Bellatrix shrugged slightly, her chin tilted upwards slightly as she regarded the Gryffindor. "I don't," she stated simply, her mind running the possible excuse for her being there. Ones that didn't inculde having a hufflepuff downstairs, waiting to be shagged.
Her only way was to lie, and lie well. Not that she did it any other way. "It's nice to come up here and look at the stars," Her tone was sugar-coated, in only the teasing way she could muster to hide the slight irritation she felt. She'd use the tone on some first years before to make them believe her on whatever she wanted.
Astronomy never interested her, it wasn't useful to her. The tower, on the other hand, had great use to her. "What brings you up here?" That didn't come off so convincingly, staying at the top of the stairs.