( ( ( ( ( As ( ( a basis ) ( for ( understanding ( this concept ) ) ) ) ) ( , ( students know ) ) ) : ) ( ( a. galaxies ) ( ( ( ( are ( clusters ( of ( billions ( of stars ) ) ) ) ) , ) and ) ( may ( have ( different shapes ) ) ) ) ) ) . )
(ROOT (S (S (PP (IN As) (NP (NP (DT a) (NN basis)) (PP (IN for) (S (VP (VBG understanding) (NP (DT this) (NN concept))))))) (, ,) (NP (NNS students)) (VP (VBP know))) (: :) (S (NP (NN a.) (NNS galaxies)) (VP (VP (VBP are) (NP (NP (NNS clusters)) (PP (IN of) (NP (NP (NNS billions)) (PP (IN of) (NP (NNS stars))))))) (, ,) (CC and) (VP (MD may) (VP (VB have) (NP (JJ different) (NNS shapes)))))) (. .)))
As a basis for understanding this concept, students know: a. galaxies are clusters of billions of stars, and may have different shapes.
(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))))))))) (. .)))