Part 8/15:
Hubble’s analysis of objects like the Andromeda Nebula (now classified as a galaxy) demonstrated that our universe extended far beyond the Milky Way. He identified Cepheid variable stars within these galaxies, measuring their brightness and distance, thus proving that they were separate "island universes" far outside our own. This proved that the universe was not static but expanding—a revelation supported by Hubble’s observation that galaxies are receding from each other, providing strong evidence for the Big Bang theory.