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1923 – Edwin Hubble resolves the Shapley–Curtis debate by finding Cepheids in the Andromeda Galaxy, definitively proving that there are other galaxies beyond the Milky Way. 1930 – Robert Trumpler uses open cluster observations to quantify the absorption of light by interstellar dust in the galactic plane ; this absorption had plagued ...
Edwin Hubble's arrival at Mount Wilson Observatory, California, in 1919 coincided roughly with the completion of the 100-inch (2.5 m) Hooker Telescope, then the world's largest. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way galaxy.
It was here, 100 years ago, that Edwin Hubble noted a light in the distance that would lead to one of science's greatest discoveries. By night, astronomers kept watch at the best telescopes on Earth.
1923 – Edwin Hubble measures distances to a few nearby spiral nebulae (galaxies), the Andromeda Galaxy (M31), Triangulum Galaxy (M33), and NGC 6822. The distances place them far outside the Milky Way, and implies that fainter galaxies are much more distant, and the universe is composed of many thousands of galaxies.
In 1927, by combining various measurements, including Hubble's distance measurements and Vesto Slipher's determinations of redshifts for these objects, Georges Lemaître was the first to estimate a constant of proportionality between galaxies' distances and what was termed their "recessional velocities", finding a value of about 600 km/s/Mpc.
A galaxy's recessional velocity is typically determined by measuring its redshift, a shift in the frequency of light emitted by the galaxy. The discovery of Hubble's law is attributed to work published by Edwin Hubble in 1929, [2] [3] [4] but the notion of the universe expanding at a calculable rate was first derived from general relativity ...
Later in the 1920s, Edwin Hubble showed that Andromeda was far outside the Milky Way by measuring Cepheid variable stars, proving that Curtis was correct. [6] It is now known that the Milky Way is only one of as many as an estimated 200 billion (2 × 10 11) [7] to 2 trillion (2 × 10 12) or more galaxies in the observable Universe.
Edwin Hubble observationally confirmed Lundmark's and Lemaître's findings in 1929. [12] Assuming the cosmological principle, these findings would imply that all galaxies are moving away from each other. Astronomer Walter Baade recalculated the size of the known universe in the 1940s, doubling the previous calculation made by Hubble in 1929.