Galactic and Extragalactic Studies: XIII. Note on the Comparative Diameters of Spheroidal and Spiral Galaxies.

From the photometric results presented in this communication it appears that contrary to some earlier indications the open spirals, with their supergiant stars and superficial structure of spiral arms, are not appreciably larger than spheroidal galaxies, which seem to be devoid of highly luminous stars and free of distributional irregularities. It follows that if in the course of time spiral arms appear in a flattened spheroidal system, they probably should not be treated as an extension or an expansion outward from the nuclear part of the galaxy, but as a development of structure well within the main body of the system. Or if, with the direction of evolution reversed, it be assumed that the spiral arms, supergiant stars and diffuse nebulosities of the spiral systems can eventually disappear into the structureless smooth form of the typical spheroidal galaxy, then that transformation also should be treated as an internal readjustment, and not as a contraction; for at any given considerable distance from the nucleus the amount of light (and probably of mass) is now found to be about the same for spheroidal and spiral systems. * Apparently there is no important "growth" in dimensions along the sequence of forms. t The spiral arms as observed are a phenomenon of only the inner half of an average galaxy. Moreover, less than twenty per cent of the light of a spiral galaxy is, on the average, in its spiral arms. The remainder is mostly in the commonly overlooked background in which the spires are embedded.