Why is a cepheid variable star referred to as a standard candle?

In astronomy, a standard candle is a light source for which the absolute magnitude is known. Standard candles are useful because we can use them to measure distances in space. Consider a 100 watt light bulb. If it is closer to us, it will appear brighter, and if it is farther away, it will appear dimmer. There is a relation between the amount of light we detect, and the distance to the light bulb.

That's why standard candles are useful tools for astronomers. If they know how bright an object should be and how bright it appears to be, they can calculate the distance to that object. Astronomers use a magnitude scale to compare the brightness of different stars. Unfortunately the magnitude of stars is often difficult to know, as stars vary in many different ways. Cepheid variable stars, however, have very predictable magnitudes.

Cepheid variable stars are stars that appear to pulsate over regular intervals. As the star expands, it cools and becomes dimmer, and as it contracts, it heats up and becomes brighter. This period of expansion and contraction can take place anywhere from 1-100 days. It turns out that the rate at which the star pulsates is correlated with how bright the star is.

If astronomers can measure the period of a Cepheid variable, they can calculate the absolute magnitude of the star, making Cepheid variables standard candles.