[innot]

This lead me to the following - what if you make a compost pile the size (mass) of Sun? Meaning, it won't be made of hydrogen, but rather some carbon-based molecules. I'm not sure about other atoms in these molecules, but I think carbon is stable enough not to initiate nuclear reactions. So probably fusion won't start. What then?

[thinkcontext]

Gravity would compress it. Depending on the exact composition fusion could start, ie it would become a star. According to Wikipedia carbon fusion requires a mass of 8 suns to start but there's a bunch of hydrogen in compost so maybe that would fuse.

[shadofx]

Carbon is capable of fusion, in a sufficiently compressed stellar mass. Everything up to Iron-56 can theoretically sustain fusion.

[jabl]

Maybe it could work via the CNO cycle: https://en.wikipedia.org/wiki/CNO_cycle ?