The Short Answer: No, a galaxy isn’t a big collection of stars, dust, and millions of planets like our own, all neatly held together by gravity. When you look at stars at night, you’re actually seeing many other stars in the Milky Way, which are also in a galaxy, and sometimes one that’s much bigger than our own. If it’s really dark, way out in space, you can also see the smoky trails of the Milky Way stretching across the sky.
The Long Answer: Not only does the Milky Way encircle billions of stars, but it also holds billions of Planets as well. And when we use our very own galaxy to explain it, we come up with something that’s called a “super Massive Black Hole”. It’s a phenomenon where extremely hot matter fills an otherwise empty space, with nothing around to stop its expansion. This is similar to a magnet, where the presence of a large amount of iron will cause a change in magnetic field, which is what causes stars to evolve.
Image credit: NASA/JPL-Caltech/Rutgers University. Note: Please see my resource box for a list of further reading on this topic. I wrote this article to give you an introduction to the subject of how big our galaxy is (and the other big ones we know about too). And if you’d like a detailed answer and explanation of all this for a living planet (not just a star), then please read my other articles on the topic.