Edge of the frame. Since stars are point sources of light any distortion from the optics, trailing from a long exposure, or even the atmosphere will distort them. IMO the center stars are what I’d consider acceptable for an untracked image, but then again I don’t really do much widefield astrophotography. It could also be your camera shifted slightly or from walking around near it?

There’s a number of YouTube channels of varying quality/expertise that are good for beginners, like AstroBackyard, Alaskan astro, cuiv the lazy geek to name a few. The cloudynights forum should If your a discord person, we do have one for this community (.gg/astrophotography) that’s pretty active with a lot of info that’s been gathered over the years, and it’s where I’ve learned most of what I know about the hobby.