The discovery of other galaxies!

 It was on December 30, 1924 when astronomer Edwin Hubble announced the discovery of other galaxies. At the time, it was thought that our Milky Way galaxy represented the entirety of the universe. Hubble was studying the Andromeda Nebula using the 100-inch Hooker Telescope at Mount Wilson observatory in California. With a weaker telescope, 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 distinguish individual 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. Astronomers 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 crunched the numbers and realized that the star he was observing was 800,000 light-years away, 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 went on to discover 23 more separate galaxies. His findings proved that, unimaginably vast though it seemed to us, our Milky Way was just one of many little islands of stars.

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