Edwin Hubble proved that many objects previously thought to be clouds of dust and gas and classified as “nebulae” were actually galaxies beyond the Milky Way.

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. Using the Hooker Telescope at Mount Wilson, Hubble identified Cepheid variables, a standard candle discovered in 1908 by Henrietta Leavitt. Comparing their apparent luminosity to their intrinsic luminosity gives their distance from Earth. Hubble found Cepheids in several nebulae, including the Andromeda Nebula and Triangulum Nebula.

His observations, made in 1923, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own; thus today they are no longer considered nebulae. This was first hypothesized as early as 1755 when Immanuel Kant‘s General History of Nature and Theory of the Heavens appeared. This hypothesis was opposed by many in the astronomy establishment of the time, in particular by Harvard University–based Harlow Shapley. Despite the opposition, Hubble, then a thirty-five-year-old scientist, had his findings first published in The New York Times on November 23, 1924, then presented them to other astronomers at the January 1, 1925, meeting of the American Astronomical Society. Hubble’s results for Andromeda were not formally published in a peer-reviewed scientific journal until 1929.

Agenda – Edwin Hubble identifies Cepheid variable star

On the α/Fe bimodality of the M31 disks

DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor

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