Editing Variable star (section)

==Extrinsic variable stars==

There are two main groups of extrinsic variables: rotating stars and eclipsing stars.

===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]].

===Eclipsing binaries===
{{Main|Binary star#Eclipsing binaries}}
[[Image:Light curve of binary star Kepler-16.jpg|right|thumb|upright=1.4|How [[eclipsing binaries]] vary in brightness]]

Extrinsic variables have variations in their brightness, as seen by terrestrial observers, due to some external source. One of the most common reasons for this is the presence of a binary companion star, so that the two together form a [[binary star]]. When seen from certain angles, one star may [[eclipse]] the other, causing a reduction in brightness. One of the most famous eclipsing binaries is [[Algol]], or Beta Persei (β Per).

====Algol variables====
{{Main|Algol variable}}

Algol variables undergo eclipses with one or two minima separated by periods of nearly constant light. The prototype of this class is [[Algol]] in the [[constellation]] [[Perseus (constellation)|Perseus]].

====Double Periodic variables====
{{Main|Double periodic variable}}
Double periodic variables exhibit cyclical mass exchange which causes the orbital period to vary predictably over a very long period. The best known example is [[V393 Scorpii]].

====Beta Lyrae variables====
{{Main|Beta Lyrae variable}}

Beta Lyrae (β Lyr) variables are extremely close binaries, named after the star [[Beta Lyrae|Sheliak]]. The light curves of this class of eclipsing variables are constantly changing, making it almost impossible to determine the exact onset and end of each eclipse.

====W Serpentis variables====
W Serpentis is the prototype of a class of semi-detached binaries including a giant or supergiant transferring material to a massive more compact star. They are characterised, and distinguished from the similar β Lyr systems, by strong UV emission from accretions hotspots on a disc of material.

====W Ursae Majoris variables====
{{Main|W Ursae Majoris variable}}

The stars in this group show periods of less than a day. The stars are so closely situated to each other that their surfaces are almost in contact with each other.

===Planetary transits===

Stars with [[extrasolar planet|planets]] may also show brightness variations if their planets pass between Earth and the star. These variations are much smaller than those seen with stellar companions and are only detectable with extremely accurate observations. Examples include [[HD 209458]] and [[GSC 02652-01324]], and all of the planets and planet candidates detected by the [[Kepler Mission]].