3.2. A Hypothetical H-R Diagram for the Inner Region
It is now possible to infer a luminosity-spectrum array that would give an integrated spectrum similar to the one observed for the nuclear region of the Andromeda Nebula. This is illustrated by the vertical lines in Figure 1, where the areas covered by these lines give the approximate loci of the principal contributors to the integrated light.
Figure 1. Hypothetical H-R diagram for the principal contributors to the light of the inner part of M 31 (vertical lines) and of NGC 4449 (horizontal lines). The plot for NGC 4449 is less complete than that for M 31, since spectra in the yellow and red regions were not available.
The question naturally arises as to how nearly unique this interpretation of the composite spectrum of M 31 may be. Although no certain answer can be given on the basis of the scanty M 31 data, the information available on the H-R diagrams and color-magnitude arrays for the older open clusters leads us to conclude that the hypothetical H-R diagram illustrated is approximately correct - as far as the principal contributors to the luminosity are concerned.
It is important to emphasize that the absence in Figure 1 of main-sequence dwarf stars of types G8 to M does not imply that such stars may not be plentiful in the inner part of M 31. In order to have an approximately equal effect in the integrated light, there would have to be of the order of 100 times as many G8 to K2 dwarfs as giants, and thus the relative number of K dwarfs to giants probably is less than 100 to 1. Any ratio smaller than about 50 to 1, however, would be consistent with the appearance of the integrated spectrum. Since the ratio of G8 to K3 dwarfs to giants is of the order of 40 to 1 in the neighborhood of the sun, it appears that a similar ratio is possible for the nuclear region of M 31. This conclusion emphasizes the fact that the principal contributors to the luminosity and to the mass of the inner part of M 31 may be entirely different in nature. Nevertheless, the existence of large numbers of K dwarfs in the system cannot yet be said to have been demonstrated.
We summarize this section as follows: The principal sources of luminosity for the inner part of the Andromeda Nebula are (a) giant stars of spectral types G8 to early M, and (b) an appreciable number of F8 to G5 stars. There is no evidence for the presence of A or early F stars in sufficient numbers to affect the integrated spectrum to an observable degree.