In 1925, a paper by Hubble described how he had measured that the Andromeda galaxy was at a far greater distance than, and thus not a part of , our Milky Way galaxy. That significantly extended the known view of the universe. His paper was read, by H.N. Russell, to a meeting of the American Astronomical Society in Washington, D.C. Hubble had observed Cepheid variable stars within the Milky and Andromeda galaxies. He applied Henrietta Leavitt’s discovery that for such stars, pulsation rate and absolute brightness were related in a predictable way. If two such stars have an equivalent pulsation rate, they have an equivalent absolute brightness. If one appears less bright, it’s further away. Comparing apparent brightness with absolute brightness (known from the pulsation rate) gives distance measurement.