Edwin Hubble Discovers the Universe

Explanation: How big is our universe? This question, among others , was debated by two leading astronomers in 1920 in what has since become known as astronomy's Great Debate . Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many . In the Great Debate , each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula , as shown on the original glass discovery plate digitally reproduced here . When Edwin Hubble compared images, he noticed that this spot varied , and on October 6, 1923 wrote “VAR!” on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. Annotated 100 years ago, the featured image may not be pretty, but the variable spot on it opened a window through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos .