astrophotography - Open source alternative for RegiStar (align, rotate, and combine images into single frame)

I'm looking for a free, open-source and Linux compatible alternative to the RegiStar software.

I need it to automatically combine several images into a single frame, where each individual image shows a portion of a large observed region.

Individual images also overlap by some amount, and need to be aligned and possibly rotated before superimposing them onto the final frame.

I believe RegiStar does all this but: a- it is not free, b- it is not open-source, and c- it seems to run on Windows only.