EPSC 201 Lecture Notes - Lecture 2: Positron, Centripetal Force, Orion Nebula

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Hydrogen through- out the universe absorbs ener- gy, and emits it back out as light. These particular wavelengths of hydrogen have been identified in the lab. We know the wavelengths that hy- drogen should emit when sta- tionary. But the hydrogen around the universe emits longer wavelengths. We know that it is moving away from us. Thus, we know the universe, and the distance be- tween stars, is expanding. Hubble determined our distance from other stars using luminosity of stars. The further a star is, the less luminous is. He used the red shift to determine how fast the stars were moving away from us. He used rel- ative luminosities by comparing different stars. Had to assume every star was the same brightness and speed was constant. He also used groups of stars to account for the fact that their brightness is not con- stant.

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