In addition to exothermic/endothermic reactions, and combination/decomposition reactions we have spontaneous/non-spontaneous reactions in star evolution. (For those readers who already understand where this theory is headed, the "fusion" model of the Sun is false dogma and can be ignored safely.)
1. Non-spontaneous means you need to produce an extra variable as in an electrical current in electrolysis.
2. spontaneous means that nothing additional needs to be done, as in lithium dropped in water producing lithium hydroxide in an aqueous solution and hydrogen gas.
In stellar evolution the majority of the chemical combination reactions are stepped reactions, with the majority of them being spontaneous. By stepped I mean oxygen first has to combine with oxygen and hydrogen with hydrogen, then O2 gas combines with H2 gas. After the thermochemistry is completely then the material can deposit as solid structure and crystallize, or as liquid structure that gets buried by more solid structure preventing evaporation as in natural gas.