Editing Red-giant branch (section)

==Variability==
Some [[red giant]]s are large amplitude variables. Many of the earliest-known variable stars are [[Mira variable]]s with regular periods and amplitudes of several magnitudes, [[semiregular variable]]s with less obvious periods or multiple periods and slightly lower amplitudes, and [[slow irregular variable]]s with no obvious period. These have long been considered to be [[asymptotic giant branch]] (AGB) stars or supergiants and the red-giant branch (RGB) stars themselves were not generally considered to be variable. A few apparent exceptions were considered to be low-luminosity AGB stars.<ref name=kiss>{{cite journal|bibcode=2003MNRAS.343L..79K|arxiv=astro-ph/0306426|title=Red variables in the OGLE-II data base – I. Pulsations and period-luminosity relations below the tip of the red giant branch of the Large Magellanic Cloud|journal=Monthly Notices of the Royal Astronomical Society|volume=343|issue=3|pages=L79|last1=Kiss|first1=L. L.|last2=Bedding|first2=T. R.|year=2003|doi=10.1046/j.1365-8711.2003.06931.x|doi-access=free |s2cid=2383837}}</ref>

Studies in the late 20th century began to show that all giants of class M were variable with amplitudes of 10 milli-magnitudes of more, and that late-K-class giants were also likely to be variable with smaller amplitudes. Such variable stars were amongst the more luminous red giants, close to the tip of the RGB, but it was difficult to argue that they were all actually AGB stars. The stars showed a period amplitude relationship with larger-amplitude variables pulsating more slowly.<ref name=jorissen>{{cite journal|bibcode=1997A&A...324..578J|title=The onset of photometric variability in red giant stars|journal=Astronomy and Astrophysics|volume=324|pages=578|last1=Jorissen|first1=A.|last2=Mowlavi|first2=N.|last3=Sterken|first3=C.|last4=Manfroid|first4=J.|year=1997}}</ref>

[[Microlensing Observations in Astrophysics|Microlensing surveys]] in the 21st century have provided extremely accurate photometry of thousands of stars over many years. This has allowed for the discovery of many new variable stars, often of very small amplitudes. Multiple [[period-luminosity relationship]]s have been discovered, grouped into regions with ''ridges'' of closely spaced parallel relationships. Some of these correspond to the known Miras and semi-regulars, but an additional class of variable star has been defined: [[OGLE]] Small Amplitude Red Giants, or [[Long-period variable star|OSARGs]]. OSARGs have amplitudes of a few thousandths of a magnitude and semi-regular periods of 10 to 100 days. The OGLE survey published up to three periods for each OSARG, indicating a complex combination of pulsations. Many thousands of OSARGs were quickly detected in the [[Magellanic Clouds]], both AGB and RGB stars.<ref name=soszynski>{{cite journal|bibcode=2007AcA....57..201S|arxiv=0710.2780|title=The Optical Gravitational Lensing Experiment. Period—Luminosity Relations of Variable Red Giant Stars|journal=Acta Astronomica|volume=57|pages=201|last1=Soszynski|first1=I.|last2=Dziembowski|first2=W. A.|last3=Udalski|first3=A.|last4=Kubiak|first4=M.|last5=Szymanski|first5=M. K.|last6=Pietrzynski|first6=G.|last7=Wyrzykowski|first7=L.|last8=Szewczyk|first8=O.|last9=Ulaczyk|first9=K.|year=2007}}</ref> A catalog has since been published of 192,643 OSARGs in the direction of the [[Milky Way]] central bulge. Although around a quarter of Magellanic Cloud OSARgs show long secondary periods, very few of the galactic OSARGs do.<ref name=soszynski2013>{{cite journal|bibcode=2013AcA....63...21S|arxiv=1304.2787|title=The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XV. Long-Period Variables in the Galactic Bulge|journal=Acta Astronomica|volume=63|issue=1|pages=21|last1=Soszyński|first1=I.|last2=Udalski|first2=A.|last3=Szymański|first3=M. K.|last4=Kubiak|first4=M.|last5=Pietrzyński|first5=G.|last6=Wyrzykowski|first6=Ł.|last7=Ulaczyk|first7=K.|last8=Poleski|first8=R.|last9=Kozłowski|first9=S.|last10=Pietrukowicz|first10=P.|last11=Skowron|first11=J.|year=2013}}</ref>

The RGB OSARGs follow three closely spaced period-luminosity relations, corresponding to the first, second and third [[overtone]]s of [[radial pulsations|radial pulsation]] models for stars of certain masses and luminosities, but that dipole and quadrupole non-radial pulsations are also present leading to the semi-regular nature of the variations.<ref name=takayama>{{cite journal|bibcode=2013EPJWC..4303013T|volume=43|pages=03013|last1=Takayama|first1=M.|last2=Saio|first2=H.|last3=Ita|first3=Y.|title=On the pulsation modes and masses of RGB OSARGs |journal=EPJ Web of Conferences |year=2013|doi=10.1051/epjconf/20134303013|doi-access=free}}</ref> The [[fundamental mode]] does not appear, and the underlying cause of the excitation is not known. [[Stochastic]] convection has been suggested as a cause, similar to [[solar-like oscillations]].<ref name=soszynski/>

Two additional types of variation have been discovered in RGB stars: long secondary periods, which are associated with other variations but can show larger amplitudes with periods of hundreds or thousands of days; and ''ellipsoidal'' variations. The cause of the long secondary periods is unknown, but it has been proposed that they are due to interactions with low-mass companions in close orbits.<ref name=nicholls2009>{{cite journal|bibcode=2009MNRAS.399.2063N|arxiv=0907.2975|title=Long Secondary Periods in variable red giants|journal=Monthly Notices of the Royal Astronomical Society|volume=399|issue=4|pages=2063–2078|last1=Nicholls|first1=C. P.|last2=Wood|first2=P. R.|last3=Cioni|first3=M.-R. L.|last4=Soszyński|first4=I.|year=2009|doi=10.1111/j.1365-2966.2009.15401.x|doi-access=free |s2cid=19019968}}</ref> The ellipsoidal variations are also thought to be created in binary systems, in this case contact binaries where distorted stars cause strictly periodic variations as they orbit.<ref name=nicholls2012>{{cite journal|bibcode=2012MNRAS.421.2616N|arxiv=1201.1043|title=Eccentric ellipsoidal red giant binaries in the LMC: Complete orbital solutions and comments on interaction at periastron|journal=Monthly Notices of the Royal Astronomical Society|volume=421|issue=3|pages=2616|last1=Nicholls|first1=C. P.|last2=Wood|first2=P. R.|year=2012|doi=10.1111/j.1365-2966.2012.20492.x|doi-access=free |s2cid=59464524}}</ref>