THE previous lecture discussed the nature of the nebulae, their intrinsic luminosities and their apparent distribution. The nebulae are stellar systems, more or less comparable with our own system, scattered through space as far as telescopes can reach. Their luminosities vary, but not widely; on the average, they are about 85 million times as bright as the sun, and, consequently, they can be observed and studied in very remote regions of space. Since they are so nearly alike, their apparent faintness measures their distances. Therefore, the apparent distribution can be determined by counting the numbers of nebulae to successive limits of apparent faintness. The extensive data now available indicate that the apparent distribution thins out with increasing distance, systematically in all directions.
However, the apparent distribution is not the true distribution. Distances are estimated on the basis of the simple law of inverse-squares. Of two equally luminous bodies, if one appears four times fainter than. the other, it is supposed to be twice as distant; if a hundred times fainter ten times as distant. The results are precise, provided the apparent luminosities are not influenced by any factors other than distance. But other factors must be considered. Space-absorption, for instance, or rapid motions in the line of sight, might presumably affect the apparent luminosities. The first, space-absorption; is now known to be negligible, but the second presents a question for investigation which, as we shall see, is still unanswered.
One other factor remains to be considered. This factor - red-shifts in nebular spectra - quite definitely affects the apparent luminosities of the nebulae, and in a very conspicuous manner. The apparent distribution of nebulae and, indeed, all results involving estimates of distances, must first be corrected for effects of red-shifts before the actual map of the observable region can be drawn. Therefore, the present lecture will be devoted to the observed behaviour of red-shifts, and their effects on apparent luminosities. We enter at once into the rather technical field of spectrum analysis, but this is a necessary preliminary for the more general discussion in the concluding lecture.