On this day in 1925 - January 1st - Edward Hubble read to the American Astronomical Society what would be a ground-breaking paper.

He showed that the Andromeda "nebula" wasn't an object in the Milky Way, but a galaxy in its own right. Here is picture of the glass plate on which Hubble discovered a Cepheid variable - marked VAR! - in the Andromeda galaxy. From this he determined that it was too far away to be located in our own galaxy.