bubble is likely due to movement of the star
through space (proper motion) since the formation of the last shock wave.

It was only the brightest part of both NGC 7635 and the Bubble’s northern rim that could
be found with the small refractor.  Even this was difficult and required a nebula filter.