Edwin Hubble

Edwin Hubble was an American astronomer whose observations established that galaxies exist beyond the Milky Way and revealed a relationship between galaxy distance and recession speed.

Vesto Slipher: An American astronomer whose measurements of spiral nebulae provided early evidence of their large radial velocities. Slipher’s redshift measurements supplied much of the velocity data Hubble used.
Cepheid variable: A pulsating star whose period of brightness variation is related to its intrinsic luminosity. Hubble identified Cepheids in Andromeda and used them to estimate its distance.
Hooker Telescope: A 100-inch reflecting telescope installed at Mount Wilson Observatory in 1917. Its resolving power made Hubble’s observations of individual stars in Andromeda possible.
Expanding universe: A cosmological model in which the universe’s large-scale distances increase over time. Hubble’s distance–velocity relation became key observational evidence for cosmic expansion.
Harlow Shapley: An American astronomer known for measuring the Milky Way’s size and locating the Sun away from its center. Shapley argued that spiral nebulae lay within the Milky Way, opposing Hubble’s extragalactic interpretation.
Period–luminosity relation: A relation connecting a pulsating star’s variation period to its intrinsic luminosity. It turned Hubble’s observed Cepheid periods into distance estimates.
Great Debate (astronomy): A 1920 debate between Harlow Shapley and Heber Curtis over the scale of the universe. The debate framed the question Hubble’s distance measurements later settled.
Big Bang: The cosmological model that describes the universe evolving from an early hot, dense state. The expansion measured by Hubble became part of the evidence for an evolving universe.
Milton Humason: An American astronomer whose observations of faint galaxies yielded recession-speed measurements. Humason extended the velocity measurements that made Hubble’s distance relation possible.
Andromeda Galaxy: A large spiral galaxy about 2.5 million light-years from the Milky Way. Cepheids observed there showed that Andromeda lay far outside the Milky Way.

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