There is one important property common to star clusters and galaxies: both are composed primarily of stars. The component stars in clusters can be observed separately; but for all except the nearest galaxies this is impossible, and methods for the study of composite radiation must be used. These methods introduce uncertainty and indeterminateness; but with care it is possible to derive some information of fairly precise interpretation.
In what follows, we shall make a descriptive comparison of some characteristics of star clusters and galaxies from the point of view of their stellar populations, as indicated by the spectroscopic evidence of their integrated light. It should be emphasized that we shall be concerned only with stars that contribute a major share to the integrated light; the principal contribution to the total mass is, of course, an entirely different matter.
In a certain sense, the history of a star cluster or a galaxy can be considered to be the history of the component stars. This approach, then, which we shall utilize, is in its essence a problem of stellar classification - classification of large groups of stars from observations of their integrated light. We adopt the spectroscopic method rather than the photometric because of (1) the possibilities of unique detection of certain kinds of stars by spectroscopic methods and (2) the complicating effects of interstellar reddening on the interpretation of photometric measures in extragalactic sources. The photometric method is a most important one; and the pioneering work of Baum (1955) should be noted especially.
We shall proceed as follows: the results of a new spectroscopic investigation of galaxies will he given as a modification of the Hubble form classification. The spectroscopic characteristics of this new classification will be described. Then, making use of the new form classification, we shall make a comparison between star clusters and galaxies. Finally, again using the new form classification, we shall note some general characteristics of the clusters of galaxies located nearest to us. We shall confine ourselves to galaxies of the brightest two to three magnitudes of absolute luminosity.