Cepheid variables are valuable as distance markers because they are and their period of brightening is directly related to their

Luminous; distance travelled

Explanation:

Cepheids, also called Cepheid Variables, are stars which brigthen and dim periodically. This behavior allows them to be used as cosmic yardsticks out to distances of a few tens of millions of light-years. a uniform function of their brightness. The reason Cepheids can be used to determine distance is because the are variable stars that change brightness according to a regular pattern Cepheid variable discovery allows one to know the true luminosity of a Cepheid by simply observing its pulsation period. This in turn allows one to determine the distance to the star, by comparing its known luminosity to its observed brightness.