James C. Glass 
Shanji [PDF ebook] 

On the planet Shanji, a ruthless emperor rules a subjugated people. Kati, raised by the lower caste Tumatsin, is taken captive by the emperor’s troops, and then saved by the Searchers, who see her as the promised Empress of Light, whose psychic powers can control the hot light of creation. But those powers can destroy a planet or star, and could be beyond everyone’s control. Kati must decide how to use her abilities when there is a planetary invasion from afar, led by a mighty empress whom she thought of as a friend and teacher. Kati must take charge of her own destiny, not only for herself, but also for Shanji and its neighboring worlds. Born with the heritage of two races, she must rise to rule them both.