Nebulae in a Spiral Galaxy
Now astronomy has progressed quite a lot since the days of Messier. Not only do we know that his spiral nebulae are in fact entire galaxies in their own right, but we can also now study the make-up of some of these galaxies. For example the local group galaxy M33.
The Hydrogen light colour has been exaggerated in this image to reveal the nebulae that exist within this spiral galaxy. This image has helped to demonstrate the theories of star formation in spiral arms, and nebulae being associated with star formation, by the revealing that this galaxies nebulae tend also to be located in the spiral arms.