( ( ( The ( sun 's ) ) ( nuclear ( fusion reactions ) ) ) ( only ( ( ( ( happen ( in ( the ( innermost core ) ) ) ) , ) ( where ( ( the ( ( pressure and ) temperature ) ) ( are ( ( high enough ) ( to ( ( fuse hydrogen ) ( into helium ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NP (DT The) (NN sun) (POS 's)) (JJ nuclear) (NN fusion) (NNS reactions)) (ADVP (RB only)) (VP (VBP happen) (PP (IN in) (NP (DT the) (JJ innermost) (NN core))) (, ,) (SBAR (WHADVP (WRB where)) (S (NP (DT the) (NN pressure) (CC and) (NN temperature)) (VP (VBP are) (ADJP (JJ high) (RB enough) (S (VP (TO to) (VP (VB fuse) (NP (NN hydrogen)) (PP (IN into) (NP (NNP helium))))))))))) (. .)))
The sun's nuclear fusion reactions only happen in the innermost core, where the pressure and temperature are high enough to fuse hydrogen into helium.
(ROOT (S (NP (DT The) (NN core)) (VP (VBZ is) (NP (NP (DT the) (NN sun) (POS 's)) (NN innermost) (NN layer))) (. .)))