什么是造父变星的周光关系？它为什么被称为丈量遥远距离的量天尺？
What is the period-luminosity relation of Cepheid variables, and why are these stars called yardsticks for measuring vast distances?
造父变星是有规律地径向胀缩的黄超巨星，脉动由氦部分电离区充当阀门来驱动：压缩时电离度升高、不透明度变大，辐射能被暂时储存，膨胀时再释放出来，从而维持稳定的振荡。造父变星光度越大，脉动周期越长；因此只需测出光变周期就能推知光度，再与视亮度比较便得到距离，把距离测量从银河系延伸到邻近星系。
Cepheids are yellow supergiants that rhythmically swell and shrink, their pulsation driven because a partially ionized helium layer acts as a valve: compression raises ionization and opacity so radiation is stored momentarily, then released during expansion, sustaining a steady oscillation. More luminous Cepheids pulse with longer periods, so the observed period reveals the true luminosity; comparing it with apparent brightness yields the distance, extending measurements from our Galaxy to nearby ones.
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Leavitt H. S., Periods of 25 Variable Stars in the Small Magellanic Cloud, Harvard College Observatory Circular 173 (1912)
Henrietta S. Leavitt, Periods of 25 Variable Stars in the Small Magellanic Cloud, Harvard College Observatory Circular No. 173 (1912)