( ( A ( neutron star ) ) ( ( is ( what ( ( remains ( after ( a ( supernova explosion ) ) ) ) ( when ( ( ( the core ) ( of ( material ( at ( ( the center ) ( of ( a star ) ) ) ) ) ) ) ( collapses ( into ( ( a ( dense mass ) ) ( of neutrons ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN neutron) (NN star)) (VP (VBZ is) (SBAR (WHNP (WP what)) (S (VP (VBZ remains) (PP (IN after) (NP (DT a) (NN supernova) (NN explosion))) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN core)) (PP (IN of) (NP (NP (NN material)) (PP (IN at) (NP (NP (DT the) (NN center)) (PP (IN of) (NP (DT a) (NN star)))))))) (VP (VBZ collapses) (PP (IN into) (NP (NP (DT a) (JJ dense) (NN mass)) (PP (IN of) (NP (NNS neutrons)))))))))))) (. .)))
A neutron star is what remains after a supernova explosion when the core of material at the center of a star collapses into a dense mass of neutrons.
(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))))) (. .)))