A New View of the Universe: Photography and Spectroscopy in Nineteenth-Century Astronomy

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A New View of the Universe: Photography and Spectroscopy in Nineteenth-Century Astronomy

Overview

The development of photography and spectroscopy in the nineteenth century allowed astronomers to record and analyze the light coming from stars and other celestial objects. This transformed astronomy from a purely descriptive science to a systematic study of the behavior of these objects, laying the foundation of the discipline we now call astrophysics. The realization that the stars are made of elements also found on Earth, and that the Sun is actually a rather ordinary star, changed the way we look at ourselves and the Universe.

Background

Leonardo da Vinci (1452-1519) and other Renaissance scientists experimented with early cameras—optical devices for projecting an image onto a surface. However, at that time there was no way to preserve the image. In 1727 Johann Schulze (1687-1744), a German physicist, discovered that silver salts are sensitive to light and created images using them. A century later, the French physicist Joseph Nicephore Niepce (1765-1833) found a way to "fix" these images onto a metal plate so that they could be kept indefinitely. The earliest surviving photograph, of the view out Niepce's window, was made in 1826. Subsequent improvements by chemists improved the process of making photographs, as well as enhanced their quality and durability.

Astronomers were quick to recognize the potential for photography to aid and publicize their work. At the 1862 World Exhibition in London, stereoscopic "three-dimensional" photographs of lunar craters and sunspots were a great success. The needs of astronomers also helped to drive photographic technology. Images took so long to develop with the earliest photographic chemicals that fast-moving shutters were not necessary for ordinary photography. However, they were invented by the Englishman Warren de la Rue (1815-1889) to enable photographs of the Sun to be taken without burning the film.

Photographic images were found to have a number of advantages over direct observation, in addition to the obvious benefit as records of permanence and portability. Photographic film can detect finer gradations of light than can the eye, allowing greater detail to be observed. Film is also sensitive to parts of the electromagnetic spectrum outside the human visual range, such as ultraviolet, and can be adjusted to emphasize particular wavelengths or colors. With a long exposure, film can detect objects far too faint for the human eye to see. Eventually photography became so essential to astronomical observation that professional astronomers rarely observed directly with the eye. Most of their work was done by studying photographic plates. This was the case until fairly recently as electronic imaging and computer analysis became the new standard for observation.

The other major new astronomical technique used during the nineteenth century was spectroscopy. During the 1660s Isaac Newton (1642-1727) had shown that the light from the Sun could be broken up into a continuous spectrum like a rainbow by using a prism. Later, it was discovered that the same effect could be achieved and more precisely measured using a diffraction grating with thousands of tiny grooves or slits. In 1802 William Hyde Wollaston (1766-1828) observed a few dark lines breaking up the solar spectrum; he assumed that these were the boundaries between colors. But beginning in 1814 the Munich optician Joseph von Fraunhofer (1787-1826) mapped hundreds of these tiny gaps, which came to be called Fraunhofer lines.

Meanwhile, in the laboratory scientists turned their spectroscopes to as many sources of light as they could find. They soon realized that passing light through a gas produced dark lines at various points on the spectrum, while burning a substance produced a spectrum with bright lines superimposed upon it. In 1859 Gustav Robert Kirchoff (1824-1887) of Heidelberg recognized that the dark and bright lines at a given wavelength were produced by the same materials. If the material was burned, bright lines were seen. If light was passed through it, there would instead be a dark line in the same place on the spectrum. The key was the temperature. A hot gas gives off light at particular wavelengths, producing bright emission lines. If the gas is cooler than the light coming from behind it, it instead absorbs light at its characteristic frequencies, and dark absorption lines are seen. The wavelengths of the lines are actually the result of changes in atomic and molecular energy levels, concepts unknown in the nineteenth century. But well before the real reason for absorption and emission lines was understood, astronomers were using them to gain a great deal of information about the composition and temperature of objects in the sky.

Impact

By the middle of the nineteenth century astronomers were taking photographs of the Sun, Moon, and stars. During the solar eclipse of 1860, de la Rue and the Italian astronomer and Jesuit priest Pietro Angelo Secchi (1818-1878) both obtained good results, which together revealed the outer corona of the Sun and the solar prominences, or great eruptions extending from the surface. While these had been observed before, some astronomers had argued that they were optical illusions; this theory was now disproved.

The permanent and detailed records of star fields that accumulated on photographic plates, including several systematic sweeps of the heavens, allowed stellar photometry, the measurement of light from the stars, to be put on a scientific basis for the first time. Previously, stellar magnitudes, or brightnesses, were estimated visually by comparing one star to another in a 2000-year-old system originated by the great astronomer Hipparchus (c.190-c.120 B.C.). Now, with special filters and plates, more accurate catalogs of stars could be produced.

Locations of objects could also be pinpointed more precisely and compared in successive photographs of the same region. Many variable stars were discovered, the brightness of which changed over time. Searching large numbers of photographic plates for small changes in detailed star fields proved a labor-intensive task, and many astronomers delegated this exacting work to female assistants. This provided an entry path for a number of women into a previously all-male field.

