![]() By observing the star's period, we know its luminosity (which is directly related to another quantity called "absolute magnitude". Luminosity is a measure of how bright the star truly is at all wavelengths. It turns out that the period is related to the star's absolute magnitude or luminosity. They vary in brightness from brightest to dimmest to brightest in certain time periods. Cepheids are stars whose total brightness (luminosity) varies with specific periods. Cepheid variables, is a method of determining cosmic distances based on the "period luminosity" characteristics of certain stars.However, parallax is detectable for many more by spacecraft above the Earth's obscuring atmosphere. Parallax is smaller the farther away a star is, and can be detected for only several thousand stars as viewed from Earth. ![]() This is the most accurate method for estimating distances to stars, but is limited to only the nearest stars.
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