[MUSIC PLAYING] What is a galaxy? Until the 1920s, we only knew of our own galaxy, the Milky Way. Even with telescopes, we could barely see little fuzzy spiral shapes in the sky, and we wondered what exactly they were. Dusty nebulas or other galaxies entirely? 

There was a great debate about the issue. Ultimately, Henrietta Swan Leavitt's work on distances to stars enabled Edwin Hubble to prove that the largest fuzzy spiral shape in the sky was actually a galaxy entirely separate from our own, the Andromeda Galaxy. So cool. It was the beginning of our discovering and grasping the vastness of space. 

Since Andromeda, we've learned there are billions more galaxies outside of the Milky Way. Galaxies are concentrations of stars, gas, dust, and dark matter, all held together by gravity. Many galaxies are spiral galaxies, like the Milky Way. But there are also elliptical and irregular galaxies. As far as we know, all large galaxies have a supermassive black hole at their centers. 

We also know that galaxies interact with each other, creating beautiful new forms over millions of years. This is true for Andromeda and the Milky Way too. Our galaxy is on a collision course with our nearest neighbor. While that may seem violent to you and me, galaxies are so huge that most stars will pass right by each other. Phew. 

Eventually, the Milky Way and Andromeda will merge into one giant new elliptical galaxy. It's not for another several billion years, but it's sure to be a spectacular show for the stargazers of the distant future. 

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