( ( As ( ( ( the ( star 's ) ) ( helium content ) ) ( builds up ) ) ) ( , ( ( the core ) ( ( ( ( contracts and ) releases ) ( ( ( gravitational energy ) , ) ( which ( ( ( ( heats up ) ( the core ) ) and ) ( increases ( ( the rate ) ( of ( hydrogen consumption ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (NP (DT the) (NN star) (POS 's)) (NNP helium) (NN content)) (VP (VBZ builds) (PRT (RP up))))) (, ,) (NP (DT the) (NN core)) (VP (VBZ contracts) (CC and) (VBZ releases) (NP (NP (JJ gravitational) (NN energy)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VP (VBZ heats) (PRT (RP up)) (NP (DT the) (NN core))) (CC and) (VP (VBZ increases) (NP (NP (DT the) (NN rate)) (PP (IN of) (NP (NN hydrogen) (NN consumption)))))))))) (. .)))
As the star's helium content builds up, the core contracts and releases gravitational energy, which heats up the core and increases the rate of hydrogen consumption.
(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)) (. .)))