3.3. The Spectrum of the Main Body
The angular dimensions of the Andromeda Nebula are so great that spectroscopic observations usually refer only to the brightest inner part in the neighborhood of the nucleus. In order to investigate the spectrum over a larger region it is convenient to have an image-forming telescope of short focal length.
Such an instrument was constructed a number of years ago at the Lick Observatory by Horace W. Babcock, who employed a short-focus six-inch mirror that had the same focal ratio of 5.8 as the Crossley reflector. 5 The nebular spectrograph was then used in conjunction with this small reflector to obtain slit spectrograms that cover a far greater angular extent of M 31 than is possible with larger telescopes. We consider it most fortunate that these small-scale spectrograms taken by Babcock were available for the investigation of the integrated spectral type of those parts of the main body of M 31 that are situated at some distance from the nuclear region.
A careful comparison of these spectrograms with those of the nuclear region itself fails to disclose any marked differences, as far as the spectral types in the violet and blue regions are concerned. It seems justifiable, therefore, to draw the tentative conclusion that the populations of the nuclear region, and of the smooth, featureless disk that extends out from the nucleus, are similar.
There is one further spectral characteristic that seems to be of interest: the giants that compose the principal source of luminosity in M 31 are normal giants; they are not the weak-lined variety observed in some globular clusters.