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.

Cecilia and Sergei Gaposchkin found that a more symmetric subset of these stars found in the Magellanic Clouds are systematically brighter than predicted, and the Baade-Wesselink Method can be used to find their size. The shortest known period of one of these stars in a binary system is 507 days in S Muscae, leading to the theory that most observed examples are on their second or high crossing of the instability strip. Metal-poor examples of these stars are called W. Virginis. A shark-fin curve represents their most distinction feature, which is caused He-plus ionizing to He-two-plus, leading to a pressure gradient and subsequent expansion. For 10 points, name this class of variable star with period longer than RR Lyrae that is often used as standard candles.
Social Science