2016-12-30. Astronomer Edwin Hubble announced the discovery of other galaxies beyond the Milky Way on this date in 1924.

posted Jan 4, 2017, 1:09 PM by Alan Gould

By Writer's Almanac. For GSS A Changing Cosmos chapter 4. Excerpt: ...Before he made his discovery, everyone thought that our Milky Way galaxy was the only galaxy in the universe, and that there wasn’t much outside it besides the Magellanic Clouds, which are visible by the naked eye in the Southern Hemisphere, and which were thought to be clouds of gas or dust. ...Hubble first published his discovery in a paper called “Extragalactic Nature of Spiral Nebulae,” which was presented on this date, in his absence, at a joint meeting of the American Astronomical Society and the American Association for the Advancement of Science. ...Hubble had written his doctoral dissertation on “Photographic Investigations of Faint Nebulae.” With older or smaller telescopes, nebulae just looked like clouds of glowing gas, but with the Hooker telescope — the most powerful telescope in the world at that time — Hubble was able to see that there were actually stars within the nebula. One of the stars in the Andromeda Nebula turned out to be a Cepheid variable: a particular type of star that pulsates and is very bright. A Harvard computationist named Henrietta Leavitt had figured out a decade earlier that, by observing a Cepheid variable and measuring its brightness and the length of time it takes to go from bright to dim and back again, they could calculate the star’s distance from the Earth. Hubble did the math and realized, to his amazement, that the star he was observing — which he called V1, or “variable number 1” — was almost 900,000 light years away. That’s more than eight times the distance of the farthest star in the Milky Way. It was then that he realized that the cloud of gas he’d been observing was really another vast galaxy that was very far away. He renamed the Andromeda Nebula the “Andromeda galaxy,” and he went on to discover 23 more separate galaxies. Within a few years of Hubble’s discovery, most astronomers came to agree that our galaxy is just one of millions. Methods to measure astronomical distance have gotten more precise, and it’s now estimated that the Andromeda galaxy is 2 million light years from Earth....  http://writersalmanac.org/episodes/20161230/