( They ( ( thought ( that ( ( the ( blue stars ) ) ( were ( ( ( composed mostly ) ( of hydrogen ) ) ( while ( ( ( cooler stars ) ( like ( the sun ) ) ) ( , ( had ( ( ( less hydrogen ) and ) ( more ( of ( the ( heavier elements ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBD thought) (SBAR (IN that) (S (NP (DT the) (JJ blue) (NNS stars)) (VP (VBD were) (VP (VBN composed) (ADVP (RB mostly)) (PP (IN of) (NP (NN hydrogen))) (SBAR (IN while) (S (NP (NP (JJR cooler) (NNS stars)) (PP (IN like) (NP (DT the) (NN sun)))) (, ,) (VP (VBD had) (NP (NP (JJR less) (NN hydrogen)) (CC and) (NP (NP (JJR more)) (PP (IN of) (NP (DT the) (JJR heavier) (NNS elements))))))))))))) (. .)))
They thought that the blue stars were composed mostly of hydrogen while cooler stars like the sun, had less hydrogen and more of the heavier elements.
(ROOT (S (NP (DT The) (NN sun)) (ADVP (RB mostly)) (VP (VBD made) (PRT (RP up)) (PP (IN of) (NP (DT the) (NN element) (NN hydrogen)))) (. .)))