Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

Delta Scuti is sometimes called a "dwarf" type of them. Photon pressure causes them to rapidly expand in size; however, when the star has expanded, the pressure gradient is then too weak to counteract gravity, leading to contraction. Their classical or Type I variety is the generally thought to be the most useful after Henrietta Leavitt discovered their period-luminosity relationship. FTP, name these stars used as a standard candle in making astronomical distance measurements, one of the major classes of variable stars.
Science