William Huggins was the father of stellar spectroscopy, the study of light given off by stars. In 1856, he built his own observatory and began photographing stars in order to break their light down into its component wavelengths.
Visible light includes a spectrum of colors with red on one end and violet on the other (the colors of the rainbow). Atoms give off and absorb light at various wavelengths. A spectroscope, a simple instrument containing a prism surrounded by two lenses, produces a spectrum of the light that enters its chamber. The spectrum can either appear as a series of dark lines indicating the wavelengths at which the light is or absorbed, or a series of bright lines indicating the wavelengths at which light is emitted. The light of an element, when passed through a spectroscope, produces a signature spectrum that identifies the element. In the same manner, the spectrum of starlight indicates which elements are present in a star's atmosphere.
Huggins used a spectroscope to determine that celestial bodies contain some of the same elements that exist on Earth. For instance, he found oxygen present in the spectrum of a star and carbon compounds in the spectrum of a comet. Likewise, he studied the spectrum of a nova (a star that suddenly becomes very bright) and found that it contained hydrogen. These findings disproved the hypothesis made by Aristotle twenty-one hundred years earlier, that objects in space were made of a special material not found on Earth.
Huggins observed the spectrum of a bright patch in the sky called a nebula and learned that it was made up of gasses, not stars, as had always been assumed. He was not successful, however, in his attempt to study the spectra of planets. The reason for this failure is that planets do not generate their own light, but merely reflect the sun's light.
Huggins was among the first to use time-exposure photographs. This method allows light from a faint object to be collected over a long period of time, thus magnifying its brightness. In this way he was able to record the spectra of very distant matter, including stars, comets, nebulae, and others.
By obtaining the spectrum of an object, Huggins showed, one can also determine its movement. He was able to detect motion by studying any shift of absorption lines, toward one end or the other of the spectrum. Huggins found that when light waves given off by an object are shifted toward the red end of the visible light spectrum, this means the object is moving away from the observer. Just the opposite is true for a blue-shifted object. It is moving closer. Huggins also established that the degree of shift indicates the object's speed.