Galileo Galilei: Galileo was the first to point a telescope to the sky to discover that the glowing Milky Way was made up of billions of stars too faint to see individually with the naked eye. Galileo boldly declared: "It is nothing else but a mass of innumerable stars planted together in clusters." This officially disproved Aristotle's earlier prediction that the glow of the Milky Way was a phenomenon of Earth's atmosphere.
William Herschel: In the late 1700s, William Herschel attempted to map out the Milky Way. His greatest setback was not being able to accurately determine the distances between stars. Regardless, he used a large reflecting telescope to produce the first General Catalog of galaxies.
Harlow Shapley: In the early 20th century, Shapely refined, Herschel's method to estimate that the Milky Way was a disc containing billions of stars and was tens or hundreds of light years across. He was particularly interested in the distribution of globular clusters within the Milky Way. Using the period-luminosity relation, Shapley determined the distances to globular clusters. Shapley found that the globular clusters form a near sphere around a point in the constellation of Sagittarius. He reasoned that this point is the center of the galaxy which he placed at a distance of 30,000 parsecs. (His distance measurements were obscured by dust clouds that decrease the luminosity of RR Lyrae stars- the modern distance is 8,000 parsecs.
Edwin Hubble: Edwin Hubble identified Cepheids in Andromeda and he derived distances even greater than those that Shapley predicted. Edwin Hubble proved that the Milky Way Galaxy did not make up the entire universe, but was merely one in a myriad of galaxies that make up the universe.
RR Lyrids and Cepheids: The period over which a Cepheid variable star fluctuates is related to its brightness (or luminosity). By measuring the period of these fluctuations, one can determine the brightness of the star. By comparing the observed brightness to the intrinsic brightness, the distance to the star can be calculated. Shapley used this method to determine the distances to clusters, while Curtis did not believe this to be an accurate method of measurement.
Immanuel Kant: In the late 18th century, Immanuel Kant speculated that the Milky Way consisted of a huge number of stars all rotating a common center. One of those stars as our very own Sun. These stars are held together in orbits around a common center by strong gravitational forces. Kant is correct in all of his hypotheses.
Henrietta Leavitt: Leavitt discovered the Period-luminosity relationship for Cepheid variable stars that allows astronomers to determine the distances of stars. Shapley used this method to determine the distances of globular clusters near the center of the Milky Way. Refer to Henrietta Leavitt Biography entry in this blog for more information.
The Great Debate: The Great Debate took place in 1920 and was officially dubbed: Shapley v. Curtis and the Scale of the Universe. The main questions they discussed were: "What is the nature of the nebulae?," "What is the size of our Galaxy?," and "Is the Sun in the center of the Galaxy?" Shapley believed that the diameter of our Galaxy was 300,000 light-years and that the Sun was not at the Galaxy's center but 60,000 light-years away. He also believed that the Milky Way was so large, it was the entire universe and that spiral nebulae were gaseous clouds repelled by the Milky Way's light pressure. Curtis believed that the diameter of the Galaxy was 30,000 light-years (ten times smaller than Shapley's prediction). He also believed that the sun was very close or at the center of the Galaxy and that spiral nebulae were galaxies (island universes). There was no clear winner in the debate because both were correct on one major point and incorrect on another. Both were incorrect in saying that interstellar absorption of starlight by dust is unimportant.
