( Galaxies ( Nebulae ( Star Clusters ) ) )
(ROOT (NP (NNP Galaxies) (NNP Nebulae) (NNP Star) (NNPS Clusters)))
Galaxies Nebulae Star Clusters
(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))))))))) (. .)))