One of the framed pictures I have hanging in my study is the 1923 photographic plate made with the Mt. Wilson Observatory's 100 inch telescope. Edwin Hubble was examining photographic plates of the Andromeda Nebula M31, looking for a novae.
On the night of October 5-6, 1923, Hubble located three novae, each marked with an "N” on this plate. Later he discovered that one was actually a Cepheid star - crossing out the "N" he wrote "Var!" (see upper right of the plate).
Harvard astronomer Henrietta Leavitt (who because of her gender was not allowed to actually use the telescope) provided one of the most important keys in astronomy discovering that Cepheids, regularly varying, pulsating stars, could be used as "standard candle" distance indicators, or in other words an objective gauge to measure distance.
So Hubble, by identifying such a star realized that Andromeda wasn’t a small cluster of stars and gas within our own galaxy, but a large galaxy in its own right at a substantial distance from the Milky Way. Right there and then, in that instant, mankind understood that the galaxy our star is in is just one galaxy in a universe filled with galaxies.
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