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How is the Structure of our Galaxy?


In his extraordinary text of 1610, the Sidereus Nuncius or The Sidereal Messenger, Galileo Galilei demonstrated that the Milky Way can be resolved into stars when observed through the telescope. These observations led to the earliest speculative cosmologies of the modern era. The ‘island universe’ model of René Descartes, published in The World of 1636, involved an interlocking jig-saw puzzle of solar systems. In 1750, Thomas Wright of Durham published An Original Theory or New Hypothesis of the Universe, in which the Sun was one of many stars which orbit the ‘Divine Centre’ of the star system. Immanuel Kant in 1755 and Johann Lambert in 1761 took these ideas further and developed the first hierarchical, or fractal, models of the Universe. Kant also 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.

Towards the end of the eighteenth century, William Herschel 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 astronomical observation. To determine the structure of the Milky Way, he counted the numbers of stars in different directions. Then, assuming that they all have the same intrinsic luminosities, he derived his famous picture for the structure of our Galaxy which consisting of a flattened disc of stars with diameter about five times its thickness, the Sun being located close to its centre(Herschel, 1785).

John Michell had already warned Herschel that the assumption that the stars have a fixed luminosity was a poor approximation. In his remarkable pioneering paper of 1767, Michell introduced statistical methods into astronomy in order to show that binary and star clusters must be real physical systems and not random associations of stars on the sky (Michell, 1767). Consequently, there must be a dispersion in the absolute luminosities of the stars from their observed range of apparent magnitudes in bright star clusters, such as the Pleiades. Despite this warning, Herschel proceeded to produce a number of different versions of his model for the structure of our Galaxy, adding appendages to account for various features of the star counts in different directions.

In 1802, Herschel measured the magnitudes of visual binary stars and was forced to agree with Michell’s conclusion about the wide dispersion in the luminosities of the stars (Herschel, 1802). Equally troubling was the fact that observations with his magnificent 40-foot telescope showed that, the fainter he looked, the more stars he continued to find. There seemed to be no edge to the Galaxy and Herschel gradually lost faith in his model. In addition, the importance of interstellar extinction by dust was not appreciated – it was only in the 1930s that its central importance for studies of our own and other galaxies was fully appreciated.

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