Why do pictures of the Milky Way look like a spiral? When I see pictures of the Milky Way they show a spiral shape. Why is this?
The objects in a galaxy orbit it's center as a planet does with it's star, they keep the same distance. They look like a spiral for something that is called density waves that make some parts of the galaxy (the arms) more dense.

You have to imagine a star system like ours orbiting the galaxy in your picture following those radial lines, so sometime the system will be inside of and arm and sometimes outside of it.

Thanks to this extra density in the arms is where most of the star formation takes place.