https://en.wikipedia.org/wiki/Edwin_Hubble
Hubble proved that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually galaxies beyond the Milky Way.[4] He used the strong direct relationship between a classical Cepheid variable's luminosity and pulsation period[5][6] (discovered in 1908 by Henrietta Swan Leavitt[7]) for scaling galactic and extragalactic distances.[8][9]
Hubble provided evidence that the recessional velocity of a galaxy increases with its distance from Earth, a property now known as Hubble's law, although it had been proposed two years earlier by Georges Lemaître.[10] The Hubble law implies that the universe is expanding.[11] A decade before, the American astronomer Vesto Slipherhad provided the first evidence that the light from many of these nebulae was strongly red-shifted, indicative of high recession velocities.[12][13]
