Herschel’s discovery of stars embedded in nebulosity, the so-called planetary nebulae, reinforced a possible interpretation of the nebulae as planetary systems in the making. Such an interpretation correlated very well with the accepted theory of the origin of the Solar System developed by Pierre Simon de Laplace towards the end of the eighteenth century. According to Laplace’s theory the Sun contracted from an immense rotating cloud; during the contraction several rings of matter detached themselves to become planets. Planetary nebulae must then be one of the final stages in the evolution of an immense cloud or nebula into a solar-type system.

Towards the middle of the nineteenth century the island universe theory was revived after observations with the large telescope of Lord Rosse (William Parsons) of Parsonstown. In February, 1845, Rosse successfully completed a 2-m mirror mounted in a 16-m tube. Together he and Robinson examined one of the nebulae in John Herschel’s list (an extension of Sir William’s) and found that it could be resolved into stars. They also verified the suggestion by John Herschel that M51 could be considered an analogue to the Milky Way system. M51 was observed later by Nichol and found to have a definite spiral pattern, a pattern also observed in several other nebulae. By 1848, fifty nebulae had been resolved by Rosse’s telescope and this brought into question once more the existence of true nebulosity. In the enthusiasm to resolve nebulae even Orion and the Crab Nebula were added to the list and the former, accord¬ing to G.P.Bond, displayed a certain amount of spiral structure! In 1852 Stephen Alexander of New Jersey suggested that the irregular appearance of the Milky Way could be explained by the existence of spiral arms and unsuccessful attempts followed to determine its exact structure and scale.

Sir William Huggins was the first to examine nebulae with a spectroscope. In 1864, he studied Orion and found it to display several bright lines, similar to the spectrum of a hot gas. This compelling evidence seemed to contradict reports that Orion could be resolved into stars, so Huggins concluded that the objects they observed must not have been stars but rather masses of tenuous gas. Similarly, photographs of Orion and the Crab revealed no stars. Huggins found that of sixty nebulae and clusters examined, one-third displayed a bright-line spectrum. Within the spiral nebulae he consistently found a stellar-type spectrum, and in the planetary nebulae, a bright-line spectrum. The existence of true nebulosity was for the first time definitively shown.

During the latter part of the nineteenth century, data on the nebulae accumulated. John Herschel’s observations of nebulae from the Cape of Good Hope, published in 1847 as the General Catalogue, extended his father’s list of 1802 by 2500. His results revived his father’s ideas on the coexistence of stars and nebulosity They also emphasized the distribution of nebulae away from the galactic plane and towards the poles, and drew attention to the peculiar zone of avoidance noted earlier by William Herschel. The strange distribution was used by some to argue for the physical association of the nebulae with the Milky Way, and occasionally by others for the opposite.

The spectroscopic results, combined with increasing interest in Laplace’s nebular results furthered the decline in popularity of the island universes. More and more evidence seemed to indicate that they were all part of our own system. Isaac Roberts, an energetic amateur astronomer specializing in astronomical photography, took photographs of M31 in 1888 which revealed not only its spiral structure but two satellite objects, M32 and NGC205. Roberts speculated that these two satellites were protoplanets in an early stage of condensation so that the system represented a perfect example of Laplace’s nebular theory at work.

The question of the nebulae was intricately linked up with the structure of the Milky Way. Three models of the Milky Way were considered during the nineteenth century the disc model of Kant and Herschel, an unlimited slab of stars, and a central cloud surrounded by a distant ring of stars. Its spiral nature was suggested by its noted similarity to M51. Which Robinson had observed to be spiral. C. Easton published the first diagram displaying the Milky Way as a spiral in 1900, in which he placed the centre in the constellation Cygnus and the Sun about a third of the way from the centre to the edge. The scale was still very dubious as there were no reliable distance indicators. At the end of the nineteenth century, its exact nature was as mysterious as that of the nebulae. The question of the uniqueness of our stellar system could not be separated from the question of the true nature of the nebulae. At this time astronomers felt the evidence weighed heavily in favour of the nebulae being part of the Milky Way.

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