How did Edwin Hubble prove that the Andromeda galaxy was beyond the Milky Way?

Edwin Hubble's groundbreaking work involved observing individual Cepheid variable stars in the Andromeda galaxy and applying the period-luminosity relation. This technique was pivotal because Cepheid variables have a well-established relationship between their pulsation periods and their intrinsic luminosities. By identifying these stars in Andromeda and measuring their brightness, Hubble could determine their distance from Earth.

Once he established the distance to these Cepheid variables, he found that Andromeda was significantly farther away than previously thought, well outside the boundaries of the Milky Way galaxy. This was crucial in demonstrating that Andromeda is indeed a separate galaxy, rather than just a nebula within our own galaxy. The application of the period-luminosity relation to measure distance was a revolutionary step in extragalactic astronomy, allowing Hubble to provide solid evidence for the existence of other galaxies in the universe.