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Variable Star

Variable star: a star whose actual or observed brightness varies periodically. These changes can occur with varying degrees of regularity and intensity, ranging from a fraction of a second to many years. Intrinsic variation occurs because of changes of the star itself, often due to internal vibration or eruptions, or to influx of nearby material. Cepheids and novae are examples of intrinsically variable stars. Extrinsic variation in a star’s observed brightness that does not reflect physical changes in the star also occur, as when a darker star periodically eclipses a brighter star in an eclipsing binary system, or with the rotation of a star.

Credit: The American Heritage Science Dictionary

Cepheid Variable

Cepheid variable: a star that brightens and dims in an extremely regular fashion. The periods of the fluctuations (the time to complete one cycle from bright to dim and back to bright) last several days, ranging from 1 to 50 days. These stars are important because the period of a Cepheid depends on its intrinsic brightness, or absolute magnitude, in a known way: the brighter the star, the longer its period. All Cepheid variables with the same period have nearly the same intrinsic brightness, but their apparent brightnesses differ because they are at different distances. By observing a Cepheid's period, one can determine how bright it actually is. By comparing this intrinsic brightness to how bright it appears to be, one can determine the star's distance. Thus Cepheids are important indicators of interstellar and intergalactic distances, and they have been called the "yardsticks of the universe." The Cepheid class takes its name from Delta Cephei, the first such star discovered in 1784. Cepheids are yellow supergiant stars, and their fluctuations in luminosity result from an actual physical pulsation, with attendant changes in surface temperature and size. The stars are hottest and brightest when expanding at maximum rate midway between their largest and smallest size. The period-luminosity relation was discovered by Henrietta Leavitt and Harlow Shapley by studying the many Cepheids in the Magellanic Clouds, the two closest galaxies; these stars are all almost equally distant. It was found that the brighter variables had the longer periods. The absolute magnitude of a few Cepheids is required to infer absolute, rather than merely relative, distances. These absolute magnitudes were measured by a statistical study of the proper motions of Cepheids within our own galaxy. In the 1950s a second class of Cepheids with different period-luminosity relations was found, leading to a dramatic doubling of estimated cosmological distances. The Hubble Space Telescope permits the observation of Cepheids in more distant galaxies, giving a more accurate picture of the size and age of the universe.

Credit: The Columbia Electronic Encyclopedia, Sixth Ed. (2012)