The distribution of young stars, clusters and cepheids in the Milky Way and M33 - a comparison

Abstract

Ever since the pioneering work by Morgan and his collaborators (1952, 1953), it has been well known that the distribution of the associations of young stars, HII regions, and young clusters defines the optical spiral features. Although considerable progress has been made in spiral structure studies during these past 25 years, the basic picture of optical spiral structure has not been significantly altered. The three spiral features first described by Morgan are still recognized. Modern work on the various optical spiral features has strengthened and improved the definition of the optical features, especially to larger distances. Most of the improvements and any additions to the basic three-arm pattern have resulted primarily from observations of the spiral tracers in the Southern Milky Way. Specifically, the Sagittarius feature is now generally recognized as the Sagittarius-Carina arm which may indeed be a major arm of the Galaxy. It can now be traced optically to very large distances, up to 6 kpc or more in the direction ℓ = 290°. The Local arm (Cygnus-Orion) probably extends to 4 kpc in the direction of Puppis (ℓ ≅ 240°), and most astronomers would probably agree that our local spiral feature is not a major arm, but an inter-arm feature.