( Rather ( , ( they ( ( occur ( in ( clusters ( star ( ( clusters and ) galaxies ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Rather)) (, ,) (NP (PRP they)) (VP (VBP occur) (PP (IN in) (NP (JJ clusters) (NN star) (NNS clusters) (CC and) (NNS galaxies)))) (. .)))
Rather, they occur in clusters star clusters and 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))))))))) (. .)))