Galactic Spiral Arms and Evolution

Abstract

It is more than 120 years since the discovery of spiral galaxies, and naturally there has been much speculation on their origin. The vital clue is the composition of the arms, and it is now known that the arms are patterns of newly formed, and hence very luminous, stars. Thus the problem of spiral arms is only part of the broader problem of the origin of stellar systems in general. The various galactic forms of the Hubble morphological classification are differentiated by their different stellar systems, and so the problem of spiral arms is part of the basic problem of the origin of the Hubble types. For this reason the potential significance of any theory of spiral arms is vastly extended.