A form of galaxy, in which most of the matter that makes up the
galaxy is flattened into a disc that rotates around a central core, or nucleus. The
forces of gravity at work within the disc create density waves, that in turn generate a spiral structure. Spiral galaxies are
classified as 'a', 'b' or 'c', depending on how tight or loose their spiral structures are.
Messier 74, the Phantom Galaxy in Pisces, is an archetypal example of a loose spiral galaxy. Seen almost exactly face-on, the structure of the two arms spiralling out from its core is clearly visible. Imagery provided by Aladin sky atlas
