Chapter 14: The Milky Way

Wilhelm Herschel, German-British astronomer.

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 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.

Carolyn Herschel

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 the 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 unrivaled familiarity with the night sky.

Uranus in natural colors

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.

William Herschel's 40-foot telescope

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.

The Milky Way galaxy as seen in Earth's sky. Photos covering the complete celestial sphere have been stitched and transformed to a panoramic image with the Milky Way as central line.

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.