<Ul> <Li> 1918 -- Harlow Shapley demonstrates that globular clusters are arranged in a spheroid or halo whose center is not the Earth , and decides , correctly , that its center is the Galactic Center of the galaxy , </Li> <Li> 26 April 1920 -- Harlow Shapley and Heber Curtis debate whether or not the Andromeda Nebula is within the Milky Way , </Li> <Li> 1923 -- Edwin Hubble resolves the Shapley -- Curtis debate by finding Cepheids in the Andromeda Galaxy </Li> <Li> 1930 -- Robert Trumpler uses open cluster observations to quantify the absorption of light by interstellar dust in the galactic plane ; this absorption had plagued earlier models of the Milky Way , </Li> <Li> 1932 -- Karl Guthe Jansky discovers radio noise from the center of the Milky Way , </Li> <Li> 1933 -- Fritz Zwicky applies the virial theorem to the Coma Cluster and obtains evidence for unseen mass , </Li> <Li> 1936 -- Edwin Hubble introduces the spiral , barred spiral , elliptical , and irregular galaxy classifications , </Li> <Li> 1939 -- Grote Reber discovers the radio source Cygnus A , </Li> <Li> 1943 -- Carl Keenan Seyfert identifies six spiral galaxies with unusually broad emission lines , named Seyfert galaxies , </Li> <Li> 1949 -- J.G. Bolton , G.J. Stanley , and O.B. Slee identify NGC 4486 ( M87 ) and NGC 5128 as extragalactic radio sources , </Li> </Ul>
<Ul> <Li> 1918 -- Harlow Shapley demonstrates that globular clusters are arranged in a spheroid or halo whose center is not the Earth , and decides , correctly , that its center is the Galactic Center of the galaxy , </Li> <Li> 26 April 1920 -- Harlow Shapley and Heber Curtis debate whether or not the Andromeda Nebula is within the Milky Way , </Li> <Li> 1923 -- Edwin Hubble resolves the Shapley -- Curtis debate by finding Cepheids in the Andromeda Galaxy </Li> <Li> 1930 -- Robert Trumpler uses open cluster observations to quantify the absorption of light by interstellar dust in the galactic plane ; this absorption had plagued earlier models of the Milky Way , </Li> <Li> 1932 -- Karl Guthe Jansky discovers radio noise from the center of the Milky Way , </Li> <Li> 1933 -- Fritz Zwicky applies the virial theorem to the Coma Cluster and obtains evidence for unseen mass , </Li> <Li> 1936 -- Edwin Hubble introduces the spiral , barred spiral , elliptical , and irregular galaxy classifications , </Li> <Li> 1939 -- Grote Reber discovers the radio source Cygnus A , </Li> <Li> 1943 -- Carl Keenan Seyfert identifies six spiral galaxies with unusually broad emission lines , named Seyfert galaxies , </Li> <Li> 1949 -- J.G. Bolton , G.J. Stanley , and O.B. Slee identify NGC 4486 ( M87 ) and NGC 5128 as extragalactic radio sources , </Li> </Ul>
when were galaxies discovered to be large separate structures in the universe
when were galaxies discovered to be large separate structures in the universe
1923
1923