( ( ( ( ( ( ( ( Approximately ( 2-3 billion ) ) years ) ( after ( the ( Big Bang ) ) ) ) ( , ( ( the galaxies ) ( began ( to ( gather ( into clusters ) ) ) ) ) ) ) , ) and ) ( ( after ( ( 4 billion ) years ) ) ( ( the ( first stars ) ) ( were formed ) ) ) ) . )
(ROOT (S (S (NP (NP (QP (RB Approximately) (CD 2-3) (CD billion)) (NNS years)) (PP (IN after) (NP (DT the) (NNP Big) (NNP Bang)))) (, ,) (NP (DT the) (NNS galaxies)) (VP (VBD began) (S (VP (TO to) (VP (VB gather) (PP (IN into) (NP (NNS clusters)))))))) (, ,) (CC and) (S (PP (IN after) (NP (QP (CD 4) (CD billion)) (NNS years))) (NP (DT the) (JJ first) (NNS stars)) (VP (VBD were) (VP (VBN formed)))) (. .)))
Approximately 2-3 billion years after the Big Bang, the galaxies began to gather into clusters, and after 4 billion years the first stars were formed.
(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))))))))) (. .)))