Answer
By measuring the period of the changing light curve of a Cepheid variable star, then using the relationship between period and luminosity, you can find its absolute luminosity.
Work Step by Step
You can see how bright it appears to be, and since you know its absolute luminosity, comparing the two tells you the distance to the star (and its galaxy), using equation 33-1.
In real life, you have to take into account the effects of light absorption by interstellar dust, etc.
