The break through was an achievement of great telescopes and especially of the greatest of them all, the loo-inch reflector on Mount Wilson. The story is briefly as follows. Nebulae have long been known as faint mysterious patches of light, scattered with the stars over the face of the sky. One, the Andromeda nebula, is readily seen by the naked eye, and the numbers increase rapidly as the luminosities diminish. In the course of time several thousand nebulae have been catalogued individually; today, perhaps 200,000 are recorded on photographic plates. The greatest telescope, under the best conditions, records as many nebulae as stars.
Study of the nebulae revealed two quite different types. The one consists of clouds of dust and gas illuminated by neighbouring stars. These object's, numbering a few scores in all, are members of our own stellar system - the galactic system, or system of the Milky Way. They show a decided preference for the plane of the Milky Way - the galactic plane - and for this reason they are known as `galactic' nebulae (or sometimes as nebulosities). They will not be further discussed in these lectures.
The other type of nebulae consists of the regular, symmetrical bodies, many of them showing a spiral structure, -found by the thousands everywhere in the sky except in the Milky Way itself. Positive information concerning their true nature began to accumulate about a quarter of a century ago, and, by 1924, their status was determined. They were demonstrated to be independent stellar systems as the theory of island universes had supposed.
The conspicuous neighbouring systems were so near that, with the 100-inch reflector, many of their brightest stars could be photographed individually. Among these stars, various types were recognized which are familiar in our own stellar system. They were all super-giants and their intrinsic luminosities were fairly well known. Therefore, their apparent faintness indicated the distances of the nebulae in which they were found. In all cases where the method could be applied the nebulae lay far beyond the boundaries of our own stellar system. They were scattered through extragalactic space, and, consequently, they have been called 'extra-galactic' nebulae. In these lectures the adjective will be dropped and the systems will be called nebulae.
Once the flood-gates were opened a wave of exploration surged forward. Already there were large accumulations of data which awaited only the essential clue, the scale of nebular distances, for their interpretation. Further accumulations followed, and now they were planned with a true perspective.
The new investigations followed two lines. In the first place, the more conspicuous nebulae were studied individually in order to determine their structure and contents, to discover their common features, and to devise general methods of estimating distances. Then, with the nature of the inhabitants known, and the scale on which they are scattered, the characteristics of the observable region as a whole were examined. The general investigation was made in two steps, a preliminary rapid reconnaissance followed by a series of accurate surveys. These lectures concern the last stage of the investigations but, in order to clarify the significance of the final results, it will be convenient to summarize the reports of the preliminary studies.