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 年代早期，天文学家 Henrietta Swan Leavitt 发现一系
造父
星，它
有规律地
暗，并且每个造父星以相同的速率脉动就会有着相同的度。
