Essentially everything astronomers have learned about stars, galaxies and nebulae has been through the analysis of the electromagnetic radiation that reaches us from them. Until the introduction of radio astronomy in the mid-twentieth century, this analysis has been restricted to light, which remains a valuable source of information regarding the composition, motion and processes occurring in distant celestial bodies.
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Visible light corresponds to electromagnetic radiation with wavelengths from 400 nm to 700 nm. [1nm = 1 x 10-9 m], while light includes the ultraviolet and infrared regions as well. Telescopes collect and focus light on a spectrograph, which uses charge-coupled devices (CCD’s) to record the information. The spectrograph with its associated data acquisition system records the amount of light received at each wavelength, from the ultraviolet to the infrared (depending on the instruments used). The arrangement of colours from the object being studied in order of wavelength is called its spectrum.