Contributions to the Nature and Structure of the Milky Way

Galileo -Determined that various stars were also at various distances from the Earth. This disproved the theory that the Universe was similar to a glass bowl. The stars are sporadically placed, and not evenly. Galileo proved this observationally.

William Herschel -  determined that Earth is located in the Milky Way amongst other fuzzy patches of stars and gas. He also determined, by observing other fuzzy patches (nebulae), that the shape of the Milky Way was a flat spiral disk. Provided observational evidence to support his claim that there were other nebulae in our universe.

Harlow Shapley - estimated the size of the Milky Way using RR Lyrids. Also found the Sun's position. Partook in the Great Debate and proposed that the Milky Way was the entire universe with other spiral galaxies contained inside. 

Edwin Hubble -observed Cepheid variable stars and calculated their distance using the research done by Henrietta Leavitt. He concluded that they were too far away from the Earth to be in the Milky Way. He classified galaxies and thought that there was some level of progression between types, for example, that ellipticals eventually became spirals. 

Immanuel Kant-published "The Universal Natural History and Theory of the Heavens" which included two theories. The first is the "Nebular Hypothesis" which covers star and planet formation, and theorizes that it happens when gas and dust clouds spin and collapse due to gravity, forming larger, denser objects (planets,etc). His second theory distinguished nebulae from galaxies, and suggested that galaxies other than the Milky Way were in existence.

Henrietta Leavitt-confirmed in 1912 that the variable stars of greater intrinsic luminostiy (Cepheid Variables) had longer periods. This became known as the "Period-Luminosity relationship" which became a vital tool used to measure distances in the universe. Her research allowed Edwin Hubble to find Cepheids in other galaxies, which radically altered our view of the universe in it's entirety.

Heber Curtis- is famous for his role in "The Great Debate" with Harlow Shapley. He argued that spiral nebulae were actually spiral galaxies outside of our own.