( Their ( ( are ( ( ( ( ( ( ( double stars ) , ) ( star clusters ) ) , ) nebulae ) and ) galaxies ) ) . ) )
(ROOT (S (NP (PRP$ Their)) (VP (VBP are) (NP (NP (JJ double) (NNS stars)) (, ,) (NP (NN star) (NNS clusters)) (, ,) (NP (NN nebulae)) (CC and) (NP (NNS galaxies)))) (. .)))
Their are double stars, star clusters, nebulae 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))))))))) (. .)))