THE NATURE OF THE NEBULAE — HUBBLE 139 



MESSIER 31 



Except for our own stellar system, the most conspicuous member of 

 the local group is the great spiral in Andromeda, M31, with its two 

 satellites, M32 and NGC205. The spiral can be seen with the naked 

 eye as a faint cloud about half the size of the moon, and with a total 

 luminosity equal to that of a fourth- or fifth-magnitude star. When 

 photographed with telescopes, the cloud takes shape as an elliptical 

 object whose long diameter is above five times that of the moon, and 

 whose ratio of axes is about 3 or 4 to 1 . The central region is relatively 

 bright but the luminosity fades outward to poorly defined edges. The 

 spiral pattern has never been seen with any telescope, although it is 

 easily recognized on the photographic plate. The image increases 

 with the focal length of the telescope, ranging from perhaps a quarter- 

 inch with small kodaks to about 6 feet at the Cassegrain focus of the 

 100-inch reflector. Thus, with powerful instruments, the nebula must 

 be explored section by section. Our detailed information, at present, 

 is restricted to selected areas, chosen for specific purposes. Photo- 

 graphs show a semistellar nucleus of about the fourteenth magnitude, 

 surrounded by a central region of structureless, unresolved nebulosity, 

 from which two arms emerge on opposite sides and wind outward to 

 form the spiral pattern. The general pattern is repeated over and 

 over among the fainter nebulae in the sky, and the variations are 

 largely concerned with the relative amount of luminous material in 

 the arms as compared with that in the central region. On the basis 

 of this criterion, the normal spirals fall into a progressive sequence with 

 M31 near the middle. In other words, M31 is a typical example of 

 the intermediate type spirals. By analogy with other members of the 

 class, we may safely conclude that the elliptical form of the image is an 

 effect of foreshortening. The fundamental plane of the thin lens- 

 shaped spiral is tilted about 15° from the line of sight. 



The outer regions of the spiral arms, iD contrast to the nuclear region, 

 are partially resolved into separate stars — presumably the brighter 

 giants and supergiants of a stellar system comparable with our own 

 galactic system. On this assumption, the unresolved portions of the 

 arms and the nuclear region consist of swarms of stars too faint to be 

 seen individually. The fragmentary data now available, such as the 

 stellar outbursts called novae, and the solar-type spectrum, are 

 thoroughly consistent with this interpretation. The recognition of 

 stars in the outer regions was the clue which solved the mystery of the 

 distance. Various types of stars were recognized which are well 

 known in our own system. Among other characteristics, we knew 

 their intrinsic luminosities, or candlepowers, and, consequently, their 

 apparent faintness indicated the distance of the nebula in which they 

 were located. Once the distance (about 700,000 light-years) was 



