When sunlight is passed through a prism a band of colors ranging from violet to red is produced. These colors correspond to wavelengths of light between 4000 and 7000 angstroms , a unit equal to 10-8 centimeters. This very small part of the entire electromagnetic spectrum is called visible light because our eyes perceive these energies as color. The visible spectrum of the Sun and stars can be photographed with a device known as a spectrograph . The resulting picture is called a spectrogram .
Some spectra also have broader bands produced by molecules.
Since each atom, ion, or molecule has a unique and characteristic set of lines or bands, the chemical composition of stars can be determined. Stars can be classified using criteria based on the appearance of their spectra. The Draper catalog, published in 1890, was the initial effort of Harvard University astronomers to classify stars based on the varied patterns of lines and bands in stellar spectra. One of the greatest women astronomers of the time, Williamina Fleming, made monumental contributions to the development of the Draper catalog, which contained spectral classifications for 10,351 stars. Most stars have very similar chemical compositions.