Final answer:
Edwin Hubble used Cepheid variables to measure the distance to faint nebulas and proved they were other galaxies.
Step-by-step explanation:
Edwin Hubble used Cepheid variables to measure the distance to faint nebulas in our sky and proved that they were actually whole other galaxies. Cepheid variables are a type of pulsating star whose brightness varies predictably over time. Using the technique pioneered by Henrietta Leavitt, Hubble measured the distance to the Andromeda galaxy and estimated it to be about 900,000 light-years away, demonstrating that it was a separate galaxy outside of the Milky Way. Today, we know that the Andromeda galaxy is actually slightly more than twice as distant as Hubble's first estimate, but his conclusion about its true nature remains unchanged.