Editing Variable star (section)

{{short description|Star whose brightness fluctuates, as seen from Earth}}
{{For|the novel|Variable Star}}
{{More citations needed|date=February 2013}}
[[File:Eso2003c.jpg|thumb|upright=1.4|Comparison of [[VLT-SPHERE]] images of [[Betelgeuse]] taken in January&nbsp;2019 and December&nbsp;2019, showing the changes in brightness and shape. Betelgeuse is an intrinsically variable star.]]

A '''variable star''' is a [[star]] whose brightness as seen from [[Earth]] (its [[apparent magnitude]]) changes systematically with time. 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:<ref>{{Citation |last=Alexeev |first=Boris V. |title=Chapter 7 - Nonlocal Theory of Variable Stars |date=2017-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780444640192000077 |work=Nonlocal Astrophysics |pages=321–377 |editor-last=Alexeev |editor-first=Boris V. |access-date=2023-06-06 |publisher=Elsevier |language=en |doi=10.1016/b978-0-444-64019-2.00007-7 |isbn=978-0-444-64019-2|url-access=subscription }}</ref>
* ''Intrinsic variables'', whose luminosity actually changes periodically; for example, because the star 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 [[Eclipsing binaries|has an orbiting companion that sometimes eclipses it]].

Many, possibly most, stars exhibit at least some [[oscillation]] in luminosity: the energy output of the [[Sun]], for example, varies by about 0.1% over an 11-year [[solar cycle]].<ref>{{cite journal|doi=10.1007/s11214-006-9046-5|title=Solar Irradiance Variability Since 1978|journal=Space Science Reviews|volume=125|issue=1–4|pages=53–65|year=2006|last1=Fröhlich|first1=C.|bibcode=2006SSRv..125...53F|s2cid=54697141}}</ref>

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