3.4. A Comparison of the Spectra of M 31 and M 33

The Crossley spectrograms of M 33 obtained by Mayall and Aller ⁶ reveal an entirely different distribution of stellar population in the H-R diagram the ultraviolet region - in both the nucleus and the surrounding area - shows conspicuously in absorption the Balmer lines of hydrogen; the cyanogen absorptions, which are well marked in the spectrum of M 31, are not observed. The spectral type in the range 3800-4100 is F, at 4340 it is G0, and thus the spectral type changes rapidly with wavelength. Since spectrograms in the green, yellow, and red regions were not available, it was not possible to construct a hypothetical H-R diagram with the same confidence as in the case of M 31. This is an important future spectroscopic task, especially since independent information on the stellar content of M 33 may be anticipated from current work by A. R. Sandage at the Mount Wilson and Palomar Observatories. ⁷

In spite of the present incomplete data, there can be little doubt that the percentage of A- and F-type stars is far higher in the main body of M 33 than in that of the Andromeda Nebula. We therefore emphasize the points that differences in the physical appearances of M 31 and M 33 are reflected in their integrated spectra, and that there is no large, bright amorphous central bulge in M 33.