The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, including light, radio waves, and X-rays (see chart at the bottom of this page). It is a continuum of wavelengths from zero to infinity. We name regions of the spectrum rather arbitrarily, but the names give us a general sense of the energy; for example, ultraviolet light has shorter wavelengths than radio light. The only region in the entire electromagnetic spectrum that our eyes are sensitive to is the visible region.

• Gamma rays have the shortest wavelengths, of less than 0.01 nanometers (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the electromagnetic spectrum. Gamma rays can result from nuclear reactions taking place in objects such as pulsars, quasars, and black holes.
  
  
• X-rays range in wavelength from 0.01 to 10 nanometers (about the size of an atom). They are generated, for example, by super-heated gas from exploding stars and quasars, where temperatures are near a million to ten million degrees.
  
  
• Ultraviolet radiation has wavelengths of 10 to 310 nanometers (about the size of a virus). Young, hot stars produce a lot of ultraviolet light and bathe interstellar space with this energetic light.
  
  
• Visible light covers the range of wavelengths from 400 to 700 nanometers (from the size of a molecule to a protozoan). The Sun emits most of its radiation in the visible range, which our eyes perceive as the colors of the rainbow. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.
  
  
• Infrared wavelengths span from 710 nanometers to 1 millimeter (from the width of a pinpoint to the size of small plant seeds). At a temperature of 37 degrees C, our bodies radiate with a peak intensity near 900 nanometers.
  
  
• Radio waves are longer than 1 millimeter. Since these are the longest waves, they have the lowest energy and are associated with the lowest temperatures. Radio wavelengths are found everywhere: in the background radiation of the universe, in interstellar clouds, and in the cool remnants of supernova explosions, to name a few. Radio stations use radio wavelengths of electromagnetic radiation to send signals that our radios then translate into sound. These wavelengths are typically a few feet long in the FM band and up to 300 yards or more in the AM band. Radio stations transmit electromagnetic radiation, not sound. The radio station encodes a pattern on the electromagnetic radiation it transmits, and then our radios receive the electromagnetic radiation, decode the pattern and translate the pattern into sound.
  

New instrumentation and computer techniques of the late 20th century allow scientists to measure the universe in many regions of the electromagnetic spectrum. We build devices that are sensitive to the light that our eyes cannot see. Then, so that we can "see" these regions of the electromagnetic spectrum, computer image-processing techniques assign arbitrary color values to the light.