In 1923, Edwin Hubble used the 100-inch Hooker Telescope at Mount Wilson to identify a Cepheid variable star on a photographic plate of the Andromeda nebula. By measuring the star's period, he found it was far dimmer than it should be if it were inside the Milky Way, meaning it had to lie far outside our galaxy. This showed that Andromeda was a separate galaxy entirely, establishing that the Milky Way is not the whole universe and that separate island universes exist.
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The famous plate showing the variable star, marked with a 'V,' is held at the Carnegie Institution.
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44:15>> 1923 discovery of the nature of galaxies, which is so incredible to me that this was so recent. Yeah, this is again the idea that the Milky Way is not the universe, >> right? >> That was a big deal. Now we know there's so many galaxies. Edwin Hubble used the 100-in hooker telescope at Mount Wilson to establish that. What he did was he he found a sephiid variable. You know that thing that Henrietta was talking about? He found that same type of star in a plate on Andromeda. There's a very famous pate that plate that's out there in Carnegie Institution where he's got a little marker that says V. It's a variable star.
44:55>> Not video assistant referee. >> Yeah. Yeah. Exactly. He found the variable star. He measured its period and he was like given this period, this star is way dimmer than it should be, >> which means that it's way outside of our galaxy. And he established that the Andromeda is a separate galaxy from the Milky Way. Very big deal. separate island universes as a concept. 1924