Editing White dwarf (section)

=== Atmosphere and spectra ===
[[File:Artist’s impression of the WDJ0914+1914 system.tif|thumb|Artist's impression of the [[WD J0914+1914]] system<ref>{{cite web |title=First Giant Planet around White Dwarf Found – ESO observations indicate the Neptune-like exoplanet is evaporating |url=https://www.eso.org/public/news/eso1919/ |website=www.eso.org |access-date=4 December 2019 |language=en |archive-url=https://web.archive.org/web/20191204214723/https://www.eso.org/public/news/eso1919/ |archive-date=4 December 2019 |url-status=live}}</ref>]]

Although most white dwarfs are thought to be composed of carbon and oxygen, [[spectroscopy]] typically shows that their emitted light comes from an atmosphere that is observed to be either hydrogen or [[helium]] dominated. The dominant element is usually at least 1000&nbsp;times more abundant than all other elements. As explained by [[Evry Schatzman|Schatzman]] in the 1940s, the high [[surface gravity]] is thought to cause this purity by gravitationally separating the atmosphere so that heavy elements are below and the lighter above.<ref>
{{cite journal
 |last1=Schatzman |first1=E.
 |date=1945
 |title=Théorie du débit d'énergie des naines blanches
 |volume=8 |page=143
 |journal=Annales d'Astrophysique
 |bibcode=1945AnAp....8..143S
}}</ref><ref name="physrev">
{{cite journal
 |last1=Koester |first1=D.
 |last2=Chanmugam |first2=G.
 |date=1990
 |title=Physics of white dwarf stars
 |journal=Reports on Progress in Physics
 |volume=53 |issue=7 |pages=837–915
 |bibcode=1990RPPh...53..837K
 |doi= 10.1088/0034-4885/53/7/001
 |s2cid=122582479
}}</ref>{{rp|§§5–6}} This atmosphere, the only part of the white dwarf visible to us, is thought to be the top of an envelope that is a residue of the star's envelope in the [[asymptotic giant branch|AGB]] phase and may also contain material accreted from the [[interstellar medium]]. The envelope is believed to consist of a helium-rich layer with mass no more than {{frac|1|100}} of the star's total mass, which, if the atmosphere is hydrogen-dominated, is overlain by a hydrogen-rich layer with mass approximately {{frac|1|{{val|10000}}}} of the star's total mass.<ref name="wden" /><ref name="kawaler"/>{{rp|§§4–5}}

Although thin, these outer layers determine the thermal evolution of the white dwarf. The degenerate electrons in the bulk of a white dwarf conduct heat well. Most of a white dwarf's mass is therefore at almost the same temperature ([[isothermal]]), and it is also hot: a white dwarf with surface temperature between {{val|8000|u=K}} and {{val|16000|u=K}} will have a core temperature between approximately {{val|5000000|u=K}} and {{val|20000000|u=K}}. The white dwarf is kept from cooling very quickly only by its outer layers' opacity to radiation.<ref name="wden" />

{| class="wikitable" style="float: right"
|+ White dwarf spectral types<ref name="villanovar4" />
|-
! colspan="2" | Primary and secondary features
|-
| A
| H lines present
|-
| B
| He I lines
|-
| C
| Continuous spectrum; no lines
|-
| O
| He II lines, accompanied by He I or H lines
|-
| Z
| Metal lines
|-
| Q
| Carbon lines present
|-
| X
| Unclear or unclassifiable spectrum
|-
! colspan="2" | Secondary features only
|-
| P
| Magnetic white dwarf with detectable polarization
|-
| H
| Magnetic white dwarf without detectable polarization
|-
| E
| Emission lines present
|-
| V
| Variable
|}
The first attempt to [[Stellar classification#White dwarf classifications|classify white dwarf spectra]] appears to have been by [[G. P. Kuiper]] in 1941,<ref name="sionspectra" /><ref>
{{cite journal
 |last1=Kuiper |first1=G. P.
 |date=1941
 |title=List of Known White Dwarfs
 |journal=Publications of the Astronomical Society of the Pacific
 |volume=53 |issue=314
 |page=248
 |bibcode=1941PASP...53..248K
 |doi= 10.1086/125335
 |doi-access=free
}}</ref> and various classification schemes have been proposed and used since then.<ref>
{{cite journal
 |last1=Luyten |first1=W. J.
 |date=1952
 |title=The Spectra and Luminosities of White Dwarfs
 |journal=The Astrophysical Journal
 |volume=116 |page=283
 |bibcode=1952ApJ...116..283L
 |doi= 10.1086/145612
}}</ref><ref>
{{cite book
 |last1=Greenstein |first1=J. L.
 |date=1960
 |title=Stellar atmospheres
 |url=https://archive.org/details/stellaratmospher0000gree |url-access=registration
 |publisher=[[University of Chicago Press]]
 |bibcode=1960stat.book.....G
 |lccn=61-9138
}}</ref> The system currently in use was introduced by [[Edward M. Sion]], Jesse L. Greenstein and their coauthors in 1983 and has been subsequently revised several times. It classifies a spectrum by a symbol that consists of an initial D, a letter describing the primary feature of the spectrum followed by an optional sequence of letters describing secondary features of the spectrum (as shown in the adjacent table), and a temperature index number, computed by dividing {{val|50400|u=K}} by the [[effective temperature]]. For example, a white dwarf with only [[Spectroscopic notation|He I]] lines in its spectrum and an effective temperature of {{val|15000|u=K}} could be given the classification of DB3, or, if warranted by the precision of the temperature measurement, DB3.5. Likewise, a white dwarf with a polarized [[magnetic field]], an effective temperature of {{val|17000|u=K}}, and a spectrum dominated by [[Spectroscopic notation|He I]] lines that also had hydrogen features could be given the classification of DBAP3. The symbols "?" and ":" may also be used if the correct classification is uncertain.<ref name="villanovar4" /><ref name="sionspectra" />

