Variable star

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A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth. 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,[1] equivalent to a change of one thousandth of a magnitude.

It is convenient to classify variable stars as belonging to one of two types:

  • 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.

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