Bubble had shown definitively that spiral nebulae were external galaxies. In the 1930s he continued work on these objects with the Mount Wilson 2.5-m reflector, which was then the world’s largest telescope. Studies over the next decade revealed that the extra-galactic nebulae could be divided into four main classes: irregular, elliptical, normal spiral and barred spiral. On the basis of their shape alone he arranged them into a sequence known as Bubble’s ‘Tuning Fork” diagram. Bubble warned that the sequence was not necessarily an evolutionary one. Be estimated the distances to two dozen ‘galaxies by 1929 and found that their velocities were in direct proportion to their distances; for example galaxies at twice the distance of other galaxies receded twice as quickly. This result caused a great stir among cosmologists since one of the deductions of general relativity is the unstable state of the Universe; that is, according to Einstein’s theory, the Universe should be either expanding or contracting. Hubble’s relationship was immediately used as evidence for a Big-Bang-type Universe. For observers it had another use: for galaxies too remote for the resolution of Cepheids or novae, the standard distance indicators, the Hubble relationship could be reversed to infer distances from velocities, easily measured from the redshift in the spectrum of the galaxy.
The new distances to the nearby spirals gave rise to several problems. First of all, the spirals in general seemed much smaller than the Milky Way, the size of which had been determined by Shapley, Secondly, the novae and globular clusters seemed much fainter than those in the Galaxy; in addition Hubble had only been able to resolve stars in the outer regions of M31, and none in the nucleus. The results conflicted with the normal assumption of astronomers-namely the uniformity of nature. Only through the assumption of the similarity of objects (i.e. Cepheids, novae) in remote galaxies with those of known distance in our own could distances be obtained.
Meanwhile, studies of the motions of stars in the solar neighbourhood provided new insight into the structure of our own Galaxy. In the early part of this century, two opposing streams of stars had been detected, one moving away from the direction of Sagittarius, the other towards it. At right angles to these two streams was a small number of stars moving at extremely high velocities. In 1921 the Swedish astronomer Lindblad explained the phenomenon by supposing the rotation of our Galaxy about some point in Sagittarius Later he proposed the existence of two separate sub-systems of stars in the Galaxy – a spherical system to which the globular clusters and high-velocity stars belonged, and the flat disc system to which the other stars belonged. Lindblad’s ideas were hypothetical, but in 1927 the Dutch astronomer, Jan Oort, provided evidence. Examining stars lying directly between us and the centre of the Galaxy, Oort found that they displayed a zero radial velocity. Stars in intermediate directions appeared to recede or approach depending on the direction. The pattern indicated rotation about a point lying in the direction of Sagittarius, the centre of Shapley’s globular cluster distribution. From his stellar velocities, Oort calculated the distance of this point from the Sun as 6kpc, a value only one-third that obtained by Shapley. Oort also confirmed the distinction pointed out by Lindblad between stars of the spherical system and those of the highly flattened system.
The discrepancy between Oort’s and Shapley’s distances for the centre of the Galaxy was explained several years later by Robert J. Trumpler of the Lick Observatory. Trumpler had been collecting data on a large number of galactic clusters, and determined distances for about one hundred of them. From the angular dimensions of these clusters he was able to calculate their actual sizes. His result indicated that remote clusters were about twice the size of nearby ones. Trumpler was not willing to accept this anomaly and explained it instead by the existence of interstellar absorption, that is, dust. This dust absorbed light, so causing more distant clusters to appear fainter and therefore more remote than they actually were. This exaggerated distance, together with the measured angular size, led to the estimated larger size of the more remote clusters. The correction due to this absorption was small for nearby objects but quite considerable for more remote ones. Shapley’s distance to the centre of our Galaxy had therefore been exaggerated by about a factor of two. Similarly, the apparently small size of M31 was explained, although there still remained some discrepancy the Galaxy still appeared to be considerably larger.
The distance scale was further revised in the 1940s when Walter Baade announced the existence of at least two distinct stellar populations in spiral galaxies. Baade reached this conclusion after long exposures of the nucleus of M31, when he succeeded in re¬solving the nucleus into great numbers of red stars, and found similar results for the two elliptical companions of M31. On plot¬ting the colour-magnitude diagram of these stars he saw that they were quite different from the red stars found by Trumpler in galactic clusters, but very similar to the bright red stars in globular clusters. A new type of system discovered by Shapley, exemplified by the Sculptor and Fornax systems (bigger than globular clusters but dwarfish compared to ellipticals) also exhibited the same kind of properties. Baade called the stars found in ellipticals, globular clusters, nuclei of galaxies Population II stars and those in the disc of spiral galaxies Population I stars. One of the results which followed was that not all Cepheid variables followed the same period-luminosity relationship; Cepheids in globular clusters were four times fainter than those in the disc of the galaxy. This meant that distances based on Cepheids in globular clusters had to be increased by a factor of two. Thus the problem which had baffled astronomers – the small size of M31 and faintness of globular clusters and novae within it – was solved, as M31 was doubled in size. Another discrepancy, the great brightness of S Andromedae which had exploded in the nucleus of M31 in 1885 was clarified by the division of such objects into two classes, novae and supernovae, an idea suggested earlier by Curtis. A study by F.Zwicky in 1936 at Palomar revealed that the latter were very rare. Once more the compelling assumption – the uniformity of nature – could be relied on to extrapolate to more remote regions of the Universe.
While the revisions to the distance scale were being made, investigations were under way to find some indication of spiral structure in our own Galaxy by comparison of the content of the Galaxy with the spirals. Baade had shown that the luminous stars in M31 lie along the spiral arms; further examination revealed that regions of ionized gas (i.e. emission nebulosities) also lie exclusively along the spiral arms. A survey of these H+ regions in the Galaxy by Morgan, Sharpless and Osterbrock produced in 1952 a map which indicated the existence of several sections of nearby spiral arms.
The long struggle to determine the structure of the Milky Way and the nature of the nebulae, begun in earnest by William Herschel, had come to a fitting conclusion. The island universes of Kant did exist and the Milky Way had unfolded itself as a very typical representative.