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Light that has been spread out into its component colors or wavelengths. Electromagnetic radiation that has been spread out into a spectrum allows an observer to study its individual colors or wavelengths. A rainbow is an example of the visible spectrum. Astronomers use spectra to study absorption and emission lines and their relative brightness to learn about the temperature, composition, speed, and distance of celestial objects. Spectra of such objects often provide very detailed information about them. To obtain spectra, astronomers use instruments called spectrographs on their telescopes. Spectrograph use diffraction gratings or prisms to refract the light into a spectrum of color.