Editing Semiregular variable star

{{Short description|Type of variable star}}
[[File:Light curve of Betelgeuse.png|thumb|right|upright=1.5|[[Light curve]] of semiregular variable star [[Betelgeuse]]]]
In [[astronomy]], a '''semiregular variable star''', a type of [[variable star]], is a [[Red giant|giant]] or [[supergiant]] of intermediate and late (cooler) [[spectral type]]. It shows considerable periodicity in its light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 [[day]]s, while the shapes of the [[light curve]]s may be rather different and variable with each cycle.  The amplitudes may be from several hundredths to several [[Apparent magnitude|magnitudes]] (usually 1-2 magnitudes in the V filter).

==Classification==
The semiregular variable stars have been sub-divided into four categories for many decades, with a fifth related group defined more recently.  The original definitions of the four main groups were formalised in 1958 at the tenth general assembly of the [[International Astronomical Union]] (IAU).  The [[General Catalogue of Variable Stars]] (GCVS) has updated the definitions with some additional information and provided newer reference stars where old examples such as [[S Vulpeculae|S Vul]] have been re-classified.

{| class="wikitable"
|+Semiregular variable subtypes
|- style="vertical-align: top;"
! Subtype<ref name=IAU/> !! IAU definition<ref name=IAU/> !! GCVS code<ref name=GCVS/> !! GCVS definition<ref name=GCVS/> !! Standard<br/>stars
|- style="vertical-align: top;"
| SRa
|| <small>''semi-regular variable giants'' of ''late'' spectral classes (M, C and S), which retain periodicity with comparative stability  and possess, as a rule, small (less than 2<sup>m</sup>.5) light-variation amplitudes. Amplitudes and forms of light curves are usually liable to strong variations from period to period.  Many of these stars differ from Mira Ceti type stars only owing to the smaller amplitude of light variation.</small>
|| SRA
|| <small>Semiregular late-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small (<2.5 mag in V) light amplitudes. Amplitudes and light-curve shapes generally vary and periods are in the range of 35–1200 days. Many of these stars differ from Miras only by showing smaller light amplitudes</small>
|| [[Z Aquarii|Z Aqr]]<ref name=IAU/><ref name=GCVS/>
|- style="vertical-align: top;"
| SRb
|| <small>''semi-regular variable giants'' of ''late'' spectral classes (M, C and S) with a poorly expressed periodicity, i.e. with a different duration of individual cycles (which leads to the impossibility of predicting the epochs of maximum and minimum brightness), or with the replacement of periodical changes by slow irregular variations, or even by the constancy of brightness. Some of them are characterised by a certain mean value of the period, given in the catalogue.</small>
|| SRB
|| <small>Semiregular late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes, and even with light constancy intervals. Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the Catalogue. In a number of cases, the simultaneous presence of two or more periods of light variation is observed</small>
|| [[AF Cygni|AF Cyg]]<ref name=IAU/><ref name=GCVS/><br/>{{nowrap|[[RR Coronae Borealis|RR CrB]]}}<ref name=IAU/><ref name=GCVS/>
|- style="vertical-align: top;"
| SRc
|| <small>''semi-regular variable super-giants'' of ''late'' spectral classes</small>
|| SRC
|| <small>Spectral-type (M, C, S or Me, Ce, Se) supergiants with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days.</small>
|| [[Mu Cephei|μ Cep]]<ref name=IAU/><ref name=GCVS/><br/>[[RW Cygni|RW Cyg]]<ref name=IAU/>
|- style="vertical-align: top;"
| SRd
|| <small>''semi-regular variable giants and super-giants'' belonging to spectral classes F, G, K</small>
|| SRD
|| <small>Semiregular variable giants and supergiants of F, G, or K spectral types, sometimes with emission lines in their spectra. Amplitudes of light variation are in the range from 0.1 to 4 mag, and the range of periods is from 30 to 1100 days</small>
|| [[S Vulpeculae|S Vul]]<ref name=IAU/><br/>[[UU Herculis|UU Her]]<ref name=IAU/><br/>[[AG Aurigae|AG Aur]]<ref name=IAU/><br/>[[SX Herculis|SX Her]]<ref name=GCVS/><br/>[[SV Ursae Majoris|SV UMa]]<ref name=GCVS/>
|- style="vertical-align: top;"
||
||
|| SRS
|| <small>Semiregular pulsating red giants with short period (several days to a month), probably high-overtone pulsators</small>
|| [[AU Arietis|AU Ari]]<ref name=GCVS/>
|}

==Pulsation==
The semiregular variable stars, particularly the SRa and SRb sub-classes, are often grouped with the [[Mira variable]]s under the [[long-period variable]] heading.  In other situations, the term is expanded to cover almost all cool pulsating stars.  The semi-regular giant stars are closely related to the Mira variables: Mira stars generally pulsate in the [[fundamental mode]]; semiregular giants pulsate in one or more [[overtone]]s.<ref name=lsp/>

Photometric studies in the [[Large Magellanic Cloud]] looking for [[gravitational microlensing]] events have shown that essentially all cool evolved stars are variable, with the coolest stars showing very large amplitudes and warmer stars showing only micro-variations.  The semiregular variable stars fall on one of five main [[period-luminosity relationship]] sequences identified, differing from the Mira variables only in pulsating in an overtone mode.  The closely related OSARG ([[OGLE]] small amplitude red giant) variables pulsate in an unknown mode.<ref name=ogle/><ref name=pl/>

Many semiregular variables show long secondary periods around ten times the main pulsation period, with amplitudes of a few tenths of a magnitude at visual wavelengths.  The cause of the pulsations is not known.<ref name=lsp/>

