Extra-galactic nebulae

Transcriber’s Note
Cover image was created by transcriber using an image of the irregular nebula N.G.C. 4214 contained in Plate XIV. The cover is placed into the public domain.
By EDWIN HUBBLE
This contribution gives the results of a statistical investigation of 400 extra-galactic nebulae for which Holetschek has determined total visual magnitudes. The list is complete for the brighter nebulae in the northern sky and is representative to 12.5 mag. or fainter.
The classification employed is based on the forms of the photographic images. About 3 per cent are irregular, but the remaining nebulae fall into a sequence of type forms characterized by rotational symmetry about dominating nuclei. The sequence is composed of two sections, the elliptical nebulae and the spirals, which merge into each other.
Luminosity relations .—The distribution of magnitudes appears to be uniform throughout the sequence. For each type or stage in the sequence, the total magnitudes are related to the logarithms of the maximum diameters by the formula,
where C varies progressively from type to type, indicating a variation in diameter for a given magnitude or vice versa. By applying corrections to C , the nebulae can be reduced to a standard type and then a single formula expresses the relation for all nebulae from the Magellanic Clouds to the faintest that can be classified. When the minor diameter is used, the value of C is approximately constant throughout the entire sequence. The coefficient of log d corresponds with the inverse-square law, which suggests that the nebulae are all of the same order of absolute luminosity and that apparent magnitudes are measures of distance. This hypothesis is supported by similar results for the nuclear magnitudes and the magnitudes of the brightest stars involved, and by the small range in luminosities among nebulae whose distances are already known.
Distances and absolute dimensions .—The mean absolute visual magnitude, as derived from the nebulae whose distances are known, is –15.2. The statistical expression for the distance in parsecs is then