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The Doppler Effect

The Doppler Effect

By 1923 he had measured the Doppler shifts of some forty-one of these star clusters. Discovered by Austrian scientist Christian Doppler, the Doppler effect describes the changes in sound or light waves transmitted from one body to another as they get closer together or farther apart. As objects move closer, waves get shorter and their frequency gets higher, and as light-wave frequencies get higher their color shifts toward the blue range. An object moving away emits longer waves with a lower frequency, and thus light waves in this category exhibit a red shift. Slipher detected red shifts in thirty-six of the galaxies he examined, meaning that they were moving away from Earth. The remaining five nebulae exhibited a blue shift, which seemed to mean that they were getting closer to Earth, but in 1925 it was discovered that the Milky Way is itself rotating rapidly. Failure to account for this spin had led to false blue-shift readings. After correcting for this factor, it was found that only two galaxies, both comparatively near to our own, showed a net blue shift. Slipher's work supported the research Edwin Hubble was doing at the same time on the expanding universe.

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