Hubble not only found a number of stars in Andromeda, he found Cepheid variable stars, which brighten and dim on a regular basis. Their brightness and periodicity had enabled a bright Harvard scholar, Henrietta Leavitt, twelve years earlier to figure out how to calculate their distances from Earth. Hubble used Leavitt’s formula to calculate that Andromeda was approximately 860,000 light years away. That’s more than eight times the distance to the farthest stars in the Milky Way. This conclusively proved that the nebulae are separate systems and that our galaxy is not the universe.
Hubble’s advances led to finding many more galaxies and the fact that they are moving away from ours – that the universe is still expanding.
Before Copernicus and Galileo, humans thought our world was the center of creation. Then (except for a few notable stragglers) we learned that the sun and planets did not revolve around the Earth, and we discovered that our sun — though the center of our solar system — was not the center of the universe or even an important star in our galaxy.
But we still grandiosely thought our own dear Milky Way contained all or most of the stars in existence. We were about to be knocked off our egotistical little pedestal once again.