( ( A ( neutron star ) ) ( ( forms ( when ( ( ( a ( massive ( star 's ) ) ) ( iron core ) ) ( ( ( collapses and ) triggers ) ( a ( supernova explosion ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN neutron) (NN star)) (VP (VBZ forms) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT a) (JJ massive) (NN star) (POS 's)) (NN iron) (NN core)) (VP (VBZ collapses) (CC and) (VBZ triggers) (NP (DT a) (NN supernova) (NN explosion)))))) (. .)))
A neutron star forms when a massive star's iron core collapses and triggers a supernova explosion.
(ROOT (S (NP (NP (DT The) (NN core)) (PP (IN of) (NP (DT a) (NN star)))) (VP (VBZ collapses) (PP (IN during) (NP (DT a) (NN supernova))) (, ,) (S (VP (VBG forming) (NP (DT a) (NN neutron) (NN star))))) (. .)))