what will happen if/when another galaxy collides with ours
On a large scale, it’s going to look a bit like stirring ice cream or another viscous liquid: https://www.youtube.com/watch?v=J_UwUuJFT3Q
On the solar system or planet scale, the night sky will change a lot (although still veeeery slowly). For the planet itself, basically nothing will change. Space between stars is so extremely empty, something solid actually hitting something else is astronomically unlikely.
“Collides” is a misnomer, galaxies are extremely empty, all considered. They will fly though each other, but gravity makes their objects interact.
Galaxies are often flat(ish) disks of stars and their surrounding stuff orbiting around a common center. A collision won’t do much to most individual star systems, as encountering another star very close (say, within one light year) is pretty unlikely. But the merger will throw many stars off their path around the galactic center. What then happens depends a lot on the details. The star might end up in the other galaxy, or fly off, or even end up in its initial galaxy but somewhere else.
On a large frame, the galaxies might pass through each other (and exchange stars and stuff), or they might merge into a larger galaxy. That new galaxy might just be an ovoid blob, or a smaller one might become a kind of ring around another ([depiction of how that looks for our galaxy right now](https://qph.cf2.quoracdn.net/main-qimg-ba258d067778267179b55ea381263508-lq)), they might create a [wheel](https://upload.wikimedia.org/wikipedia/commons/f/f0/Cartwheel_Galaxy_JWST_NIRCam%2BMIRI_Full_Res.png), or [many](https://cdn.sci.news/images/enlarge7/image_8308_1e-TXS-2116-077.jpg) [other](https://www.nao.ac.jp/en/contents/news/science/2019/20190618-alma-fig.jpg) [options](https://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/Cosmic_%E2%80%9Cflying_V%E2%80%9D_of_merging_galaxies.tif/lossy-page1-946px-Cosmic_%E2%80%9Cflying_V%E2%80%9D_of_merging_galaxies.tif.jpg).
When not if.
The Andromeda Galaxy will collide with our in about 4.5 billion years.
For comparison 4.5 billion years is about how old the Earth is.
Our Earth will still be there in 4.5 billion years, but life will no longer be possible on it, it will simply be too hot for that.
Still if there was anyone alive in the solar system at that point they would not notice much. The whole process is likely to take a really long time. As the galaxies flow into one another and merge into a bigger one.
Galaxies are mostly made up out of nothing, so despite there being 100s of billions of solar systems involved in the crash, the chance of any stars actually colliding is extremely low.
A few stars might get flung out of the newly formed galaxy, but that will be so slow that anyone on a planet in one such system will not notice too much of it.
There is a good chance that some existing stuff may happen when the black holes in the center of the two galaxies start interact with each other. if they merge that will certainly be something energetic enough that you wouldn’t want to be anywhere near that.
Out here in the our spiral arm we will probably be safe, but the collision might end with our sun closer to the core than we are now, which would be a problem.
But stated earlier Earth will be a dead planet by then anyway.
effectively…nothing worth noting. there is simply too much space in between stars to the point where collisions are a footnote at worst.
the night sky definetely gets more interesting tho.
Not a whole lot. In fact, statistically, you’d never even notice.
There’s so much space between stars, any of them coming close enough to interact with each other would be quite rare