Editing Long-period variable star

{{short description|Cool luminous pulsating variable star}}
[[File:Evolution of the mira-type star S Orionis.jpg|thumb|upright=2.0|Pulsations of [[S Orionis]], showing dust production and [[maser]]s (ESO)]]
The descriptive term '''long-period variable star''' refers to various groups of cool luminous pulsating [[variable star]]s.  It is frequently abbreviated to '''LPV'''.

==Types of variation==
The [[General Catalogue of Variable Stars]] does not define a long-period variable star type, although it does describe [[Mira variable]]s as long-period variables.<ref name=gcvs/>  The term was first used in the 19th century, before more precise classifications of variable stars, to refer to a group that were known to vary on timescales typically hundreds of days.<ref name=parkhurst/>  By the middle of the 20th century, long period variables were known to be cool giant stars.<ref name=merrill/>  The relationship of Mira variables, [[semiregular variable]]s, and other pulsating stars was being investigated and the term ''long period variable'' was generally restricted to the coolest pulsating stars, almost all Mira variables.  Semiregular variables were considered intermediate between LPVs and [[Cepheid]]s.<ref name=rosino/><ref name=smak/>

After the publication of the General Catalogue of Variable Stars, both Mira variables and semiregular variables, particularly those of type SRa, were often considered as long period variables.<ref name=merrill1960/><ref name=harrington/>  At its broadest, LPVs include Mira, semiregular, slow irregular variables, and [[OGLE]] small amplitude red giants (OSARGs), including both giant and supergiant stars.<ref name=ogle/>  The OSARGs are generally not treated as LPVs,<ref name=osarg/> and many authors continue to use the term more restrictively to refer just to Mira and semiregular variables, or solely to Miras.<ref name=tuthill/>  The [[AAVSO]] LPV Section covers "Miras, Semiregulars, RV Tau and all your favorite red giants".<ref name=aavso/>

The [[AAVSO]] LPV Section covers the Mira, SR, and L stars, but also [[RV Tauri variables]], another type of large cool slowly varying star.  This includes SRc and Lc stars which are respectively semi-regular and irregular cool supergiants.  Recent researches have increasingly focused on the long period variables as only AGB and possibly red giant tip stars.  The recently classified OSARGs are by far the most numerous of these stars, comprising a high proportion of red giants.<ref name=ogle/>

==Properties==
[[File:Eso0315i.jpg|thumb|right|upright=1.2|Light curves of four [[Mira variable]]s in the [[galaxy]] [[Centaurus A]]]]
Long period variables are pulsating cool [[giant star|giant]], or [[supergiant]], [[variable star]]s with periods from around a hundred days, or just a few days for OSARGs, to more than a thousand days.  In some cases, the variations are too poorly defined to identify a period, although it is an open question whether they are truly non-periodic.<ref name=ogle/>

LPVs have [[stellar classification|spectral class]] F and redwards, but most are spectral class M, [[S-type star|S]] or [[carbon star|C]]. Many of the reddest stars in the sky, such as [[La Superba|Y CVn]], [[V Aquilae|V Aql]], and [[VX Sagittarii|VX Sgr]] are LPVs.

Most LPVs, including all Mira variables, are thermally-pulsing [[asymptotic giant branch]] stars with luminosities several thousand times the sun.  Some semiregular and irregular variables are less luminous giant stars, while others are more luminous supergiants including some of the [[List of largest stars|largest known star]]s such as [[VY Canis Majoris|VY CMa]].

==Long secondary periods==
Between a quarter and a half of long period variables show very slow variations with an amplitude up to one magnitude at visual wavelengths, and a period around ten times the primary pulsation period.  These are called long secondary periods.  The causes of the long secondary periods are unknown.  Binary interactions, dust formation, rotation, or non-radial oscillations have all been proposed as causes, but all have problems explaining the observations.<ref name=lsp/>

==Pulsation modes==
Mira variables are mostly [[fundamental mode]] pulsators, while the semiregular and irregular variables on the [[asymptotic giant branch]] pulsate in the first, second, or third [[overtone]].  Many of the less regular LPVs pulsate in more than one mode.<ref name=wood/>

