( It ( ( ( becomes , ) ( not ( ( ( ( a ( white dwarf ) ) , ) but ) ( a ( neutron star ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VP (VBZ becomes)) (, ,) (NP (RB not) (NP (DT a) (JJ white) (NN dwarf)) (, ,) (CC but) (NP (DT a) (NN neutron) (NN star)))) (. .)))
It becomes, not a white dwarf, but a neutron star.
(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))) (. .)))