TABLE XVII ========+=======================+=======+===================== Type | Diameter in Parsecs | Type | Diameter in Parsecs +-----------+-----------+ | | Obs. | Cal. | | --------+-----------+-----------+-------+--------------------- E0 | 360 | 340 | Sa | 1450 E1 | 430 | 380 | Sb | 1900 E2 | 500 | 430 | Sc | 2500 E3 | 590 | 490 | | E4 | 700 | 570 | SBa | 1280 E5 | 810 | 680 | SBb | 1320 E6 | 960 | 850 | SBc | 2250 E7 | 1130 | 1130 | Irr | 1500 --------+-----------+-----------+-------+--------------------- MASSES OF EXTRA-GALACTIC NEBULAE 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 Mass = 2.6L. (9) The application of this method of determining orders of masses seems to be justified, at least in the case of the later-type spirals and irregular nebulae, by the many analogies with the galactic system itself. Moreover, when applied to M 31, where the distance is fairly well known, it leads to a mass of the same order as that derived from the spectrographic rotation: MASS OF M 31 Spectrographic rotation 3.5×10^9 ☉ Öpik’s method 1.6×10^9 The distance of N.G.C. 4594 is unknown, but the assumption that it is a normal nebula with an absolute magnitude of –15.2 places it at 700,000 parsecs. The orders of the mass by the two methods are then MASS OF N.G.C. 4594 Spectrographic rotation 2.0×10^9 ☉ Öpik’s method 2.6×10^8 Here again the resulting masses are of the same order. They can be made to agree as well as those for M 31 by the not unreasonable assumption that the absolute luminosity of the nebula is 2 mag. or so brighter than normal.