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Variable star
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This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2013)
Trifid Nebula contains Cepheid variable stars
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:*Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.*Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.
^ Fröhlich, C. (2006). "Solar Irradiance Variability Since 1978". Space Science Reviews 125: 53. Bibcode:2006SSRv..125...53F. doi:10.1007/s11214-006-9046-5.
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