Gentleman Astronomers

Many of the great astronomical discoveries of the 17th and 18th centuries were made by men with rich patrons: university professors or other academics who received stipends from kings or princes to continue their researches.

The 19th century, though, was the century of the gentleman-astronomer: men who amassed fortunes through business or family connections, and devoted much of it to their own scientific pursuits.

Such was the case of William Parsons, the third Earl of Rosse, who built what was then the world's largest telescope at his family's castle in Ireland.

He earned a master's degree in mathematics from Oxford, served in the House of Lords, and married an heiress. He joined the Royal Astronomical Society at age 24, and by age 36 he had retired to Birr Castle to pursue astronomy and other interests.

Contemporary engraving of the newly completed Leviathan.

Rosse designed and built his own reflecting telescope, a 36-inch model that was one of the largest and grandest of the day. The copper-tin mirror was cast on the estate, and ground to the proper shape and polished with a steam-powered machine of the Parsons' design. The telescope allowed him to study the Moon, star clusters, and other objects in great detail. It also began to show ghostly details in the objects known as nebulae — oddly shaped puffs of light scattered throughout space.

Rosse and others thought that seeing the full details of these objects, which Rosse thought were star clusters, required an even larger telescope, so he set about building one.

The telescope itself featured a three-ton primary mirror that was 72 inches in diameter. Since it tarnished easily in the damp Irish weather, Rosse actually cast two mirrors, allowing him to continue his observations while one of the mirrors was cleaned and polished. The mirror was mounted in a 60-foot tube that was braced between two 50-foot walls. The telescope could move up and down but not from side to side, limiting its view of an astronomical object to an hour or two at a time.

Rosse's sketch of M51, today known as the Whirlpool Galaxy

The telescope, which was nicknamed "the Leviathan of Parsonstown," first saw the stars in February of 1845. Its view was unequaled — it could see stars that were at least 10,000 times too faint to detect with the unaided eye.

A key target for the telescope was the Orion Nebula, which is a cloud of gas and dust that is giving birth to new stars. At the time, its nature was unknown. Rosse saw many of the stellar nursery's newborn stars and concluded that the entire nebula was simply a swarm of stars.

Satisfied with his Orion observations, Rosse turned to other nebulae, and discovered that dozens of them exhibit a spiral structure, like little pinwheels spinning through the firmament. He thought that perhaps they were agglomerations of stars, but his giant telescope could see no individual stars in them. Yet his sketches became valuable resources as astronomers debated the nature of these objects, which are galaxies that contain billions of stars.

The Leviathan continued its work for decades under Rosse and his son. By the end of the century, though, its mirror had been removed and its mounting fell into disrepair. But no telescope surpassed it until the 100-inch telescope at Mount Wilson, California, entered service in 1917.

In the late 20th century, Rosse's descendants restored the telescope's mounting and built a replica of the Leviathan, which is now open for public inspection — and occasionally takes a peek at the stars.

For more than two centuries, the telescope allowed astronomers to see wonders beyond compare, from crowded star clusters to the rings of Saturn to the “spiral nebulae” that appeared to spin across the sky. Yet the telescope could reveal little about the true nature of these wonders. It was as though astronomers could see a big slice of chocolate cake, but couldn’t taste it or smell it.

In 1859, though, Gustav Kirchhoff and Robert Bunsen invented a device that eventually would allow astronomers to decipher many of the secrets of the stars: the spectroscope.

More than three decades earlier, German scientist Josef Fraunhofer had created a mystery when he directed the light from the Sun and Sirius, the brightest star in the night sky, through a prism attached to a telescope. As he expected, the prism split the starlight into a rainbow of colors. But Fraunhofer also saw dark lines imprinted on the rainbow pattern like stripes on a cosmic animal. Scientists eventually decided that the lines revealed something about the chemistry of the stars, but they had no way of knowing which elements the lines represented.

That changed after Kirchhoff, a German physicist, analyzed the light from different gases that were burned in Bunsen’s new laboratory burner. He saw that each element yielded a pattern of bright lines in its “spectrum” — the rainbow of colors produced when the light was passed through a prism. He also saw that passing the light of the flame through a dark gas imprinted dark lines in the spectrum.

This illustration shows how the Doppler shift is applied to astronomical objects. The center panel shows a basic spectrum with simulated dark spectral lines. In the top panel, the lines are shifted toward the blue end of the spectrum, indicating that the astronomical object is moving toward us. In the bottom panel, the lines are shifted toward the red, indicating that the object is moving away from us. How far these lines are shifted from their ‘default’ positions indicates the object’s speed. [Tim Jones]

Kirchhoff concluded that the dark lines that Fraunhofer saw in the Sun’s spectrum were the imprints of cooler elements in the Sun, which “absorbed” the bright light from the surface of the Sun.

Using the new spectroscope that he and Bunsen created, Kirchhoff could measure the precise pattern of bright and dark lines from each chemical element, allowing scientists to begin identifying the composition of the Sun and other stars. In the 1860s, Kirchhoff himself found 16 elements in the Sun’s spectrum, and he and Bunsen identified several elements in their laboratory that had never been seen before.

