William Herschel Biography
William Herschel biography essentials: born Hanover, 1738, became the greatest astronomer of the 18th century, died 1822.
William Herschel began working life as a musician, following his father into the Hanoverian Guard. But, after bad experiences under fire, he moved to Britain and earned his living as a musician in Bath. He then developed an all-consuming interest in astronomy.
Although competing with the lavish court observatories of Europe, Herschel produced the the best telescopes in the world. In 1781 he discovered Uranus using a home-made telescope. This was the first planet discovered since ancient times.
Herschel measured the approximate distance to hundreds of stars, using the distance to Sirius, the dogstar, as his yardstick. He assumed all stars were equally bright and, because Sirius was the brightest star, that made it the nearest star and its distance the most plausible unit of measurement. He did not believe all stars were actually the same brightness, but he had to start with this approximation.
Brightness falls off with the square of the distance, so a star 1/9 the brightness of Sirius, emitting the same amount of light, is 3 times the distance of Sirius (3 siriometers). From this he concluded the stars were clumped together in a flat disc 1000 siriometers in diameter and 100 siriometers thick. This is compatible with the appearance of the Milky Way. The ancients did not know the Milky Way was composed of individual stars, that observation had to wait for the first telescopes.
"Imagine that we are embedded somewhere within the pancake of stars, then we would see lots of stars to the left, right, ahead, and behind, but we would see fewer stars above and below us because the pancake is thin ... and indeed such a band can be seen arching across the night sky (as long as you are far away from bright city lights)." Big Bang by Simon Singh, p.173.
Herschel suggested that everything was within the Milky Way, but other astronomers began to think that nebulae might be outside.
- The Milky Way is the name of our galaxy, as well as the name of the milky band of stars that runs across the night sky. So we are in the Milky Way.
Herschel died without knowing how many kilometres there were in a siriometer, it was not until 1838 that the German astronomer Friedrich Wilhelm Bessel measured the distance to the star 61 Cygni. Comparing the brightness of 61 Cygni to Sirius gave a ballpark conversion of William Herschel's siriometers, and the diameter of our galaxy in kilometres. Unfortunately, Sirius is much brighter than 61 Cygni so the estimate was a tenth that of today's figure of 105 light years.