Once the galaxies get close enough, the actual merger takes millions of years. But the process is quite common. When galaxies merge, they create new, bigger galaxies. For example, when two spiral galaxies come together, they can create an elliptical galaxy. One day in the distant future, some 4.5 billion years from now, the Milky Way and our nearby neighbor, the Andromeda galaxy, will likely merge as well.
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