Between 1922 and 1923 Edwin Powell Hubble (20th November, 1889 - 28th September, 1953), an American astronomer, identified Cepheid variables in the Andromeda Nebula, proving that there were galaxies outside our own. Up until this point the idea of there being galaxies outside the Milky Way had been disputed by astronomers, most famously by Harlow Shapley (2nd November, 1885 - 20th October, 1972). In April 1920, Shapley had taken part in the 'Great Debate' with American astronomer Heber D. Curtis (27th June, 1872 - 9th January, 1942), with Shapley arguing there were no galaxies other than our own.
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