Spiral

Spiral galaxies usually consist of two major components: A flat, large disk which often contains a lot of interstellar matter (visible sometimes as reddish diffuse emission nebulae, or as dark dust clouds) and young (open) star clusters and associations, which have emerged from them (recognizable from the bluish light of their hottest, short-living, most massive stars), often arranged in conspicuous and striking spiral patterns and/or bar structures, and an ellipsoid ally formed bulge component, consisting of an old stellar population without interstellar matter, and often associated with globular clusters.

The young stars in the disk are classified as stellar population I, the old bulge stars as population II. The luminosity and mass relation of these components seem to vary in a wide range, giving rise to a classification scheme. The pattern structures in the disk are most probably transient phenomena only, caused by gravitational interaction with neighboring galaxies.