Astronomy 162
1. With for example a prism you can break light up into its separate colors, in a so-called spectrum. When astronomers did this to the light of stars, they found that the stars weren't all the same. Some had more blue light, and others had more red light. Also, thin dark lines (spectral lines) could be seen in the spectra and those weren't the same for all stars, but there were groups of stars that had roughly the same spectra. The astronomers started dividing the stars into classes based on their spectra. They began with class A and worked their way down the alphabet. Some of these classes were later combined so eventually only classes A, B, F, G, K, M, N, O, R, and S remained. The Sun is a star of class G. Many years later it was discovered that most spectral classes depend mainly on the surface temperature of the stars. When you put the classes in descending order of surface temperature, then you get O B A F G K M and that is the order in which they're usually......
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