Galaxies

There are lots of ways to think of galaxies. We might look at them as cosmic cities, each with a densely populated “downtown” and more thinly settled suburbs. More poetically, we might think of them as “island universes” — collections of stars, gas, and all the other stuff that makes up the universe, all in one big, bright, beautiful package.
However you look at them, galaxies are the biggest individual objects in the universe. They contain anywhere from a few million stars to trillions. And they span anywhere from a few thousand light-years to a million or more.
These giant agglomerations are dynamic — they grow and evolve. Their clouds of gas and dust collapse to give birth to stars, for example. That can happen steadily, like the drip of a leaky faucet — just what’s happening now in our home galaxy, the Milky Way. Or it can happen in a hurry, like the rush of a fire hose — usually when two galaxies interact with each other — passing close to each other, or even merging.
In fact, big