( ( ( ( ( Billions ( of galaxies ) ) , ) ( ( each ( of which ) ) ( is ( ( a ( ( gravitationally bound ) cluster ) ) ( of ( billions ( of stars ) ) ) ) ) ) ) , ) ( now ( ( form ( most ( of ( ( the ( visible mass ) ) ( of ( the universe ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NP (NNS Billions)) (PP (IN of) (NP (NNS galaxies))) (, ,) (SBAR (WHNP (NP (DT each)) (WHPP (IN of) (WHNP (WDT which)))) (S (VP (VBZ is) (NP (NP (DT a) (ADJP (RB gravitationally) (VBN bound)) (NN cluster)) (PP (IN of) (NP (NP (NNS billions)) (PP (IN of) (NP (NNS stars))))))))) (, ,)) (ADVP (RB now)) (VP (VBP form) (NP (NP (JJS most)) (PP (IN of) (NP (NP (DT the) (JJ visible) (NN mass)) (PP (IN of) (NP (DT the) (NN universe))))))) (. .)))
Billions of galaxies, each of which is a gravitationally bound cluster of billions of stars, now form most of the visible mass of the universe.
(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))))))))) (. .)))