Editing White dwarf (section)

== Variability ==
{{Main|Pulsating white dwarf}}
{{Seealso|Cataclysmic variables}}

{| class="wikitable" style="float: right"
|+ Types of pulsating white dwarf<ref>
 {{cite web
  |title=ZZ Ceti variables
  |publisher=Association Française des Observateurs d'Etoiles Variables
  |website=Centre deDonnées astronomiques de Strasbourg
  |url=http://cdsweb.u-strasbg.fr/afoev/var/ezz.htx
  |access-date=6 June 2007
  |archive-url=https://web.archive.org/web/20070205132930/http://cdsweb.u-strasbg.fr/afoev/var/ezz.htx
  |archive-date=5 February 2007
 }}
</ref><ref name="quirion" />{{rp|§§1.1, 1.2}}
|-
| '''DAV''' ([[General Catalog of Variable Stars|GCVS]]: ''ZZA'') || DA [[#Atmosphere and spectra|spectral type]], having only hydrogen [[absorption line]]s in its spectrum
|-
| '''DBV''' (GCVS: ''ZZB'') || DB spectral type, having only [[helium]] absorption lines in its spectrum
|-
| '''GW Vir''' (GCVS: ''ZZO'') || Atmosphere mostly C, He and O; may be divided into '''DOV''' and '''PNNV''' stars
|}

Early calculations suggested that there might be white dwarfs whose luminosity [[variable star|varied]] with a period of around 10 seconds, but searches in the 1960s failed to observe this.<ref name="physrev" />{{rp|§7.1.1}}<ref>
{{cite journal
 |last1=Lawrence |first1=G. M.
 |last2=Ostriker |first2=J. P.
 |last3=Hesser |first3=J. E.
 |date=1967
 |title=Ultrashort-Period Stellar Oscillations. I. Results from White Dwarfs, Old Novae, Central Stars of Planetary Nebulae, 3c 273, and Scorpius XR-1
 |journal=The Astrophysical Journal
 |volume=148 |pages=L161
 |bibcode=1967ApJ...148L.161L
 |doi= 10.1086/180037
}}</ref> The first variable white dwarf found was [[HL Tau 76]]; in 1965 and 1966, and was observed to vary with a period of approximately 12.5&nbsp;minutes.<ref>
{{cite journal
 |last1=Landolt |first1=A. U.
 |date=1968
 |title=A New Short-Period Blue Variable
 |journal=The Astrophysical Journal
 |volume=153 |page=151
 |bibcode=1968ApJ...153..151L
 |doi= 10.1086/149645 |doi-access=free
}}</ref> The reason for this period being longer than predicted is that the variability of HL Tau 76, like that of the other pulsating variable white dwarfs known, arises from non-radial [[gravity wave]] pulsations.<ref name="physrev" />{{rp|§7}} Known types of pulsating white dwarf include the ''DAV'', or ''ZZ Ceti'', stars, including HL Tau 76, with hydrogen-dominated atmospheres and the spectral type DA;<ref name="physrev" />{{rp|891, 895}} ''DBV'', or ''V777&nbsp;Her'', stars, with helium-dominated atmospheres and the spectral type DB;<ref name="wden" />{{rp|3525}} and ''[[GW Vir stars]]'', sometimes subdivided into ''DOV'' and ''PNNV'' stars, with atmospheres dominated by helium, carbon, and oxygen.<ref name="quirion">
{{cite journal
 |last1=Quirion |first1=P.-O.
 |last2=Fontaine |first2=G.
 |last3=Brassard |first3=P.
 |date=2007
 |title=Mapping the Instability Domains of GW&nbsp;Vir Stars in the Effective Temperature–Surface Gravity Diagram
 |journal=The Astrophysical Journal Supplement Series
 |volume=171 |issue=1
 |pages=219–248
 |bibcode=2007ApJS..171..219Q
 |doi= 10.1086/513870
 |doi-access=free
}}</ref><ref>
{{cite journal
 |last1=Nagel |first1=T.
 |last2=Werner |first2=K.
 |date=2004
 |title=Detection of non-radial g-mode pulsations in the newly discovered PG&nbsp;1159 star HE&nbsp;1429-1209
 |journal=Astronomy and Astrophysics
 |volume=426 |issue=2 |pages=L45
 |arxiv= astro-ph/0409243
 |doi= 10.1051/0004-6361:200400079
 |bibcode=2004A&A...426L..45N
 |s2cid=9481357
}}</ref> GW Vir stars are not, strictly speaking, white dwarfs, but are stars that are in a position on the [[Hertzsprung–Russell diagram]] between the asymptotic giant branch and the white dwarf region. They may be called ''pre-white dwarfs''.<ref name="quirion" /><ref>
{{cite journal
 |last1=O'Brien |first1=M. S.
 |date=2000
 |title=The Extent and Cause of the Pre–White Dwarf Instability Strip
 |journal=The Astrophysical Journal
 |volume=532 |issue=2 |pages=1078–1088
 |arxiv= astro-ph/9910495
 |bibcode=2000ApJ...532.1078O
 |doi= 10.1086/308613
 |s2cid=115958740
}}</ref> These variables all exhibit small (1%–30%) variations in light output, arising from a superposition of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations gives [[asteroseismology|asteroseismological]] evidence about the interiors of white dwarfs.<ref>
{{cite journal
 |last1=Winget |first1=D. E.
 |title=Asteroseismology of white dwarf stars
 |date=1998
 |journal=Journal of Physics: Condensed Matter
 |volume=10 |issue=49 |pages=11247–11261
 |bibcode= 1998JPCM...1011247W
 |doi=10.1088/0953-8984/10/49/014
 |s2cid=250749380
}}</ref>