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How is the Structure of our Galaxy? 2


How is the Structure of our Galaxy? 1

Even before the discovery of the telescope, it had been realised that there exist ‘nebulous’ objects which differ from the stars in having a diffuse or fuzzy appearance. Kant, Lambert, Swedenborg and Wright argued that these objects were ‘island universes’ similar to the Milky Way, but too distant to be resolved into stars. There was, however, no observational basis for this hypothesis. Herschel also inferred that the nebulae were island universes similar to our Galaxy. A test of this picture was to show that the nebulae could be resolved into stars and he believed that this had been achieved in a number of cases. In others, he assumed that the nebulae were too distant to be resolved into individual stars. This picture came into question, however, when he discovered that, among the nebulae were the planetary nebulae, which consist of a central star surrounded by a shell of gas. Herschel recognised that these nebulae were unlikely to be resolved into stars but rather consisted of ‘luminous fluid’ surrounding the central star.

The cataloguing of the bright nebulae was begun by Charles Messier whose catalogue of 109 objects was compiled during the years 1771 to 1784. Messier’s
interest was primarily in comets and his objective in compiling the catalogue was to enable him to distinguish between diffuse nebulae and comets. The catalogue contains a mixture of what we now know are the brightest Galactic and extra-galactic nebulae and they are still commonly referred to by their Messier, or M, numbers.

The systematic cataloguing of the nebulae was begun by William Herschel and his sister Caroline and was continued through the first half of the nineteenth century by his son John Herschel. The results of these huge endeavours was the publication by John Herschel in 1864 of the General Catalogue of Nebulae and Clusters of Stars containing 5079 objects. These catalogues were based upon visual observations long before photography became a standard tool of the astronomer. In 1888, John Dreyer published an expanded catalogue which was known as the New General Catalogue of Nebulae and Clusters of Stars which, together with the two supplementary Index Catalogues of 1895 and 1908, contain some 15,000 objects. Objects in these catalogues are still commonly referred to by their NGC or IC numbers.

While the cataloguing of the nebulae proceeded apace, their nature remained a mystery. Undoubtedly, some of them were gas clouds, as demonstrated by William Huggins’ pioneering spectroscopic observations of diffuse nebulae in the 1860s (Huggins and Miller, 1864). The big question was whether or not the ‘spiral nebulae’ were objects within our own Galaxy or were more distant systems. These nebulae were beyond the distances at which conventional techniques of distance measurement could be used. This problem culminated in what became known as ‘The Great Debate’ and concerned two related issues. Firstly, what is the size of our own Galaxy and, secondly, are the spiral nebulae members of our Galaxy or are they separate ‘island universes’, well beyond the confines of our Galaxy? This key episode in the history of modern astronomy should be required reading for all observers and theorists (Sandage, 1961b; Hoskin, 1976; Smith, 1982; Trimble, 1995).

To illustrate the nature of the problem, by 1920, Jacobus Kapteyn had determined the luminosity function of stars near the Sun and so, from star counts in different directions, determined the structure of the Galaxy which he found to be highly flattened with dimensions 1500 pc perpendicular to the plane and about 8 times that size in the Galactic plane (Fig. 1.2) (Kapteyn, 1922).

Meanwhile, Harlow Shapley had adopted a quite different approach to the de- termination of Galactic structure. In 1912, Henrietta Leavitt had discovered the remarkable period–luminosity relation for Cepheid variable stars in the Magellanic Clouds (Fig. 1.3). This discovery provided a powerful means of measuring astronomical distances because the Cepheid variables are intrinsically luminous stars and their distinctive light curves can be recognised in stars in distant systems. The Cepheid variables were the tools used by Harlow Shapley to determine the structure of the Galaxy through his studies of globular clusters. He found the scale of the

Fig. 1.2. Kapteyn’s model for the distribution of stars in the Galaxy (Kapteyn, 1922). The diagram shows the distribution of stars in a plane perpendicular to the Galactic plane. The curves are lines of constant number density of stars and are in equal logarithmic steps. The Sun S is slightly displaced from the centre of the system globular cluster system to be enormous, the most distance globular cluster having a distance of 67 kpc. Furthermore, the globular cluster system was not centred upon the Solar System, but rather most of the globular clusters were found in a direction centred upon the constellation of Sagittarius (Fig. 1.4) (Shapley, 1918).

The course of the debate was complex, but the issues were resolved finally and conclusively in 1925 by Edwin Hubble’s observations of Cepheid variables in the Andromeda Nebula. Using the period–luminosity relation for Cepheid variables, he established to everyone’s satisfaction that the spiral nebulae are distant extragalactic systems.

Within a year, Hubble had published the first major survey of the properties of galaxies as extragalactic systems. In his remarkable paper (Hubble, 1926), he introduced an early version of his classification of galaxies into ellipticals, spirals and irregulars, estimated mass-to-light ratios for these different types of galaxies used number counts of galaxies to show that they are uniformly distributed in space and hence estimated the mean density of matter in the Universe in the form of galaxies. Adopting Einstein’s static model of the Universe, he found that the radius of curvature of its spherical geometry was 27,000 Mpc. He estimated that, with the

Fig. 1.4. The distribution of globular clusters in the Galaxy according to Shapley’s distance measurements (Shapley, 1918). The scales on the abscissa and ordinate are in units of 100 pc and correspond to distances in and perpendicular to the Galactic plane respectively. The Sun, located at zero coordinates on the abscissa and ordinate, lies towards one edge of the globular cluster system

100-inch Hooker telescope, he could observe typical galaxies to about 1/600 of the
radius of the Universe. He concluded with the remark that
. . . with reasonable increases in the speed of plates and sizes of telescopes,
it may become possible to observe an appreciable fraction of the Einstein

This paper marked the beginning of extragalactic astronomy. It comes as no surprise to learn that George Ellery Hale began his campaign to raise funds for the Palomar 200-inch telescope in 1928 – before the year was out, he had secured a grant of $6 million from the Rockefeller Foundation for the telescope, the construction of which was completed in 1949.

In 1929, Hubble made his second fundamental contribution to cosmology. He showed that the extragalactic nebulae are all moving away from our own Galaxy and that their recessional velocities v are proportional to their distances r from our Galaxy (Fig. 1.5a) (Hubble, 1929). It is remarkable that he was able to deduce this key result from such a small sample of nearby galaxies but, within five years, he and Humason had extended the relation to very much greater velocities and distances using the apparent magnitudes of the fifth brightest members of clusters of galaxies as distance indicators (Fig. 1.5b). The velocity–distance relation v = H 0 r is commonly referred to as Hubble’s law and H 0 as Hubble’s constant. The significance of this discovery was that, combined with the isotropy of the Universe, Hubble’s law demonstrates that the whole system of galaxies is partaking in a uniform expansion.

Fig. 1.5. a Hubble’s first velocity–distance relation for nearby galaxies (Hubble, 1929). The filled circles and the full line represent a solution for the solar motion using the nebulae individually; the open circles and the dashed line represent a solution combining the nebulae into groups. The cross is an estimate of the mean distance of the other 20 galaxies for which radial velocities were available. b The velocity–apparent magnitude relation for the fifth brightest member of clusters of galaxies, corrected for galactic obscuration (Hubble and Humason, 1934). Each cluster velocity is the mean of the various individual velocities observed in the cluster, the number being indicated by the figure in brackets.

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