The electromagnetic spectrum is the full range of light, ordered by wavelength, from radio waves metres long to gamma rays smaller than an atomic nucleus. Visible light occupies a sliver of it, which is why most of astronomy is invisible to the eye. Each band reveals different physics: radio traces cold gas and pulsars, infrared pierces dust to show forming stars, X-rays and gamma rays mark the hottest and most violent processes. Earth's atmosphere blocks most of these wavelengths, which is the reason so many observatories are flown into space.