Long secondary periods cannot be caused by fundamental mode radial pulsations or their harmonics, but strange mode pulsations are one possible explanation.<ref name=lsp/>

==References==
{{Reflist|refs=

<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=2063–2078|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=parkhurst>{{cite journal|bibcode=1893AnHar..29...89P|title=Observations of variable stars|journal=Annals of the Astronomical Observatory of Harvard College |volume=29|issue=4|pages=89|last1=Parkhurst|first1=Henry Martyn|last2=Pickering|first2=Edward Charles|year=1893}}</ref>

<ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|display-authors=etal|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009}}</ref>

<ref name=merrill>{{cite journal|bibcode=1936PA.....44...62M|title=Long-period variable stars and the stellar system|journal=Popular Astronomy|volume=44|pages=62|last1=Merrill|first1=Paul W.|year=1936}}</ref>

<ref name=rosino>{{cite journal|bibcode=1951ApJ...113...60R|title=The Spectra of Variables of the RV Tauri and Yellow Semiregular Types|journal=Astrophysical Journal|volume=113|pages=60|last1=Rosino|first1=L.|year=1951|doi=10.1086/145377|doi-access=free}}</ref>

<ref name=smak>{{cite journal|bibcode=1966ARA&A...4...19S|title=The Long-Period Variable Stars|journal=Annual Review of Astronomy and Astrophysics|volume=4|pages=19–34|last1=Smak|first1=Józef I.|year=1966|doi=10.1146/annurev.aa.04.090166.000315}}</ref>

<ref name=merrill1960>{{cite journal|bibcode=1960ApJ...131..385M|title=Periods and Lights-Ranges of Long-Period Variable Stars|journal=Astrophysical Journal|volume=131|pages=385|last1=Merrill|first1=Paul W.|year=1960|doi=10.1086/146841|doi-access=free}}</ref>

<ref name=harrington>{{cite journal|bibcode=1965AJ.....70..569H|title=Variations in the maxima of long-period variables|journal=Astronomical Journal|volume=70|pages=569|last1=Harrington|first1=J. Patrick|year=1965|doi=10.1086/109783|doi-access=free}}</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|issue=3|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}}</ref>

<ref name=tuthill>{{Cite journal | last1 = Tuthill | first1 = P. G. | last2 = Haniff | first2 = C. A. | last3 = Baldwin | first3 = J. E. | doi = 10.1046/j.1365-8711.1999.02512.x | title = Surface imaging of long-period variable stars | journal = Monthly Notices of the Royal Astronomical Society | volume = 306 | issue = 2 | pages = 353 | year = 1999 |bibcode = 1999MNRAS.306..353T | doi-access = free }}</ref>

<ref name=osarg>{{Cite journal|arxiv=1303.7059|author1=Masaki Takayama|author2=Hideyuki Saio|author3=Yoshifusa Ita|title=On the pulsation modes of OGLE small amplitude red giant variables in the LMC|journal=Monthly Notices of the Royal Astronomical Society|volume=431|issue=4|pages=3189|year=2013|doi=10.1093/mnras/stt398|doi-access=free |bibcode = 2013MNRAS.431.3189T }}</ref>

<ref name=aavso>{{cite web|title=AAVSO Observing Sections|url=https://www.aavso.org/observers#sections|access-date=August 3, 2016}}</ref>

<ref name=wood>{{cite journal|bibcode=2000PASA...17...18W|title=Variable Red Giants in the LMC: Pulsating Stars and Binaries?|journal=Publications of the Astronomical Society of Australia|volume=17|issue=1|pages=18–21|last1=Wood|first1=P. R.|year=2000|doi=10.1071/AS00018|doi-access=free}}</ref>

}}

==External links==
* [http://www.sai.msu.su/gcvs/gcvs/gcvs5/vartype.htm GCVS Variability Types]
* [https://sites.google.com/site/aavsolpvsection/Home AAVSO Long Period Variable Section]
* [http://ogle.astrouw.edu.pl/atlas/LPVs.html OGLE Atlas of Variable Star Light Curves - Long Period Variables]

{{Variable star topics}}

[[Category:Pulsating variables]]