Astronomy and the electromagnetic spectrum. The EM spectrum runs from gamma rays (left), via X-rays, ultraviolet, optical, and infrared, to radio waves (right). Some spectral bands (radio, IR, optical) can employ sea-level or mountain-top telescopes. Other bands (notably UV, gamma, X-ray) suffer from absorption of EM radiation by Earth's atmosphere. They use balloons, planes and satellites to take telescopes into the upper atmosphere or outer space. Powerful sources of EM radiation are active galaxies (top left) in the gamma and X-ray bands, star-forming regions in infrared (top centre), supernova remnants and the glow of radiation from the Big Bang (top right) in the radio band.