( ( ( ( ( b -RRB- ) ( ( The age ) ( of ( ( the Galaxy ) ( is ( estimated ( to ( be ( ( ( 14-18 billion ) years ) ( from ( ( the rate ) ( of evolution ) ) ) ) ) ) ) ) ) ) ) ) ( of stars ) ) ( in ( ( ( globular clusters ) , ) ( which ( are ( thought ( to ( be ( ( the ( oldest stars ) ) ( in ( the Galaxy ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (NP (NP (LST (LS b) (-RRB- -RRB-)) (NP (DT The) (NN age)) (PP (IN of) (NP (NP (DT the) (NN Galaxy)) (SBAR (S (VP (VBZ is) (VP (VBN estimated) (S (VP (TO to) (VP (VB be) (NP (NP (QP (CD 14-18) (CD billion)) (NNS years)) (PP (IN from) (NP (NP (DT the) (NN rate)) (PP (IN of) (NP (NN evolution)))))))))))))))) (PP (IN of) (NP (NNS stars))) (PP (IN in) (NP (NP (NN globular) (NNS clusters)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBP are) (VP (VBN thought) (S (VP (TO to) (VP (VB be) (NP (NP (DT the) (JJS oldest) (NNS stars)) (PP (IN in) (NP (DT the) (NNP Galaxy))))))))))))) (. .)))
b) The age of the Galaxy is estimated to be 14-18 billion years from the rate of evolution of stars in globular clusters, which are thought to be the oldest stars in the Galaxy.
(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))))))))) (. .)))