The irregular galaxy NGC 4449 has often been considered the northern hemisphere counterpart of the Large Magellanic Cloud. Little symmetry of structure is visible, and the most striking features are apparently large gaseous emission regions, which bear some resemblance to the H II regions in the Galaxy and in the Andromeda Nebula. Crossley spectrograms show that the system is, in fact, either immersed in a gaseous stratum, or is composed of numerous unresolved H II regions in which the Balmer emission lines originate. In addition, the chief nebular [O III] lines are well marked, and the emission spectrum resembles that to be expected from a gigantic H II region. The absorption spectrum is also of great interest: the Balmer lines are strong in the ultraviolet, the spectral type in the range 3850-4100 is around A7, and at 4340 the spectral type is middle F.
It may be seen, therefore, that the three systems NGC 4449, M 33, and M 31 possess widely differing spectral and physical characteristics. The irregular galaxy NGC 4449 has a spectrum suggestive mainly of spiral-arm population, M 33 is intermediate in structural symmetry and spectrum, while M 31 is a well-developed Sb spiral having a pronounced central bulge that contains a relatively great number of CN giants.
A hypothetical H-R diagram for NGC 4449 also is illustrated in Figure 1 by the horizontal lines. A comparison with M 31 shows how different the stellar population may be in these two galaxies - as far as the contribution to the luminosity is concerned. Moreover, there may be a significant similarity between these two arrays and those for galactic clusters in the solar neighborhood that are considered to be of greatly different ages. Thus the hypothetical diagram for NGC 4449 appears to correspond to that for a young galactic cluster, while that for M 31 resembles the kind expected for an old cluster, whether galactic or globular. Despite its interest, we do not pursue this line of reasoning further at the present time, because of the fragmentary nature of the observations of spectra of galaxies.