Complex sentence: The five brightest stars in this cluster have masses on the order of 15-30 solar masses, lie within a diameter of 1.5 light-years and are responsible for much of the illumination of the surrounding nebula.
Split into simpler sentences: The five brightest stars are on the order of 15-30 solar masses in size. They are within a diameter of 1.5 light-years of each other and are responsible for much of the illumination of the surrounding nebula.