By the 1870s, astronomers were using another new technology, photography, to capture the spectra of the Sun and other stars for detailed analysis. The combination of these technologies helped spawn a new discipline: astrophysics, which is the study of the physics at work in astronomical objects.

Today, spectroscopy is the most powerful tool in all of astronomy. When attached to a telescope, a spectroscope (or spectrograph, as it is more commonly called today) reveals not only the chemical composition of objects, it can reveal their motion through space, whether they rotate, whether they have companion objects (such as planets), and much more. In essence, it allows astronomers to “taste” the tantalizing treats they see through their telescopes.

Above: The Kirchhoff-Bunsen spectroscope

While early spectroscopes used glass prisms, modern instruments use "diffraction gratings," which are series of thousands of tiny lines engraved in a glass surface. These lines act like the prism, breaking the light from an astronomical object into its component colors.

Light passing through a spectrograph attached to the 107-inch telescope at McDonald Observatory produces a rainbow of colors. These are views of the instrument splitting the light from an ordinary incandescent bulb. [Martin Harris/McDonald Observatory]

In addition to revealing an object's chemical composition, spectroscopy can reveal other details through the Doppler effect.

This effect compresses lightwaves that are traveling toward us to shorter wavelengths, making them look bluer (a "blueshift), and stretches lightwaves that are traveling away from us to longer wavelengths, making them look redder ("redshift"). The same thing works with sound waves, so that the pitch of a siren or train horn rises as it approaches you, but drops as it moves away from you.

From the size of the shift, astronomers can measure how fast an object is moving toward or away from us. Distant galaxies are all moving away from us because the universe is expanding, so their redshift reveals their distance, which in turn allows scientists to measure how fast the universe is expanding both today and at earlier times in the universe.

The Doppler shift can also reveal an object's rotation rate, because the side that is moving toward us is blueshifted, while the side that is moving away is redshifted. And tiny back-and-forth "wobbles" in the spectrum can reveal the presence of a companion star or planet, which exerts a gravitational pull on the star, causing it to move slightly toward us or away from us at different times.

In the late 19th century, the royalty of telescope makers was Alvan Clark & Sons of Cambridgeport, Massachusetts. And the leading jewel in Clark's crown was the 40-inch refractor at Yerkes Observatory.

George Ellery Hale
During a career that spanned four decades, George Ellery Hale masterminded four giant telescopes, each of which held the title of "world's largest" for a decade or more. A graduate of MIT and son of a wealthy industrialist, he built his own observatory, which eventually earned him a professor's appointment at the University of Chicago. There, he established Yerkes Observatory and supervised the construction of its 40-inch refractor. In the early 1900s he moved to California, where he established the Mount Wilson Solar Observatory and pursued his own studies of the Sun. At Mount Wilson, he built 60- and 100-inch reflectors, each of which was the world's largest at the time it entered service. And after retiring in 1923, he began designing and raising funds for the 200-inch telescope at Palomar Mountain, California. Dedicated in 1948, the telescope today bears Hale's name.

Alvan Clark came from a New England whaling family, but began making telescope lenses and mirrors after failing as a portrait painter. He and his sons learned the business on their own, and soon began making the finest optical glasses in the country. And during the latter half of the 19th century, they had produced the lenses for the four largest refracting telescopes in the world.

The company had started on an even bigger telescope, for Lick Observatory in California. But the project stalled when its benefactor went bankrupt.

The 40-inch refractor today [Yerkes Observatory]

In 1892, one of the sons, Alvan G. Clark, told George Ellery Hale, the director of the as-yet-unbuilt observatory at the University of Chicago, that the two 40-inch glass blanks were still sitting in the factory where they were cast, in France. Hale quickly convinced the university's president to snatch up the lenses, and convinced Charles Yerkes, a Chicago streetcar operator with a less-than-stellar reputation, to pay for telescope and the entire new observatory.

Clark & Sons began grinding and polishing the lenses in 1892, and completed them three years later. They were mounted in a 63-foot tube (which survived a fire while on display at the Chicago world's fair) at the new Yerkes Observatory at Williams Bay, Wisconsin.

The 40-inch telescope was the largest refractor in the world when it began its scientific observations in 1897, and remains the largest today. The list of scientists who have used the telescope includes Edwin Hubble, for whom Hubble Space Telescope is named, who photographed distant galaxies. (At the time, there was debate about whether they were separate galaxies or motes of matter inside our own galaxy. Hubble himself proved that they were separate galaxies a few years later, at Mount Wilson Observatory.)

In recent years, among other projects, astronomers have compared modern pictures of starfields taken with the telescope to those taken as long as a century ago to plot tiny changes in star positions.

More Information

History of Yerkes Observatory

Yerkes Observatory: National Park Service

Yerkes Observatory, 1892-1950, by Donald E. Osterbrock (University of Chicago, 1997)