1. Harlow Shapley
1.1. Big Galaxy
1.1.1. Nebula M101
1.1.1.1. Adriaan Van Maanen determined that M101 must have a rotational period of 85 000 years.
1.1.1.2. Shapley plugged in a diameter of 100 000 light-years for it. Where it's circumference is around 300 000 light-years.
1.1.2. Shapley determined that the Milky Way to be 300 000 light-years in diameter.
1.1.2.1. Globular Clusters
1.1.2.1.1. These clusters had been estimated to be about 500 light-years in diameter.
1.1.2.1.2. Shapley then calculated that they are about 20 000 to 200 000 light-years distant.
1.1.3. The idea of the Big Galaxy is that it had enough room to hold every observable object.
1.1.4. Many stars that varied in their luminousity had been discovered in the 20th century. It was then called Capheid Variable Stars, in which their variability were studied and demontrated by Henrietta Leavitt in 1912.
1.1.4.1. Shapley further studied these stars and began using them to measure distances to globular clusters. Shapley had been calculating the distances to globular clusters one by one, in hopes to show that their distances all fit in the Milky Way
2. The one that ended the Great Debate
2.1. Edwin Hubble
2.1.1. Found Capheid Variable Stars in the Andromeda Nebula. With the distance of over 900 000 light-years.
2.1.1.1. With Hubble's and other astronomer's evidences. A concensus emerged that large nabulae were indeed Island Universes and galaxies on their own.
2.1.2. By the end of the decade, he concluded that the universe is expanding.
3. Heber D. Curtis
3.1. Island Universe
3.1.1. Nebulae
3.1.1.1. Unidentified class of fuzzy objects in space
3.1.1.2. As telescopes improved throughout the 1800s and revealed more detail of the nebulae, they were further clasified into 'spirals' and 'globular clusters' based on their appearance.
3.1.1.3. Some astronomers speculated that nebulae comprised many stars, while others proposed some kind of luminous matter
3.1.1.3.1. In 1840s, Lord Rosse used his giant telescope to discern individual stars within the nebulae.
3.1.1.3.2. Other astronomers argued that nebulae were made of stars based on an 1885 "nova" in the Andromeda Nebula.
3.1.1.3.3. In 1864, William Huggins attached a spectroscope to his telescope and aimed it at a nebula. He concluded that the nabulae is not an aggregation of stars, but a luminous gas.
3.1.1.3.4. John P. Nichol swore that the clouds were some sort of luminous fluid.
3.1.1.4. By 1900, over 100,000 nebulae had been detected. Astronomers began to study whether these nabulae are outside the Milky Way or within the Milky Way. They began focusing on the distances of the nebulaes.
3.1.1.4.1. Astronomer F.W. assumed that the Milky Way and Andromeda Nebula were of the same size, by 120 light-years in diameter. He concluded that the two were 3800 light-years apart by comparing their brightness.
3.1.1.4.2. Max Wolf argued that most nebulaes were about 1000 light-years in diameter.
3.1.2. Andromeda Nebula
3.1.2.1. William Wallace Campbell determined a correlation between the age of stars and their velocities. The older the star, the higher the velocity. With Slipher's help and observation, they've come up with a strong evidence that the Andromeda Nebulae is an Island Universe.
3.1.2.2. Vesto Slipher measured radial velocities of nebulae. He measured Andromeda Nebula to be moving toward the Milky Way at a rate of 300 kilometers per second.