Hubble establishes Andromeda as an external galaxy
Hubble establishes Andromeda as an external galaxy
Overview
In 1925 Edwin Hubble announced the identification of Cepheid variables in the Andromeda Nebula using the 100-inch Hooker telescope at Mount Wilson. Applying Leavitt's period–luminosity relation, he showed Andromeda lies roughly a million light-years away — far beyond any plausible boundary of the Milky Way. The discovery established that the universe is populated by countless galaxies, expanding the known cosmos by orders of magnitude in a single step.