==Bright examples==
[[Eta Geminorum|η Gem]] is the brightest SRa variable, and also an eclipsing binary. [[57 Pegasi|GZ Peg]] is an SRa variable and [[S-type star]] with a maximum magnitude of 4.95.  [[T Centauri|T Cen]] is listed as the next-brightest SRa example,<ref name=GCVS/> but it is suggested that it may actually be an [[RV Tauri variable]], which would make it by far the brightest member of that class.<ref name=vsx/>

There are numerous naked-eye SRb stars, with third-magnitude [[L2 Puppis|L<sup>2</sup> Pup]] being the brightest listed in the GCVS.  [[Sigma Librae|σ Lib]] and [[Rho Persei|ρ Per]] are also third-magnitude SRb stars at maximum brightness.  [[Beta Gruis|β Gru]] is a second magnitude star classified as a [[slow irregular variable]] by the GCVS, but reported to be of SRb type by later research.<ref name=jaavso/>  These four are all class M giants, although some SRb variables are [[carbon star]]s such as [[UU Aurigae|UU Aur]] or S-type stars such as [[Pi1 Gruis|Pi<sup>1</sup> Gru]].<ref name=GCVS/>

Catalogued SRc stars are less numerous, but include some of the brightest stars in the sky such as [[Betelgeuse]] and [[alpha Herculis|α Her]].  Although SRc stars are defined as being supergiants, a number of them have giant spectral [[luminosity class]]es and some such as α Her are known to be [[asymptotic giant branch]] stars.<ref name=GCVS/>

Many SRd stars are extremely luminous [[hypergiant]]s, including the naked-eye [[Rho Cassiopeiae|ρ Cas]], [[V509 Cassiopeiae|V509 Cas]], and [[Omicron1 Centauri|ο<sup>1</sup> Cen]].  Others are classified as giant stars, but the brightest example is the seventh-magnitude [[LU Aquarii|LU Aqr]].<ref name=GCVS/>

Most SRS variables have been discovered in deep large-scale surveys, but the naked-eye stars [[V428 Andromedae|V428 And]], [[AV Arietis|AV Ari]], and [[57 Piscium|EL Psc]] are also members.<ref name=GCVS/>

==See also==
*[[List of semiregular variable stars]]
*[[Low-dimensional chaos in stellar pulsations]]
*[[Variable star designation]]

== References ==
{{reflist|refs=

<ref name=vsx>{{cite journal|bibcode=2006SASS...25...47W|title=The International Variable Star Index (VSX)|journal=The Society for Astronomical Sciences 25th Annual Symposium on Telescope Science. Held May 23–25|volume=25|pages=47|last1=Watson|first1=C. L.|year=2006}}</ref>

<ref name=pl>{{cite journal|bibcode=2007AcA....57..201S|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|arxiv = 0710.2780 }}</ref>

<ref name=ogle>{{cite journal|bibcode=2009AcA....59..239S|arxiv=0910.1354|title=The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IV. Long-Period Variables in the Large Magellanic Cloud|journal=Acta Astronomica|volume=59|pages=239|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=Szewczyk|first7=O.|last8=Ulaczyk|first8=K.|last9=Poleski|first9=R.|year=2009|issue=3 }}</ref>

<ref name=lsp>{{cite journal|doi=10.1111/j.1365-2966.2009.15401.x|title=Long Secondary Periods in variable red giants|journal=Monthly Notices of the Royal Astronomical Society|volume=399|issue=4|pages=2063|year=2009|last1=Nicholls|first1=C. P.|last2=Wood|first2=P. R.|last3=Cioni|first3=M.-R. L.|last4=Soszyński|first4=I.|doi-access=free |arxiv = 0907.2975 |bibcode = 2009MNRAS.399.2063N |s2cid=19019968 }}</ref>

<ref name=IAU>{{cite journal|doi=10.1017/S0251107X00020988|title=27. Commission des Etoiles Variables|journal=Transactions of the International Astronomical Union|volume=10|pages=398–431|year=2016|last1=Kukarkin|first1=B. V.|doi-access=free}}</ref>

<ref name=GCVS>{{cite web
  |date=12 Feb 2009
  |title=GCVS Variability Types
  |publisher=[[General Catalogue of Variable Stars]] @ [[Centre de données astronomiques de Strasbourg|Sternberg Astronomical Institute, Moscow, Russia]]
  |url=http://www.sai.msu.su/gcvs/gcvs/iii/vartype.txt
  |access-date=2010-11-24}}</ref>
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<ref name="AAVSO">{{cite web 
  |title=Variable Star of the Month&mdash;December, 2000: Alpha Orionis
  |author=Davis, Kate
  |date=2000-12-01
  |publisher=[[American Association of Variable Star Observers]] (AAVSO)
  |url=http://chart.aavso.org/sites/default/files/vsots/1200.pdf
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<ref name=jaavso>{{cite journal | last1=Otero | first1=S. A. | last2=Moon | first2=T. | title=The Characteristic Period of Pulsation of β Gruis | journal=The Journal of the American Association of Variable Star Observers | volume=34 | issue=2 | pages=156–164 |date=December 2006 | bibcode=2006JAVSO..34..156O }}</ref>

}}

==External links==
* [https://www.aavso.org/vsots_eudel EU Delphini and the Small-Amplitude Pulsating Red Giants]
* [https://britastro.org/node/7224 Y Lyncis]
* [http://chandra.harvard.edu/edu/formal/variable_stars/bg_info.html Pulsating variable stars and the H-R diagram]
* [http://ogle.astrouw.edu.pl/atlas/SRVs.html OGLE Atlas of Variable Star Light Curves - Semiregular Variables]

{{Variable star topics}}

{{DEFAULTSORT:Semiregular Variable Star}}
[[Category:Semiregular variable stars| ]]