Question: how does a galaxy evolve?

Nicolas Bonne answered on 20 Jun 2019:

There are lots of things that can change a galaxy. The two really big things are changes in colour (this has to do with whether a galaxy is making any new stars, blue stars are new, red stars are usually old), or changes in shape (usually because a galaxy has passed too close (or even straight through!) another galaxy and the gravity from both has pulled some of their stars and gas in odd directions