Probably exactly that. If you take a single 10 minute exposure (or really, anything more than a few seconds) you'll get noticeable star trails if you don't put your camera on a rotating mount. Stacking multiple exposures also has other nice benefits such as noise canceling itself out and being able to remove satellite trails.

Last time I did astrophotography was a few years ago, before Starlink made the problem considerably worse, but satellite trails were relatively easy to remove with stacking. I'm sure it's harder now but definitely still possible, so I'm assuming in this case leaving them in was done on purpose to highlight the problem.

EDIT: Looking better at the picture, I belive this was taken with a star tracker and then composited with a shorter exposure of the foreground. Notice how the foreground, even far away, looks considerably blurrier than the stars, and how the tower in the background has some light streaks. This is exactly what you'll see if you use a star tracker. Rather than star trails, you'll have "foreground trails". This would explain why there are relatively few gaps in the satellite trails, since the exposures can be much longer.

My time to shine! I've spent yesterday morning to track the photo down and answer this question. The APOD description is lacking. Yes, this was an exaggerated stack of 153 four-second exposures (the rejection map of the satellite trails was added on top of the image), and the gaps happened when the camera took its time to save between two exposures.

Here is a link to the original photo and it's description (German) by Uli Fehr: https://www.facebook.com/groups/Nachtfotografie/posts/264063...

My guess is the camera itself was taking photos of shorter exposure and the final image was composed in post-production, yes.

I am guessing, but I think it likely has to do with the shape and orientation of the satellite with respect to the sun and the camera. Depending on the relative positions, the brightness reflected off the satellite and reaching the camera will change over time.

I've taken long exposures using film (analog, so no stacking or any other funny business) and saw the same thing. I always thought they were planes but now it seems they may have been satellites. I'm curious if someone knows why this happens

I'm not aware of a digital camera that can take a 10 minute continuous exposure, but maybe there are special astronomy cameras that can?

Maybe, but also a lot of satellites rotate and so their brightness changes over time.

Passing clouds?

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