( ( ( the stars ) ( in ( these clusters ) ) ) ( ( ( ( ( formed ( ( perhaps ( ( 15 billion ) years ) ) ago ) ) , ) and ) ( have ( witnessed ( everything ( that ( ( has ever ) ( happened ( in ( our Galaxy ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT the) (NNS stars)) (PP (IN in) (NP (DT these) (NNS clusters)))) (VP (VP (VBD formed) (ADVP (NP (RB perhaps) (QP (CD 15) (CD billion)) (NNS years)) (RB ago))) (, ,) (CC and) (VP (VBP have) (VP (VBN witnessed) (NP (NP (NN everything)) (SBAR (WHNP (WDT that)) (S (VP (VBZ has) (ADVP (RB ever)) (VP (VBN happened) (PP (IN in) (NP (PRP$ our) (NN Galaxy))))))))))) (. .)))
the stars in these clusters formed perhaps 15 billion years ago, and have witnessed everything that has ever happened in our 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))))))))) (. .)))