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The above image shows Henrietta Leavitt. Her work at the Harvard College Observatory early in this century was instrumental in the cataloguing of variable stars and in the realization that Cepheid variables could be used to establish a distance scale.
In 1917, she used Cepheid variables to show that the Magellanic Clouds were too distant to be part of our galaxy and thus must be separate galaxies in their own right. Later, Hubble used the same methods to show that the "spiral nebulae" were outside our own galaxy and thus must really be independent spiral galaxies external to our own.