Please, please do not write sci-fi as if solar (or stellar) system is synonymous with galaxy!
“But they’re the same thing… right?”
*slightly despairing sigh*
Not even remotely. And really, this is the sort of thing you should have learned in high school (if not sooner), especially if you’re now writing science fiction with an outer-space setting and asking potential readers to pay you money for your stories.
Below, friends, is a diagram of the Solar system. (I capitalized Solar here because it’s derived from the proper name of our sun, Sol, and I wanted to make that fact clear. Technically, any other system of planets and whatnot around a star is a stellar system, because whatever their sun is, it isn’t Sol.) Notice how it contains a single star (some systems have more than one) with planets and stuff in orbit around it. Notice how the thing in the middle is a star, not a black hole. (That’s what set off this particular science rant: seeing a sci-fi writer claim that ‘we now know that there’s a black hole at the center of most solar systems.’ [emphasis added by me] *facepalm*)
Now take a look at this, an image of a galaxy (courtesy of NASA):
Notice how it’s not just one star (or two or three stars, very close together) with its own planets. Notice how it’s made of (as Carl Sagan would say) billions and billions of stars. And yes, it almost certainly has a black hole in its center. The difference in size alone… Our stellar system is a mere thirty-something light-hours across, and our galaxy (which is not the biggest that we know of) is so big that it takes light more than one hundred thousand years to go from one edge to the other.
How can anyone mistake one for the other?
Unfortunately necessary disclaimer/explanation: Yes, it is remotely possible for a stellar system to have a black hole at its center, if the star’s transition into a black hole somehow managed not to result in the destruction of its planets first, which is much more likely, since super-massive stars tend to eat their planets, if they have any, well before they go BOOM! and then collapse. (Contrary to what popular science fiction sometimes likes to use as a disaster — kinda literally, since the word means bad star — trope, a black hole has no more gravitic pull than did the star it used to be, so a star becoming a black hole doesn’t mean it’ll suddenly start pulling the entire universe into itself…) Do not take this to mean that most stellar (“solar”) systems have black holes in their centers, though, ’cause that’s ridiculous.