Astronomer Edwin Hubble was born #OTD in 1889.
Hubble is best known for combining galactic distance measurements with Slipher’s redshift data to obtain a linear relation between distance and recession velocity. This is now understood as evidence for an expanding universe.
Before that, Hubble used Henrietta Swan-Leavitt's period-luminosity relation for Cepheid variable stars to establish spiral nebulae as separate galaxies situated outside the Milky Way.
In 1923, Hubble took a photographic plate of the spiral nebula M31. It showed what he at first thought was a nova. Over subsequent observations he realized it was actually a Cepheid variable star. He crossed out “N” and excitedly wrote “VAR!”
Using Henrietta Swan-Leavitt's relationship between the period and brightness of Cepheids, Hubble established that M31 was situated outside the Milky Way. Spiral nebula were in fact distinct galaxies.
IMAGE: Carnegie Observatories
Robert McNees
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