( ( CLUSTER ( A cluster ) ) ( ( is ( ( a group ) ( of ( ( stars or ) galaxies ) ) ) ) . ) )
(ROOT (S (NP (JJ CLUSTER) (DT A) (NN cluster)) (VP (VBZ is) (NP (NP (DT a) (NN group)) (PP (IN of) (NP (NNS stars) (CC or) (NNS galaxies))))) (. .)))
CLUSTER A cluster is a group of stars or galaxies .
(ROOT (S (NP (DT A) (NN galaxy)) (VP (VBZ is) (VP (RBS best) (VBN described) (PP (IN as) (NP (NP (DT a) (NN cluster)) (PP (IN of) (NP (NP (NNS billions)) (PP (IN of) (NP (NNS stars))))))))) (. .)))