( ( ( ( ( There ( are ( ( three ( main types ) ) ( of ( emission nebulae ) ) ) ) ) , ) and ) ( all ( have ( ( very low ) ( gas densities ) ) ) ) ) . )
(ROOT (S (S (NP (EX There)) (VP (VBP are) (NP (NP (CD three) (JJ main) (NNS types)) (PP (IN of) (NP (NN emission) (NN nebulae)))))) (, ,) (CC and) (S (NP (DT all)) (VP (VBP have) (NP (ADJP (RB very) (JJ low)) (NN gas) (NNS densities)))) (. .)))
There are three main types of emission nebulae, and all have very low gas densities.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))