How Does an Astronomer Use Cepheid Variables to Determine Distance

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The Answer is Measurements of brightness can determine stellar distances up to about 10,000 parsecs. Measurements of even greater distances come from the study of special kinds of stars that pulsate, meaning that they expand and contract periodically. When a pulsating star expands, it has a larger surface area, but its temperature doesn't change, so its apparent brightness increases. Henrietta Leavitt discovered that one class of pulsating stars, known as Cepheid variables, display a predictable relationship between true brightness variation and the periodicity of their pulsations. So, if astronomers know the periodicity of a Cepheid variable star's pulsations, they also know its true brightness. Then, by comparing this true brightness with the apparent brightness of the star, astronomers can calculate the star's distance. If a Cepheid variable star is within a distant galaxy, the distance from Earth to the galaxy can be measured by examining the brightness of that star.

How Does an Astronomer Use Cepheid Variables to Determine Distance

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