( ( When ( ( ( ( ( ( the ( hydrogen supply ) ) ( in ( the core ) ) ) ( begins ( to ( run out ) ) ) ) , ) and ) ( ( the star ) ( ( is ( no longer ) ) ( ( generating heat ) ( by ( nuclear fusion ) ) ) ) ) ) ) ( , ( ( the core ) ( ( ( ( becomes unstable ) and ) contracts ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (S (NP (NP (DT the) (NN hydrogen) (NN supply)) (PP (IN in) (NP (DT the) (NN core)))) (VP (VBZ begins) (S (VP (TO to) (VP (VB run) (PRT (RP out))))))) (, ,) (CC and) (S (NP (DT the) (NN star)) (VP (VBZ is) (ADVP (RB no) (RB longer)) (VP (VBG generating) (NP (NN heat)) (PP (IN by) (NP (JJ nuclear) (NN fusion)))))))) (, ,) (NP (DT the) (NN core)) (VP (VP (VBZ becomes) (ADJP (JJ unstable))) (CC and) (VP (VBZ contracts))) (. .)))
When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts.
(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)) (. .)))