The Velocity-Distance Relation among Extra-Galactic Nebulae

Methods of determining distances of extra-galactic nebulae are discussed, and the mean absolute magnitude is revised on the basis of (i) Shapley's revision of the zero- point of the period-luiuinosity curve for Cepheids, and (2) more extensive observations of stars involved in nebulae. The revised value is M (vis) -14.9. The mean color-index of the nearer extra-galactic nebulae appears to be of the order of +1.1 mag., hence M(pg) = -13.8. A color-excess is suggested which is independent of distance but shows some relation to galactic latitude. The velocity-distance relation is re-examined with the aid of 40 new velocities, 26 of which refer to nebulae in 8 clusters or groups. Distances of the clusters, ranging out to about 32 million parsecs, have been derived from the most frequent apparent mag- nitudes. The velocity displacements reduce the apparent magnitudes by amounts which become appreciable for the more distant clusters. The new data extend out to about eighteen times the distance available in the first formulation of the velocity-distance relation, but the form of the relation remains un- changed except for the revision of the unit of distance. The relation i