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When galaxies collide, they don't actually smash into each other; they swirl around each other and eventually become a single galaxy.

Galaxies collide very often, although they take millions of years to do so. In fact now scientists believe that collisions and merges between multiple galaxies make up some of the main elements causing evolution as time passes.

Most galaxies in the universe are made of two or more other galaxies that have collided together.

A galaxy is made of about 40 to 250 billion stars, although our galaxy, the Milky Way, has many more. So you might think that during a collision, numerous collisions will occur between a galaxy's stars, right? But actually the chance of any two stars colliding in the entire two galaxies is almost a straight zero. This is because even though, as I said, there are hundreds of billions of stars in a galaxy, about 99.9995% of material that makes up a galaxy is not made up of stars. That gives you another really big hint of about how big the universe really is.

However, this seems false when colliding galaxies are observed through a telescope. This is because the 99.9995% of space in a galaxy not consisting of stars consist of gases and dust. When the galaxies collide, this material will do so by gravity, the same way we get pulled down to Earth. There also is lots of friction in both of the colliding galaxies, causing shock waves that can trigger star formation in the galaxies that make the collision look astonishing.

These processes can radically affect the galaxies. For example, two spiral galaxies, like our home galaxy-The Milky Way-- can collide to form an elliptical galaxy. As I said, it takes millions of years for galaxies to collide, but that's really short when it comes to astronomy! For More, copy and paste this in the web address bar: http://curious.astro.cornell.edu/question.php?number=351
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15y ago

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