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The description of the word you requested from the astronomical dictionary is given below.
Edwin Hubble (1889 - 1953) was the first person to prove (in 1923) that what we now call spiral galaxies are outside of our Milky Way Galaxy, so that our Galaxy does not fill the entire Universe. He found that far-away galaxies move away from us at a speed that on average is proportional to their distance from us. This relationship is now called Hubble's Law, and the constant of proportionality (the ratio of the Doppler speed to the distance) is Hubble's Constant.