The Universe Beyond Our Galaxy ( Major Trends In The History of Astronomy)

The Nature of Nebulae

Certain nebulous patches, such as the Magellanic Clouds in the south and the Andromeda Nebula in the north, are clearly visible to the naked eye and have undoubtedly been observed from early times. Comments on the Andromeda Nebula date back to the Arabs in the middle of the tenth century. Similarly, the nebulous band of light stretching across the sky, called the Milky Way, is also a conspicuous feature of the night sky. (The name ‘Galaxy’, which we now use to refer to our stellar system comes from the Greek ‘galaxias’ which translates as Milky Way.) In the seven¬teenth century, Galileo applied his telescope to this hazy band and found much to his surprise that it could be resolved into myriads of faint stars. On examining some of the nebulous patches, he found that they also were dense regions of stars. From this he concluded that most nebulosities could be resolved into stars.

Those that could not be resolved were simply called nebulae and by 1700 about 10 of these were known. The first extensive list of these objects, comprising 42 southern nebulae, was the work of Nicolas de Lacaille in 1755. Often, in fact, the discovery of nebulae was the by-product of another goal – the search for comets. The next extensive list of nebulae, consisting of 103 nebulous objects and clusters, was compiled by Charles Messier and Pierre Mechain in 1784 in order to keep track of nebulae during their regular comet searches. In general, eighteenth-century astrono¬mers were too concerned with the problems of the Solar System to be bothered about the curiosities in Messier’s list. Certain eighteenth-century philosophers, on the other hand, considered these objects and the Milky Way to be quite worthy of speculation.

Emanuel Swedenborg, the Swedish philosopher, described the Milky Way as a rotating mass of stars in the shape of a sphere. According to him, the Universe was filled with such spheres. Thomas Wright, the English instrument-maker and mathematician, also considered that the Milky Way was one among many, but he presented an entirely different view of its structure. In his ‘Original Theory’, or ‘New Hypothesis of the Universe’ he con¬sidered the Milky Way as a vast disc containing concentric rings of stars. In the same work he proposed another model closer to Swedenborg’s; however later writers preferred the former model, which became known as the ‘grindstone theory’ of the Milky Way. Later works of Wright, discovered by a London book dealer in 1966, presented views very reminiscent of the mediaeval philo¬sophers and inclined to a theological view of the Universe.

Wright’s ideas were drawn to the attention of Immanuel Kant who became interested in a correct interpretation of the Milky Way, and he furthered the disc theory by replacing Wright’s poetic ideas with Newtonian physics. In the ‘General History of Nature and Theory of the Heavens’, published in 1755, Kant com¬pared the Milky Way to Saturn, a system with swarms of particles rotating about it. The shape of the Milky Way and the nebulous stars could be explained by their rotation. Kant considered the nebulae to be other universes or Milky Ways and the term ISLAND UNIVERSES is usually attributed to him. He argued that a flattened, circular array of stars would look smooth and elliptical if seen from a distance, thus accounting for patches such as the Andro¬meda Nebula.

William Herschel was the first astronomer who carefully examined the heavens to study the stars for their own sake. Largely self-taught, he constructed his own reflecting telescope (1773) and began observations. In this, and most of his later endeavours, he was aided by his sister Caroline who became an astronomer in her own right. As his enthusiasm increased, he spent much time grind¬ing and polishing mirrors for bigger and better telescopes. To¬gether he and Caroline swept the skies systematically, and during one of these surveys in 1781 he discovered Uranus, an achievement for which he was amply rewarded.

Herschel had obtained a copy of Messier’s catalogue of nebulae and star clusters and he proceeded to examine each of the objects on the list with his 6-m focal-length telescope of 40-cm aperture. He was able to resolve many of them into stars and by 1785 had concluded that all nebulae must consist of stars. Over a period of seven years spent in the regular sweeps of the sky, he and Caroline were able to add 2000 nebulae to the list. From their distribution, in general away from the plane of the Milky Way, he concluded that they must be separate systems.

In addition to his interest in the nebulae, Herschel was intent on determining the structure and scale of the Milky Way. He had developed his own model by 1784 and was determined by the method of ‘star-gauging’ to obtain some scale to this model. His method involved counting the number of stars in the field-of-view of the telescope over a large number of regions. Assuming the stars to be uniformly distributed and their faintness to be an indication of their distance, the number of stars in a particular region was then proportional to the extent of that region. Since no stellar distance had yet been obtained, Herschel used the distance of Sirius as his measuring stick. The relationship between distance and faintness was a poor one and Herschel realized from his dis¬covery of binary systems that it was crude. However, there was no other method available for estimating distance. His results indi¬cated a diameter for the Milky Way of 800 times the distance of Sirius, or 2kpc. Later, Herschel became more sceptical of this method because of the limitations of his 6-m telescope, which he felt had led him to underestimate considerably the size of the Milky Way. He revised his diameter to 2300 times the distance of a first magnitude star, or about 6 kpc.

In later years, Herschel turned from a spatial description of the Milky Way and the nebulae to an evolutionary one. He began classifying his objects into an evolutionary sequence, with the start of the sequence represented by a uniform, static Universe of scattered stars. However, the planetary nebulae, which he dis¬covered and named such because of their appearance, would not fit into his stellar scheme. As he studied these objects more closely, it became more and more obvious that they were stars in actual association with nebulosity. Therefore, true nebulosity did exist and all nebulae were not necessarily unresolved stellar systems at remote distances. The existence of island universes could no longer be assumed, and, because of Herschel’s great authority, it suffered a decline in popularity in the first quarter of the nineteenth century.

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