Complex sentence: K2 W. Strohmeier, "Variable Stars", Pergamon (1972), they are 5--20 times as massive as the Sun and up to 30000 times more luminous.
Split into simpler sentences: K2 W. Strohmeier, "Variable Stars", Pergamon (1972). The stars are 5--20 times more massive than the Sun and up to 30000 times more luminous.