[MUSIC PLAYING] Are all galaxies the same? We live on a planet in a single solar system, inside a large galaxy made up of a hundred billion stars. A supermassive black hole equal to the weight of a few million suns anchors the center of our galaxy. Our universe has hundreds of billions of galaxies, each containing stars like our sun. 

Galaxies range in size from ones with 1,000 stars to ones with a trillion or more stars. That means we have a lot of places to explore. Edwin Hubble was one of the first astronomers to discover that our universe isn't just sitting still, but it's expanding. 

Hubble also grouped galaxies by how they looked because not all galaxies are the same. He described a few distinct types. Elliptical galaxies, which have no easily defined shape with stars that move chaotically. And spirals or disk galaxies, which are flat with pinwheel spiral arms that spread out from the center. 

As Hubble began to classify galaxies, he quickly discovered many galaxies didn't fit neatly into his categories. Was it possible that galaxies change from one type to another over eons? Even our own Milky Way galaxy has been changing. 

There are a handful of smaller galaxies, such as the Magellanic clouds, Fornax, Sculptor, Leo and others that orbit around us. The powerful gravity of the Milky Way affects these nearby galaxies, either causing them to be ripped apart, or pulling them into merge with ours. 

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