Star whose observed brightness varies noticeably in intensity. Pulsating variables expand and contract in cycles, pulsating rhythmically in brightness and size. Explosive variables include novas and supernovas, which brighten rapidly due to sudden outbursts of radiant energy; the increased brightness lasts a short time, followed by relatively slow dimming. Eclipsing variable stars are variable only because light from one star is blocked by another in Earth's direction. Hundreds of thousands of variable stars are known. Seealso binary star; Cepheid variable; flare star; pulsar; T Tauri star.
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Binary star in an orbit whose plane passes through or very near Earth. An observer on Earth sees one star pass periodically in front of the other and diminish its light through an eclipse. The star Algol, in the constellation Perseus, was the first such star recognized (1782); thousands are now known. By combining the brightness variations with spectroscopic information for both stars, astronomers can determine the mass and size of each star. Seealso variable star.
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