Unlike the Crab Nebula, the rendition of the Sombrero Galaxy (above) was produced without utilizing any pattern from the images I downloaded from the Internet. The cropped image was my model since it had more detail and color to work with (below). The full image (far below) is no doubt more like the natural exposure directly from a telescope, and in fact looks very much like the one from the Hubble site.
The process I used to create this image was first to fill my canvas with black and once again borrow a star field from one of my space images. This image also only had ten major stars in the field, and I placed those by guessing.
To create the pattern for the galaxy I then opened another canvas, this time square so I could create a circle. With the elliptical marquee, I held the shit key and made a circle nearly the size of my canvas, then feathered it 50 pixels. I used the lighter blue and painted the circle with a splatter brush, then went to Filters>distort>twirl and applied that to create the rotational effect.
The next step is to drag the circle into the star field, then distort the circle until it is a flat ellipse. Also add some tilt. I created a new layer and painted the front edge with black and some highlight colors. I created another layer for the dome, then duplicated it to work with the top dome and bottom dome separately. This is done by erasing the upper portion for the bottom, and the opposite for the top.
I found that some of the wet media brushes worked quite well for creating the textures. This was one of those artistic endeavors that seemed easy enough to recreate, unlike the Crab Nebula, which proved to be a major challenge.