Because of its nearness, and the fact that it is visible from the northern hemisphere, M33 is one of the two most intensively studied extragalactic objects. Observations of M33 figured prominently in the controversy over the island-universe theory, and in the proof that spiral nebulae are in fact galaxies similar in size to the Milky Way. As a giant stellar system, M33 displays many similarities with our own Galaxy. M33 is a weak nonthermal radio source. It is highly flattened and approaches circular symmetry both in the distribution and motions of its component stars and gas. The abundances of the chemical elements are similar to those found in the solar neighbourhood; both young and old populations of objects are represented. The most abundant gaseous constituent, neutral atomic hydrogen, is distributed in a broad ring with its maximum projected surface density well outside the most prominent optical features of the galaxy. These features, noted as a result of their high surface brightness, contribute only a small fraction of the total luminosity of M33. Most of the light is emitted by a smooth distribution of vast numbers of individually less luminous stars.
The differences between M33 and the Milky Way are mostly Ones of degree rather than kind. M33 is the less massive of the two galaxies. the kinematics, the explanation of which is very important to the understanding of the same puzzling phenomenon in our own Galaxy. The gathering of more quantitative data on the distribution of the various gaseous constituents of M33, in addition to the neutral atomic hydrogen, may shed light on the problems of the interstellar medium and of star formation. Without doubt there are many fertile fields for investigation, even in the nearest galaxies.
It is a pleasure to thank Dr M. S. Roberts for many helpful discussions and suggestions. I am also indebted to Mr M. A. Smith and Dr C. M. Wade for aid in compiling the bibliography. This study forms a portion of a doctoral thesis for The University of Michigan. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation.