William Herschels Milky

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The views of Wright, Kant, and Lambert about the Milky Way resulted simply from a visual impression of the distribution of stars in the sky. The first proper survey of the Milky Way using a telescope was started by William Herschel. Herschel moved from Germany to Britain at the age of 19 where he earned his living as a musician (later he obtained the position of organ player in the chapel of Bath). In 1773, at the age of 35, he happened to buy a book on astronomy. "When I read about the many enchanting discoveries made using a telescope, I became so fascinated by the subject that I wanted to see the sky and the planets by my own eyes through one of those instruments."

Herschel learned how to grind mirrors working on building telescopes day and night. He was assisted by his musician brother Alexander and sister Caroline. In the following years, music was left more in the background while Herschel learnt how to build bigger and better telescopes. He also started making systematic notes of what he saw in the sky. We already told about the discovery of the planet Uranus in 1791 (Chap. 11). This brought Herschel fame as an astronomer. The hobby turned into profession - he started receiving a salary from the King as the first Astronomer Royal. In scientific circles, Herschel was still rather unknown. For example J. E. Bode, the leading German astronomer, wondered if his name was Mersthel, Hertschel, Herrschel, or Hermstel.

In the study of the Milky Way, Herschel pioneered statistical methods. His brilliant idea was to chart its outline using star counts. Herschel trusted that his 47 cm telescope was powerful enough to see the edges of the Milky Way. The number of stars seen in the telescope tells how far the edge is in that particular direction: the more stars, the further the edge. Figure 20.4 shows the result of the star counts when translated into the outline of the Milky Way. This cross-section perpendicular to the plane of the Milky Way resulted from the study of 683 regions placed on an arc of a great circle in the sky. It agrees with the visual conclusion that the Milky Way is a flat stellar system.

Later Herschel became suspicious of the correctness of his picture. It was questionable whether the telescope was actually powerful enough to see the stars at the edge of the system. His new telescope of 120 cm diameter showed many more stars than what he had seen with the 47 cm tube. Otherwise this large telescope was not entirely satisfactory: it was clumsy to handle, and the assistant was injured several times while operating it. Also his studies of star clusters made Herschel believe that his initial assumption of uniform distribution of stars in space was far from the truth. However, the idea of the disk-like Milky Way survived until the twentieth century when it became possible to confirm it using more advanced methods.

Herschel made also good progress in the study of stars and nebulae. He discovered binary stars and his systematic "sweeps of the sky" revealed about 2,500 star clusters and nebulae (Chap. 21). Previously, only about one hundred such objects were known.

Astrophysics William Herschel

Fig. 20.4 (a) A portrait of William Herschel, painted at the time of his discovery of Uranus. (b) Cross-section of the Milky Way based on Herschel's star count using his telescope equipped with a 47 cm mirror (an illustration from the year 1785). This great astronomer spent numberless nights observing the starry sky

Fig. 20.4 (a) A portrait of William Herschel, painted at the time of his discovery of Uranus. (b) Cross-section of the Milky Way based on Herschel's star count using his telescope equipped with a 47 cm mirror (an illustration from the year 1785). This great astronomer spent numberless nights observing the starry sky

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