In the early 1900s, astronomer Henrietta Swan Leavitt realized a class of stars called Cepheids pulsate, bright and dim, and every Cepheid that pulsates at the same rate has the same brightness.

1900 代早期，天文学家 Henrietta Swan Leavitt 发现一系列名为造父变星，它有规律地变亮和变暗，并且每个造父变星以相率脉动就会有着相亮度。