( ( As ( ( ( ( ( the ( star 's ) ) ( hydrogen supply ) ) ( begins ( to ( run out ) ) ) ) and ) ( ( its ( energy output ) ) falters ) ) ) ( , ( ( the star ) ( ( begins ( to contract ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (S (NP (NP (DT the) (NN star) (POS 's)) (NN hydrogen) (NN supply)) (VP (VBZ begins) (S (VP (TO to) (VP (VB run) (PRT (RP out))))))) (CC and) (S (NP (PRP$ its) (NN energy) (NN output)) (VP (VBZ falters))))) (, ,) (NP (DT the) (NN star)) (VP (VBZ begins) (PP (TO to) (NP (NN contract)))) (. .)))
As the star's hydrogen supply begins to run out and its energy output falters, the star begins to contract.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (DT the) (NN core)) (PP (IN of) (NP (DT a) (NN star)))) (VP (VBZ begins) (S (VP (TO to) (VP (VB run) (ADJP (JJ low)) (PP (IN on) (NP (NN hydrogen))))))))) (, ,) (NP (PRP it)) (VP (VBZ contracts)) (. .)))