Variable star

Comparison of VLT-SPHERE images of Betelgeuse taken in January 2019 and December 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 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:^[1]

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 the Sun, for example, varies by about 0.1% over an 11-year solar cycle.^[2]

^ Alexeev, Boris V. (2017-01-01), Alexeev, Boris V. (ed.), "Chapter 7 - Nonlocal Theory of Variable Stars", Nonlocal Astrophysics, Elsevier, pp. 321–377, doi:10.1016/b978-0-444-64019-2.00007-7, ISBN 978-0-444-64019-2, retrieved 2023-06-06
^ Fröhlich, C. (2006). "Solar Irradiance Variability Since 1978". Space Science Reviews. 125 (1–4): 53–65. Bibcode:2006SSRv..125...53F. doi:10.1007/s11214-006-9046-5. S2CID 54697141.

[1] Alexeev, Boris V. (2017-01-01), Alexeev, Boris V. (ed.), "Chapter 7 - Nonlocal Theory of Variable Stars", Nonlocal Astrophysics, Elsevier, pp. 321–377, doi:10.1016/b978-0-444-64019-2.00007-7, ISBN 978-0-444-64019-2, retrieved 2023-06-06

[2] Fröhlich, C. (2006). "Solar Irradiance Variability Since 1978". Space Science Reviews. 125 (1–4): 53–65. Bibcode:2006SSRv..125...53F. doi:10.1007/s11214-006-9046-5. S2CID 54697141.

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