The American astronomer Henrietta Swan Leavitt (1868–1921) and the Danish astronomer Ejnar Hertzsprung (1873–1967) studied Cepheid variable stars in the Small Magellanic Cloud in 1913. That work led to the first period-luminosity relation calculation for Cepheid variables and their potential use as standard candles for determining distances beyond the Milky Way. A decade later, Edwin Hubble used their work to determine that Andromeda was far outside the Milky Way, leading to the birth of modern extragalactic astronomy.