Spectroscopic rotations are available for the spirals M 31[20] and
N.G.C. 4594,[21] and from these it is possible to estimate the masses
on the assumption of orbital rotation around the nucleus. The distances
of the nebulae are involved, however, and this is known accurately
only for M 31; for N.G.C. 4594 it must be estimated from the apparent
luminosity.
Another method of estimating masses is that used by Öpik[22] in
deriving his estimate of the distance of M 31. It is based on the
assumption that luminous material in the spirals has about the same
coefficient of emission as the material in the galactic system. Öpik
computed the ratio of luminosity to mass for our own system in
terms of the sun as unity, using Jeans’s value[23] for the relative
proportion of luminous to non-luminous material. The relation is