Since the turn of the nineteenth century it had been known that there existed a number of minor planets, or asteroids, in the large gap between the orbits of Mars and Jupiter. A few had actually been observed through telescopes and their orbits calculated. In 1891 Max Wolf (1863-1932) of Heidelberg applied photography to the search by using long exposures. An asteroid, so much closer to Earth than the stars in the background, moves fast enough across the field to leave a short but noticeable track on the plate. With this method asteroids were catalogued by the hundreds over the next few years.

Spectroscopic techniques were quickly recognized as being important in astronomy because of the ability to study hot gaseous objects like the Sun and stars. The absorption lines observed by Fraunhofer indicated that sunlight comes from hot gases beneath the surface of the Sun, and is absorbed at some wavelengths by cooler gases higher up. Thus spectroscopy provided clues to the layered structure of the gases in the Sun, which would be better understood in the twentieth century in terms of the nuclear reactions that take place in stellar cores.

Known substances were studied in the laboratory so that the positions of their lines could be compared to those observed in space. By the end of the century, dozens of elements had been identified in the Sun, including hydrogen, sodium, calcium, magnesium, carbon, and iron. The eclipse of 1868 had allowed astronomers to take advantage of the blocked solar disc to take the spectrum of the outer regions of the Sun. The English astronomer Sir Joseph Lockyer (1836-1920) found that, as well as hydrogen, there was another previously unknown element in the Sun, which he called helium after helios, the Greek name for the Sun. Helium was not discovered on the Earth until 1895, when it was found in the mineral clevite.

Soon astronomers learned to adjust their spectroscopes so that they could be focused on a specific region of the Sun without waiting for an eclipse. More frequent observations made clear the dynamic nature of the Sun, as sunspots waxed and waned, and disturbances in the outer atmosphere were associated with changes in the lines that were seen.

With better focusing techniques, spectra could be obtained from more distant stars. As early as 1823, Fraunhofer had written that other stars had lines in their spectra similar to that of the Sun. In 1864 Sir William Huggins (1824-1910) had identified these lines with elements on Earth. The fact that the Sun and stars are made of the same materials we find around us, and that there exist a great many stars similar to the Sun, contributed to a shift away from the generally accepted worldview that our Solar System is unique in character. Astronomers began to classify the stars in terms of the details of their spectra, and made inferences as to their stage of development, the beginning of the concept that the large-scale Universe has changed over time rather than having originated as we see it today.

Of course, there are other objects in the sky besides stars. In 1864 Giovanni Battista Donati (1826-1873) first applied spectroscopy to the observation of a comet, detecting lines but making no identification. Four years later Huggins saw similar lines and realized that they indicated the presence of hydrocarbons. The discovery of simple organic compounds in these "visitors" from outer space would lead some to speculate that life may have come to Earth as the result of an impact. However, this theory just removed the riddle of life's origins on Earth to an unknown outer space location.

Other objects that elicited great curiosity were the nebulae, or clouds. In 1781 the French observer Charles Messier (1730-1817) recorded the positions of 103 of these fuzzy patches in the sky so that he wouldn't confuse them with the comets he was seeking. The eighteenth-century philosopher Immanuel Kant (1724-1804) and a few other advanced thinkers considered the idea that these nebulae might be "island universes" outside the Milky Way. This first hypothesis of the existence of other galaxies was later rejected by prominent astronomers like William Herschel (1738-1822). Herschel's three great catalogues of nebulae included some that seemed to be halos of what he called "shining fluid" surrounding stars, and he assumed that this was the material of which all the nebulae were made.

Later generations of astronomers were to discover that, without a reliable distance scale, the early catalogues of nebulae lumped together separate entities such as the hot gases from exploding stars relatively nearby, clusters of stars within the Milky Way, and fuzzy glimpses of distant galaxies. The better resolution and understanding provided by nineteenth-century photographic and spectroscopic observations helped to sort out these different types of objects.

Another major contribution of spectroscopy to the science of astronomy was the information it provided about the motion of objects. Most people are familiar with the way the pitch of an ambulance siren falls as it recedes into the distance. This is an example in sound waves of an effect that Christian Doppler (1803-1853) first detailed for light in 1842. He showed that as a luminous body approaches, the light it gives off decreases in wavelength, shifting toward the blue end of the spectrum. Conversely, as it recedes, the light shifts toward the red. Changes in the expected positions of lines in a spectrum could therefore be interpreted as indicating motion in the object producing those lines.

Applying Doppler's principle to observations of the Sun, astronomers clocked violent disturbances in its outer atmosphere, which reached speeds of 300 miles per second. The rotation of the Sun was also confirmed by observing a red shift at one edge and a blue shift at the other. Many stars were discovered to be binaries, showing pairs of lines that shifted as the two stars orbited each other, receding from and approaching us in turn.

In 1868 Huggins observed a shift in a hydrogen absorption line in the spectrum of Sirius, and interpreted it as indicating that the star was moving away from the Solar System at a considerable speed. Other such observations followed, with everything seeming to be flying away from us. It was left to the twentieth-century astronomer Edwin Hubble (1889-1953) to realize that this was evidence of an expanding Universe, with all its components getting farther from each other like dots on the surface of a balloon being blown up. Projecting this scenario backward in time, scientists began to postulate the theory that the Universe resulted from a massive explosion, or "Big Bang," billions of years ago.

SHERRI CHASIN CALVO