The pattern is most likely caused by very slight differences between frames being amplified by stacking, not by the stars themselves. Two likely contributors from the answers are:

1. Lens correction or other preprocessing changed the scale/geometry of some frames slightly before stacking.
2. Fine sensor/demosaicing detail (from the Bayer pattern) became visible as a low-frequency “beat” pattern when nearly identical images were overlaid with tiny scaling differences.

To avoid it:

• Stack frames that are processed identically and avoid any per-frame geometric changes before stacking.
• If you use lens correction, apply it after the stack, not before.
• Minimize resizing, warping, or mixing differently rendered versions of the same image.
• If testing, try a different focal length or lens to see whether the issue is tied to a particular correction profile or optical setting.

In short: the moiré is usually a stacking/alignment/rendering artifact caused by slight frame-to-frame scaling differences, often introduced during raw processing or lens correction.