( It ( ( is ( ( caused ( by ( ( the buildup ) ( on ( ( ( a ( white ( dwarf 's ) ) ) surface ) ( of ( ( hydrogen gas ) ( from ( a ( companion star ) ) ) ) ) ) ) ) ) ) ( to ( ( the point ) ( where ( ( the hydrogen ) ( ( fuses explosively ) ( into helium ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (NP (NP (DT the) (NN buildup)) (PP (IN on) (NP (NP (NP (DT a) (JJ white) (NN dwarf) (POS 's)) (NN surface)) (PP (IN of) (NP (NP (NN hydrogen) (NN gas)) (PP (IN from) (NP (DT a) (NN companion) (NN star))))))))) (PP (TO to) (NP (NP (DT the) (NN point)) (SBAR (WHADVP (WRB where)) (S (NP (DT the) (NN hydrogen)) (VP (VBZ fuses) (ADVP (RB explosively)) (PP (IN into) (NP (NNP helium)))))))))) (. .)))
It is caused by the buildup on a white dwarf's surface of hydrogen gas from a companion star to the point where the hydrogen fuses explosively into helium.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NNP helium)) (VP (VBZ is) (VP (VBN gone))))) (, ,) (NP (DT the) (NNS stars)) (VP (VBP become) (NP (JJ white) (NNS dwarfs))) (. .)))