( Stars ( ( are ( classified ( by ( their ( surface temperature ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Stars)) (VP (VBP are) (VP (VBN classified) (PP (IN by) (NP (PRP$ their) (NN surface) (NN temperature))))) (. .)))
Stars are classified by their surface temperature.
(ROOT (S (NP (DT The) (NN sun)) (VP (VBZ is) (VP (VBN classified) (PP (IN as) (NP (DT a) (NN star))))) (. .)))
The sun is classified as a star.