The second part of our history concerns the understanding of the large-scale distribution of matter in the Universe. At the beginning of the period 1900 to 1939, little was known even about the structure of our own Galaxy; by the end of it, the Universe of galaxies was established, the system was known to be expanding and general relativity provided a theory capable of describing the distribution of matter in the Universe on the very largest scales.

‘Island universes’ and the cataloguing of the nebulae

The earliest cosmologies of the modern era were speculative conjectures. The ‘island universe’ model of René Descartes (1596–1650), published in The World of 1636, involved an interlocking jigsaw puzzle of solar systems. In 1750, Thomas Wright of Durham (1711–1786) published An Original Theory or New Hypothesis of the Universe, in which the Sun was one of many stars which orbit about the ‘Divine Centre’ of the star system. Immanuel Kant (1724–1804) in 1755 and Johann Lambert (1728–1777) in 1761 took these ideas further and developed the first hierarchical, or fractal, models of the Universe. Kant made the prescient suggestion that the flattening of these ‘island universes’ was due to their rotation. The problem with these early cosmologies was that they lacked observational validation, in particular because of the lack of information on the distances of astronomical objects.

Towards the end of the eighteenth century, William Herschel (1738–1822) was one of the first astronomers to attempt to define the distribution of stars in the Universe in some detail on the basis of careful observation. To determine the structure of the Milky Way, he counted the number of stars in different directions, assuming they all have the same intrinsic luminosities. In this way, he derived his famous picture for the structure of our Galaxy, consisting of a flattened disc of stars with diameter about five times its thickness, the Sun being located close to its centre (Figure 5.1) (Herschel, 1785).