M 31 Cepheids periods

Cepheid variables can be used for more than just a distance indicator. Since the age can be determined from the period, they can be used to trace the star formation history of a...造父变星（Cepheid variables）的应用远不止于作为距离指示器。由于可通过其周期测定恒星年龄，它们可被用于追溯……的恒星形成历史。