Galileo was able to resolve the Milky Way into myriads of stars. William Stukeley (1687-1765) pictured the Sun and the stars we see as individuals as forming a spherical cluster, all surrounded by a flattened ring containing the stars of the Milky Way (i.e. looking similar to Saturn and its rings).
Thomas Wright of Durham (1711-86) in his An Original Theory or New Hypothesis of the Universe (1750) suggested that the Sun is one of innumerable stars occupying a thin layer of space surrounding a Divine centre. Looking across the ring we see the dense concentration of stars of the Milky Way but looking away from the ring we see very few stars. Immanuel Kant (1724-1804) believed that the stars were a rotating body held together by gravitational forces, like the Solar System but on a much larger scale.
William Herschel noted that the density of the stars varied in different parts of the sky. He assumed that the stars were really more or less equally spaced and where the stars seemed to be densest they extended out the farthest. On this assumption and by counting the stars in different parts of the sky he was able to produce a map of the stars that had the shape of a disc or grindstone and resembled the late twentieth-century model of our Galaxy. In this he was fortunate because he had worked from unsound assumptions. First he assumed that all the stars have essentially the same absolute magnitude. He then assumed that if all the stars were the same distance as the brightest star in the sky, Sirius, they would look as bright. Finally, the extent to which a star appeared to differ in brightness from Sirius he used as a gauge of its distance.
Having used the star Sirius as his standard he decided to call his unit of distance a siriometer, i.e. the distance to Sirius. In 1785 he calculated that his model to be a thousand siriometers across and a hundred siriometers thick. Modern estimates of the dimensions of the Galaxy have found it to be more than ten times bigger than Herschel’s measurements.
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