( ( ( ( stars ( are ( born ( in ( clouds ( of gas ) ) ) ) ) ) and ) ( dust ( called ( ( nebulae stars ) ( are ( clustered ( into ( groups ( of ( billions ( of ( stars ( called galaxies ) ) ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NNS stars)) (VP (VBP are) (VP (VBN born) (PP (IN in) (NP (NP (NNS clouds)) (PP (IN of) (NP (NN gas)))))))) (CC and) (S (NP (NN dust)) (VP (VBD called) (SBAR (S (NP (JJ nebulae) (NNS stars)) (VP (VBP are) (VP (VBN clustered) (PP (IN into) (NP (NP (NNS groups)) (PP (IN of) (NP (NP (NNS billions)) (PP (IN of) (NP (NP (NNS stars)) (VP (VBN called) (NP (NNS galaxies))))))))))))))) (. .)))
stars are born in clouds of gas and dust called nebulae stars are clustered into groups of billions of stars called 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))))))))) (. .)))