( ( 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 (NN Neutron) (NNS stars)) (VP (VBP are) (NP (NP (DT the) (NNS corpses)) (PP (IN of) (NP (ADJP (VBN left) (RP over)) (NN supernova) (NNS explosions))))) (. .)))