With the advent of telescopic astronomy in the seventeenth century, observers began to detect many small cloudy or fuzzy objects in the sky called nebulae, so named from the Latin word nebula, for "cloud." As we saw in the last chapter, successively more detailed catalogs of the stars and nebulae were made, culminating around 1800 in the work of William Herschel with his large reflecting telescopes. In 1755 Immanuel Kant (1724—1804) published Universal Natural History and the Theory of the Heavens. Following a suggestion of his English contemporary Thomas Wright (1711—1786), Kant speculated that the Sun and the other stars in the sky make up a connected system bound by gravity, in much the same way (except on a much larger scale) as the Sun and the planets form a system. It was known that the band of the Milky Way consisted of a myriad of stars, with the number of stars in the sky increasing as one approached the band of the Milky Way and decreasing as one moved away from it. According to Kant, the Sun and the other stars formed a flat, thin disk lying in the plane defined by the great circle of the Milky Way. As we look out from the solar system within the disk, we observe many stars in the plane of the disk, while progressively fewer stars are seen as we direct our gaze in a direction perpendicular to the plane. Kant further suggested that the white nebulae were themselves conglomerations of stars similar to the Milky Way and situated throughout very distant space. He suggested the observed oval shape of many of the nebulae was a perspective effect resulting from their disk structure. The theory that the nebulae populate space as so many island systems of stars, our Milky Way system being just one instance, would become known as the island-universe theory. Although Kant's writings were only speculative, he provided the first clear statement of the island-universe theory, and the idea of extragalactic nebulae was implanted in the minds of astronomers.
Kant's book contained many other ideas and speculations. He believed that it was probable that the other planets of the solar system were populated by intelligent beings and gave detailed descriptions of their personalities and temperaments. These fantastic suggestions were typical of Enlightenment speculation and were influenced by exotic facts about traditional societies revealed by European voyages of exploration. More important for the subsequent development of science, Kant outlined a theory of the origin of the solar system. The solar system was supposed to have begun as a nebulous mass of swirling gas and dust. As the mass contracted under gravitational attraction and its rotational speed increased, a dense central object formed, while a series of smaller objects were cast off from the center. The center coalesced into the Sun, and the cast-off bodies became the planets. The theory explained why the planets all revolve around the Sun in the same direction and why their orbits all lie in a thin plane with the Sun as center. A conception similar to Kant's was formulated by the great French mathematician and physicist Simon Laplace at the end of the century. This explanation of the solar system became known as the nebular hypothesis and was important historically for introducing ideas of evolution and development in time into scientific thinking about the natural world.
Johann Lambert (1728-1777) was a Swiss mathematician, who, in 1761, published a theory of the Milky Way similar to Kant's. Lambert conceived of the galaxy as a collection of smaller star systems that physically interacted through the action of gravity. The galaxy in turn was only one of a much larger collection of island universes. In his initial investigations of nebulae William Herschel subscribed to the island-universe hypothesis, believing that since many of the nebulae were resolvable into stars, this would be true of all of them. However, his discovery of the planetary nebula in Taurus (chapter 6, figure 6.4) led to an important change in his thinking. It as well as other similar nebula continued to appear diffuse as they were examined under larger telescopes. Herschel openly questioned the hypothesis of island universes and instead tried to explain the planetary nebulae as the first stage in the formation of new stars.
The construction of ever larger telescopes in the nineteenth century brought new information about the detailed structure of nebulae. In 1850 the Irish astronomer and aristocrat Lord Rosse (1800-1867) discerned with his large reflecting telescope, the Leviathan of Parsonstown, that many of the white nebulae possessed a definite spiral structure. One example is an object close to the Big Dipper, numbered 51 in the Messier catalog (figure 7.1). This object is observed from the Earth face on, and M 51 is sometimes called the Whirlpool nebula. Figures 7.1(a) and 7.1(b) provide a comparison of Rosse's original sketch and a modern photograph of M 51. Another very bright nebula that is observed more obliquely is M 31 in the constellation of Andromeda, often referred to as the Great Andromeda nebula. It may be sighted easily with the naked eye in the autumn from locations in the northern hemisphere. It turned out that the class of nebulae possessing an oval or spiral shape was very large indeed, involving myriad objects distributed throughout the sky in regions away from the band of the Milky Way.
Although the island-universe hypothesis continued to attract occasional adherents, it largely lost favor among astronomers as the nineteenth century
came to a close. Several pieces of evidence counted against the hypothesis. First, as was noted above, the nebulae are not distributed randomly in the sky but congregate in regions away from the band of the Milky Way. This "zone of avoidance" seemed to indicate that the nebulae were systemically connected to the Milky Way galaxy and were not independent objects distributed in distant space. In 1885 a new star or nova in the Andromeda nebula for a short period of time outshone in brightness the entire nebula. It was later realized that this star was a supernova, an incredibly energetic and short-lived event in which a massive star explodes. At the time, astronomers reasoned that the brightness of the Andromeda nova meant that it must be nearby, celestially speaking, certainly within the vicinity of the Milky Way system. A final piece of evidence against the island-universe hypothesis emerged with the invention of stellar spectroscopy and the discovery that several of the nebulae classified by Herschel as "planetary" showed only emission lines, indicating that they were composed primarily of gas. This finding confirmed Herschel's own conclusion about such objects. It was later determined that planetary nebulae are of a special sort, being in fact the gaseous remnants of exploded stars within the galaxy. However, it was not apparent at the time that the nebulae were of such radically different types, and the existence of emission lines in some of them was regarded as evidence that the nebulae as a class were not distant objects composed of an immense number of stars.
In 1887 Agnes Clerke (1842—1907), an influential American writer on astronomy, published A Popular History of Astronomy during the Nineteenth Century, in which she confidently rejected the island-universe hypothesis: "There is no maintaining nebulae to be simply remote systems of stars ... it becomes impossible to resist the conclusion that both nebular and stellar systems are parts of a single scheme" (Crowe 1994, 196). This conclusion was reiterated in 1907 by the German astronomer Max Wolf (1863—1932), who wrote that "the nebulae and clusters of stars represent an essential part of our star-island and perhaps lie relatively close to us. They all form, together with the stars of the Milky Way, an organic whole. Distant, isolated Milky Ways have never been sighted by man" (Crowe 1994, 197).
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