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14.20 William Herschel

The man who established the place of the Sun in the realm of the stars was William Herschel. Born in 1738 into a musical family in Hanover, Herschel was an accomplished musician himself. He deserted from the German army during the Seven Years' War and got a job in England as the organist in a chapel in Bath. In his study of musical harmony, Herschel read a mathematics book by Robert Smith, a Cambridge professor. This led him to another book by Smith, on astronomical optics. Spurred by his long-time fascination in the stars, Herschel began to build his own telescopes.

Astronomy soon became a passion for Herschel. Throughout the year, he would spend each clear night scanning the skies. Caroline Herschel, William's sister, was his constant companion in this work, helping to document his observations. Herschel worked in weather so cold he had to break through ice on his inkwell to make notes. He rushed home during the intermissions of his concerts to squeeze in a few more observations. He developed an unrivalled familiarity with the night sky.

Herschel's dedication paid off in 1781. As he was scanning pairs of stars to look for the effect of parallax, he saw an object noticeably larger than a star that "did not belong" in the surrounding pattern of stars. His excellent telescope and sharp eye clearly resolved the disk of what was in fact a planet, even though it has a tiny angular diameter no larger than 3.7 seconds of arc as seen from Earth. In fact, this planet had been observed more than a dozen times previously, but each time the observer had marked it down as a star. Other astronomers soon determined that the planet's orbit was far beyond the distance of Saturn. The new planet was named Uranus after the oldest of the Greek gods. Herschel was the first person since antiquity to discover a new planet. At a stroke, he doubled the size of the solar system by finding a planet twice as far away as Saturn.

Herschel became famous after his discovery of Uranus. Backed by what we would call federal support — an annual stipend from King George III beginning in 1782 — Herschel built a series of larger and larger telescopes. He ground his own mirrors and fashioned the fittings from fine wood using the same techniques he had used to make cellos and oboes years before. Using the reflector telescope design of Isaac Newton, he built a telescope with a 1.2-m mirror in a few years. This was forty times the size of Newton's first reflector and a size that would not be surpassed until the 1840s. The construction was sometimes hazardous. Once, Herschel himself cast a mirror from molten metal using a mold made of dried horse dung after no foundry would take on the project. The mold cracked under the intense heat and Herschel and his colleagues had to run to escape the pool of liquid metal. Herschel lived and worked in the English country town of Bath, and his house and the workshop where he ground and polished his mirrors can still be visited.

Herschel wanted to figure out what he called "the construction of the heavens" — how many stars there are in the galaxy and how they are distributed. To do so, Herschel allowed the telescope to sweep the skies, counting stars in parallel strips. (These were the days before telescopes had motors that could compensate for the Earth's rotation and keep an object fixed in view.) Turning his attention to the Milky Way, Herschel was struck by the vast number of stars that were revealed by his telescope. "We find that the stars are crowded beyond imagination along the extent of the Milky Way,…so that, in fact, its whole light is composed of nothing but stars of every magnitude from such as are visible to the naked eye down to the smallest points of light perceptible with the best telescope." His painstaking and indefatigable approach allowed him to create the largest catalogs of stars and nebluae in history.

Herschel developed a method to compare the brightness of different stars. With two telescopes of the same size pointed at different stars, he would partially cover the aperture of one telescope until the stars appeared to have the same brightness. He discovered that the ratio of the uncovered areas of the two telescopes was equal to one divided by the relative brightness of the two stars. Herschel then assumed that all stars have the same true brightness, so the fainter a star appears, the farther away it is. He found far more stars and fainter stars in the Milky Way than in other directions. Herschel deduced from this observation that the stars in the direction of the Milky Way are distributed over larger distances than stars in other directions. He correctly mapped the Milky Way system as a disk of stars, with us inside it, but he did not know the size of the system of stars.

Herschel was part of an illustrious astronomical family. His sister Caroline was a tireless assistant and for a long time, an unsung hero of astronomy. She wrote in her memoir: "I did nothing for my brother but what a well-trained puppy dog would have done, that is to say, I did what he commanded me." But over time, her contributions grew and he clearly appreciated her expertise. She discovered several comets was was the first woman to be awarded the Gold Medal of the Royal Astronomical Society. Despite a bout of typhus at age ten, which stunted her growth, she lived to the age of 97 and was active in astronomy well into her nineties. Herschel's only son John also became a famous astronomer. John Herschel did valuable work in mathematics, chemistry, and botany, but astronomy was his passion. He made major contributions to the early science of photography and was the first to make photographs on glass plates and the first to experiment with color. Like his father, he was knighted for his contributions to science.

As he scanned the skies, Herschel was like a cartographer. Hundreds of years earlier, brave adventurers had mapped out the size and shape of the globe we live on. Herschel continued the exploration by detecting stars across vast regions of space and by mapping the distribution of "this magnificent collection of stars."