I doubt that anyone can answer that question via the internet. It might be fine, it might be serious. It has to be viewed in person. You could hire a structural engineer to inspect.

That plaster does not look like it is imminently going to fall. Have you done anything about it yet?

Hard to tell from the photos but we live in a1940 house that had cracking plaster on ceiling too. Has bugged me for years but husband says the house is just settling.

Plaster cracks are evidence that the keys that hold the plaster to the lathe in that area have broken. Those aren't bad cracks at all, but the forces of gravity and other banging and jarring might eventually break them loose. I highly recommend you buy a kit of Big Wally's Plaster Magic. It's a simple and effective way to fix them.

Plaster cracks. It cracks because of humidity changes, settling, etc.

You can generally knock on plaster with your knuckles and tell if it's solid, or hollow sounding. If it's solid, it's fine.

You can mud and tape plaster cracks with paper drywall tape - just like a drywall joint - and do a very a very good job. I've done it on many old houses. It both hides the crack and provides stability to the plaster if it's tight. It's obviously not going to support a large chunk of plaster that's ready to fall. I've also used drywall screws to help secure plaster that felt mostly secure - but I where wanted a little more assurance.

My house was built in the 1880's -90's and had all plaster walls when I bought it and the ceilings and walls were badly cracked. I never had an issue with the ceiling falling down. Over the years I ended up gutting it and taking down all of the old plaster and rewiring the electricity and insulating the walls correctly. It's a messy job but was totally worth it.