Transcriber’s Note
Cover image was created by transcriber using an image of the irregularnebula N.G.C. 4214 contained in Plate XIV. The cover is placed into thepublic domain.
EXTRA-GALACTIC NEBULAE1
By EDWIN HUBBLE
[Transcriber’s Note: This etext was produced from
The Astrophysical Journal, Vol. LXIV, pp. 321-369, 1926.]
ABSTRACT
This contribution gives the results of a statistical investigation of400 extra-galactic nebulae for which Holetschek has determined totalvisual magnitudes. The list is complete for the brighter nebulae in thenorthern sky and is representative to 12.5 mag. or fainter.
The classification employed is based on the forms of the photographicimages. About 3 per cent are irregular, but the remaining nebulae fallinto a sequence of type forms characterized by rotational symmetryabout dominating nuclei. The sequence is composed of two sections, theelliptical nebulae and the spirals, which merge into each other.
Luminosity relations.—The distribution of magnitudes appears tobe uniform throughout the sequence. For each type or stage in thesequence, the total magnitudes are related to the logarithms of themaximum diameters by the formula,
where C varies progressively from type to type, indicating a variationin diameter for a given magnitude or vice versa. By applyingcorrections to C, the nebulae can be reduced to a standard type andthen a single formula expresses the relation for all nebulae fromthe Magellanic Clouds to the faintest that can be classified. Whenthe minor diameter is used, the value of C is approximately constantthroughout the entire sequence. The coefficient of log d correspondswith the inverse-square law, which suggests that the nebulae are all ofthe same order of absolute luminosity and that apparent magnitudes aremeasures of distance. This hypothesis is supported by similar resultsfor the nuclear magnitudes and the magnitudes of the brightest starsinvolved, and by the small range in luminosities among nebulae whosedistances are already known.
Distances and absolute dimensions.—The mean absolute visualmagnitude, as derived from the nebulae whose distances are known, is–15.2. The statistical expression for the distance in parsecs is then
where mT is the total apparent magnitude. This leads to mean valuesfor absolute dimensions at various stages in the sequence of types.Masses appear to be of the order of 2.6×108 ☉.
Distribution and density of space.—To apparent magnitude about16.7, corresponding to an exposure of one hour on fast plates withthe 60-inch reflector, the numbers of nebulae to various limits oftotal magnitude vary directly with the volumes of space representedby the limits. This indicates an approximately uniform density ofspace, of the order of one nebula per 1017 cubic parsecs or 1.5×10–31in C.G.S. units. The corresponding radius of curvature of thefin