A recent investigation has called attention to a rather close correlation between the forms of certain galaxies and their stellar content as estimated from composite spectra. (1) The correspondence between form and spectral appearance is especially close for two categories of galaxies: (1) Irregular systems of the Magellanic Cloud type and spirals having an insignificant central concentration of luminosity, and (2) giant ellipticals such as those found in the Virgo cloud, and spiral systems like M 31, in which the major share of the luminosity of the main body is clue to a bright amorphous central region. In the first category, early-type stars and emission nebulosities have a pronounced effect on the spectrum in the blue and violet regions; in the second, the luminosity of the brighter parts is due principally to yellow giant stars.
Systems possessing an intermediate degree of central concentration of light (M 51) give spectroscopic evidence of an intermediate kind of stellar population: the degree of compositeness is very high, and it appears likely that most of the luminosity is due to a mixture of F to G main-sequence stars and K giants.
These results are only approximate; programs now being initiated will determine the relative contribution of different kinds of stars in galactic population over as large a range in wavelength as possible. However, the general relationship between central concentration of light and spectral appearance (stellar population) appears definite; and a preliminary attempt at a classification of galactic forms in terms of their stellar populations seems to be justified.
1
W. W. Morgan and N. U. Mayall,
Pub. A.S.P., 69, 291, 1957.