It’s bigger than we thought
In 1923 Edwin Hubble used a variable star to prove that the Andromeda Nebula was actually the Andromeda Galaxy. Before his measurements, it was widely believed that our galaxy, the Milky Way, was the only island of stars in the universe. Boy, did we get it wrong!
Have a look at NASA’s page about when they pointed the telescope named after Edwin Hubble at the star he used to measure the distance to M31—Andromeda.