Editing Variable star (section)

===Rotating variable stars===

Stars with sizeable [[sunspot]]s may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view. Bright spots also occur at the magnetic poles of magnetic stars. Stars with ellipsoidal shapes may also show changes in brightness as they present varying areas of their surfaces to the observer.<ref>{{Cite web |title=Rotating Variables: Mapping the Surfaces of the Stars {{!}} aavso |url=https://www.aavso.org/rotating-variables-mapping-surfaces-stars |access-date=2023-12-12 |website=www.aavso.org}}</ref>

==== Non-spherical stars ====

=====Ellipsoidal variables=====

These are very close binaries, the components of which are non-spherical due to their tidal interaction. As the stars rotate the area of their surface presented towards the observer changes and this in turn affects their brightness as seen from Earth.

====Stellar spots====

The surface of the star is not uniformly bright, but has darker and brighter areas (like the sun's [[Sun spot|solar spots]]). The star's [[chromosphere]] too may vary in brightness. As the star rotates we observe brightness variations of a few tenths of magnitudes.

=====FK Comae Berenices variables=====
[[File:FKComLightCurve.png|thumb|right|[[Light curve]]s for FK Comae Berenices. The main plot shows the short term variability plotted from [[Transiting Exoplanet Survey Satellite|''TESS'']] data;<ref name=MAST/> the inset, adapted from Panov and Dimitrov (2007),<ref name="Panov"/> shows the long term variability.]]
These stars rotate extremely rapidly (~100&nbsp;km/s at the [[equator]]); hence they are [[ellipsoidal]] in shape. They are (apparently) single giant stars with [[spectral type]]s G and K and show strong [[chromosphere|chromospheric]] [[emission line]]s. Examples are [[FK Comae Berenices|FK Com]], [[V1794 Cygni]] and [[UZ Librae]]. A possible explanation for the rapid rotation of FK Comae stars is that they are the result of the merger of a [[contact binary|(contact) binary]].<ref name=Livio_Stoker_1988>{{cite journal
 | title=The Common Envelope Phase in the Evolution of Binary Stars
 | last1=Livio | first1=Mario | last2=Soker | first2=Noam
 | journal=Astrophysical Journal | volume=329 | page=764
 | date=June 1988 | doi=10.1086/166419 | bibcode=1988ApJ...329..764L
}}</ref><!-- Ref. for last statement. -->

===== BY Draconis variable stars =====
{{main|BY Draconis variable}}
BY Draconis stars are of spectral class K or M and vary by less than 0.5 magnitudes (70% change in luminosity).

====Magnetic fields====

===== Alpha<sup>2</sup> Canum Venaticorum variables =====
{{Main|Alpha2 Canum Venaticorum variable}}

Alpha<sup>2</sup> Canum Venaticorum (α<sup>2</sup> CVn) variables are [[main sequence|main-sequence]] stars of spectral class B8–A7 that show fluctuations of 0.01 to 0.1 magnitudes (1% to 10%) due to changes in their magnetic fields.

=====SX Arietis variables=====
{{Main|SX Arietis variable}}
Stars in this class exhibit brightness fluctuations of some 0.1 magnitude caused by changes in their magnetic fields due to high rotation speeds.

=====Optically variable pulsars=====
{{Main|Pulsar}}

Few [[pulsar]]s have been detected in [[visible light]]. These [[neutron star]]s change in brightness as they rotate. Because of the rapid rotation, brightness variations are extremely fast, from milliseconds to a few seconds. The first and the best known example is the [[Crab Pulsar]].