By 1929, an important time in cosmology, Edwin Hubble used the spiral nebulae data collected by Vesto Slipher from 1912 to 1925 to introduce the world to galaxies.

Prior to the 20th century, astronomers debated whether spiral nebulae were merely gaseous clouds within the Milky Way, possibly forming solar systems. This view changed dramatically following the “Great Debate” in 1920 between Harlow Shapley, who believed the Milky Way constituted the entire universe, and Heber Curtis, who argued these nebulae were distant “island universes” or separate galaxies. This debate was conclusively settled by Edwin Hubble in the mid-1920s through his observations using the 100-inch Hooker Telescope at Mount Wilson Observatory. By identifying Cepheid variable stars in the Andromeda nebula between 1923 and 1924, Hubble determined Andromeda’s considerable distance from the Milky Way, thus supporting Curtis’s hypothesis. By 1929, Hubble had not only confirmed that spiral nebulae were independent galaxies but also discovered the relationship between a galaxy’s redshift and its distance, formulating Hubble’s Law and providing strong evidence for the expanding universe—a cornerstone of the Big Bang theory.