Understanding light, and how it interacts with matter, is very important for astronomers, because their science usually is done passively, by analyzing light from stars. They can't perform experiments, they can only look. Although we usually think of "light'' as visible light, that's just a tiny fraction of the electromagnetic spectrum, which includes radio waves and X-rays. One of the reasons astronomy has been so successful in gaining knowledge about the Universe is that astronomers have become more adept at collecting and analyzing light throughout the electromagnetic spectrum.
Light is made up of packets of energy called photons. All photons travel at the same speed - the speed of light - but the amount of energy in a photon determines what kind of light it is. White light from the Sun can be spread into a spectrum of different colors, like a rainbow. The spectrum reveals the unique fingerprints of elements and compounds that have affected the light. The instrument that breaks the light into a spectrum is called a spectrometer.
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