Evolution of Galaxies

After galaxies and the first stars have formed, called Population III stars, rapid burning of Hydrogen fuel causes heavy elements, the first in the Universe in form. Then, when these stars die (most were massive enough to cause supernovas), heavier elements are dispersed into space, adding a slight amount of each to the next generation. Over the next two billion years, key structures are seen in the galaxy (see Types of Galaxies) and as more and more supernovas occur, the abundance of heavy elements slowly increases, eventually allowing the formation of solar systems. Then, as time goes on, formation of new stars slows. Sometimes, especially near the centers of galaxy clusters, two galaxies merge. This is how an elliptical galaxy forms.

This is a photo of two merging galaxies, collectively called NGC 4676.
Image provided by Wikipedia.