Progress on the nebulae in the early twentieth century was stim lated by the construction of large telescopes in good climates and the regular use of permanent records such as photographic plates and spectrograms. The Harvard telescope in Peru, the Crossley 1-m reflector at the Lick Observatory and the Mount Wilson 1.5-m played an important role in the closer examination of the nebulae. The photographic plates revealed hitherto unsuspected numbers of fainter nebulae, a large portion of which appeared to be spiral. The nature of the spiral became one of the searching questions in astronomy.

Spectrograms of M31 by Scheiner and, independently, by Hug-gins, showed a continuum due to stars. Similarly, a 12-hour exposure of M31 by Wilson revealed bright patches which looked very much like star clusters; Wilson considered them excellent confirmation of Laplace’s nebular hypothesis. This observation was taken by Sir James Jeans as evidence for the spirals being early stages in the formation of globular dusters. Moulton used the spiral shape to add weight to his and Chamberlain’s theory of the origin of the, Solar System, which explained the formation of the planets as due to material pulled on” the Sun by a passing star. He argued that, the, tidal material pulled off by the star would form a double branched logarithmic spiral, so that spiral nebulae were interpreted as planetary systems in the making. Coincident with these ideas, though somewhat subdued, was the idea that the spirals were stellar systems like the; Milky Way. We can see that although there was no shortage of ideas on the nebulae at the turn of the century, no accurate means of estimating the scale of the Universe existed which would enable models to be tested.

At about this time, however, a ‘standard candle’ became avail¬able for obtaining distances. In 1885 a dramatic flare-up of a nova in M31 indicated that, unless this nova, S Andromedae, was very unlike galactic novae, the Andromeda Nebula must be fairly close. In 1895, a second extremely bright nova was found in the spiral NGC5253. In 1911, Very estimated the size of the Milky Way from the expansion parallax, of nova Persei to be about 35 pc; assuming S Andromedae to be similar to this he calculated a distance of 500pc for M31. In the following year, Wolf applied a different method. He compared the size of dark holes in M31 with some in the Galaxy to get a distance for M31 of 8kpc. At this point, another distance indicator became available. Henrietta Leavitt at Harvard, while examining the Small Magellanic Cloud for variables, dis-covered the period-luminosity law for Cepheid variables, which enabled distances to be found from apparent magnitudes. E.Hertz-sprung obtained a distance of 8kpc for the Small Magellanic Cloud after he had calibrated the period-luminosity law.

In 1914, V.Slipher of Lowell Observatory found that M31 was approaching us at a velocity of 300kmsr1, the largest radial velocity then known. Slipher continued his study of radial velocities and found that of 15 nebulae, 11 showed velocities of recession; the average velocity was 800 km sr1. The high velocities made it clear that the spirals were very different from ordinary stars. Slipher also noted that the spectral lines of certain spirals were tilted, which indicated rotation. The high velocities and rotation rates suggested the intriguing possibility that spirals might have measurable proper motions and angular rates of rotation. H.D.Curtis of the Lick Observatory obtained an average proper motion of 0.033 arc seconds per year from 66 spirals, and this result combined with the average radial velocity, indicated an average distance of 3kpc. Angular rates of rotation, became the long-term project of A. van Maanen of Mount Wilson. For the first spiral he examined, van Maanen found a period of rotation of 85,000 years. Similar results followed over a period of 10 years and van Maanen’s high rotation measures became the main evidence against the spirals being external galaxies.

Harlow Shapley revolutionized the scale of the Galaxy by using the globular clusters as boundary markers to plot its extent. On the assumption that the Cepheids in globular clusters were the same as those defining the period-luminosity relationship, he arrived at the incredible diameter of 80kpc for the Milky Way with the Sun 20kpc from the centre. The average distance of spirals of 3-6kpc put them well inside this Galaxy.

Meanwhile, there was more evidence for the spirals being very remote. In July 1917 Ritchey at the Mount Wilson Observatory discovered a nova in the spiral NGC6946, which, on comparison with galactic novae, implied a vast distance. This discovery encouraged a closer study of stars associated with the spiral nebulae by Curtis. He became convinced that spiral systems were like our own Galaxy, and that the spectacular novae of 1885 and 1895 were unusual; Curtis later (1921) suggested that the novae could be divided into two magnitude classes. In March 1917 he had dis¬covered a nova in NGC4527 and two more in NGC4321, all of about 14 magnitude. Curtis, always a very cautious astronomer, awaited more evidence before publishing his results; an examination of plates files soon revealed many more novae, all of which indicated the existence of island universes.

In 1920 a meeting was arranged by the National Academy of Sciences to discuss the question of the nebulae openly: were the spirals island universes or part of our own Galaxy? The two sides were represented by H.D.Curtis and H.Shapley respectively. Essentially the two men defended different aspects of the problem -Shapley arguing for the large size of the Galaxy, Curtis, for spirals being external galaxies. Shapley felt, partly on the basis of van Maanen’s large rotations, the spirals had to be nearby. Curtis con¬sidered Shapley’s distance measures to be unreliable and preferred to trust the old accepted size of the Milky Way, which meant that the nebulae could be well outside its boundaries. The meeting did not answer the question because insufficient data were available.

However the required information soon followed. In 1922 J.C. Duncan found variables in M31, and, from photographs of NGC 6822 showed it to contain both stars and nebulae. Edwin Hubble became interested in the latter and took a series of photographs with the 2.5-metre telescope which resulted in the discovery of 15 variables, 11 of which were Cepheids. In 1923, Hubble identified one of the variables in M31 as a Cepheid and several more in M31 and M33 soon afterward. Using the period-luminosity relationship he calculated a distance of 230 kpc for NGC6822. With the continuation of his search for Cepheids with the 2.5-m telescope, he soon discovered 35 in M33 from which he calculated a distance of 250 kpc. The spirals were definitely island universes far beyond our Galaxy.

One question had been answered but many remained. The nature of our own system was still uncertain. The nebulae had been separated into two separate categories, the galactic objects -planetary and diffuse nebulae – and the extragalactic systems, but little was understood of their structure or evolution.

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