( ( ( ( ( ( When ( ( the star ) ( exhausts ( ( the ( hydrogen fuel ) ) ( in ( its core ) ) ) ) ) ) ( , ( ( nuclear reactions ) ( ( can ( no longer ) ) continue ) ) ) ) and ) so ) ( ( the core ) ( begins ( to ( contract ( due ( to ( its ( own gravity ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NN star)) (VP (VBZ exhausts) (NP (NP (DT the) (NN hydrogen) (NN fuel)) (PP (IN in) (NP (PRP$ its) (NN core))))))) (, ,) (NP (JJ nuclear) (NNS reactions)) (VP (MD can) (ADVP (RB no) (RB longer)) (VP (VB continue)))) (CC and) (RB so) (S (NP (DT the) (NN core)) (VP (VBZ begins) (S (VP (TO to) (VP (VB contract) (ADJP (JJ due) (PP (TO to) (NP (PRP$ its) (JJ own) (NN gravity))))))))) (. .)))
When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its own gravity.
(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)) (. .)))