White dwarfs whose primary spectral classification is DA have hydrogen-dominated atmospheres. They make up the majority, approximately 80%, of all observed white dwarfs.<ref name="wden" /> The next class in number is of DBs, approximately 16%.<ref name="sdsswd" /> The hot, above {{val|15000|u=K}}, DQ class (roughly 0.1%) have carbon-dominated atmospheres.<ref>
{{cite journal
 |last1=Dufour |first1=P.
 |last2=Liebert |first2=James
 |last3=Fontaine |first3=G.
 |last4=Behara |first4=N.
 |date=2007
 |title=White dwarf stars with carbon atmospheres
 |journal=Nature
 |volume=450 |issue=7169 |pages=522–4|pmid=18033290
 |bibcode=2007Natur.450..522D
 |doi=10.1038/nature06318
 |arxiv = 0711.3227 |s2cid=4398697
}}</ref> Those classified as DB, DC, DO, DZ, and cool DQ have helium-dominated atmospheres. Assuming that carbon and metals are not present, which spectral classification is seen depends on the effective temperature. Between approximately {{val|100000|u=K}} to {{val|45000|u=K}}, the spectrum will be classified DO, dominated by singly ionized helium. From {{val|30000|u=K}} to {{val|12000|u=K}}, the spectrum will be DB, showing neutral helium lines, and below about {{val|12000|u=K}}, the spectrum will be featureless and classified DC.<ref name="kawaler"/>{{rp|§2.4}}<ref name="wden" />

[[Molecules in stars|Molecular]] hydrogen ([[Molecular hydrogen|H<sub>2</sub>]]) has been detected in spectra of the atmospheres of some white dwarfs.<ref name="dwarf">{{cite journal |bibcode=2013ApJ...766L..18X |title=Discovery of Molecular Hydrogen in White Dwarf Atmospheres |last1=Xu |first1=S. |last2=Jura |first2=M. |last3=Koester |first3=D. |last4=Klein |first4=B. |last5=Zuckerman |first5=B. |journal=The Astrophysical Journal |year=2013 |volume=766 |issue=2 |pages=L18 |doi=10.1088/2041-8205/766/2/L18 |arxiv=1302.6619 |s2cid=119248244 }}</ref> While theoretical work suggests that some types of white dwarfs may have [[stellar corona]], searches at X-ray and radio wavelengths, where coronae are most easily detected, have been unsuccessful.<ref name=Weisskopf2007>{{cite journal |last1=Weisskopf |first1=Martin C. |last2=Wu |first2=Kinwah |last3=Trimble |first3=Virginia |last4=O’Dell |first4=Stephen L. |last5=Elsner |first5=Ronald F. |last6=Zavlin |first6=Vyacheslav E. |last7=Kouveliotou |first7=Chryssa |date=2007-03-10 |title=A Chandra Search for Coronal X-Rays from the Cool White Dwarf GD 356 |url=https://iopscience.iop.org/article/10.1086/510776 |journal=The Astrophysical Journal |language=en |volume=657 |issue=2 |pages=1026–1036 |doi=10.1086/510776 |issn=0004-637X|arxiv=astro-ph/0609585 |bibcode=2007ApJ...657.1026W }}</ref><ref name=Route2024>{{cite journal|last1=Route|first1=Matthew|title=The Decline and Fall of ROME. V. A Preliminary Search for Star-disrupted Planet Interactions and Coronal Activity at 5 GHz among White Dwarfs within 25 pc|journal=The Astrophysical Journal|date=20 December 2024|volume=977|issue=1|page=261|doi=10.3847/1538-4357/ad9567|arxiv=2411.13718|doi-access=free |bibcode=2024ApJ...977..261R }}</ref>

A few white dwarves have been observed to have inhomogeneous atmosphere with one side dominated by hydrogen and the other side dominated by helium.<ref>{{Cite journal |last1=Moss |first1=Adam |last2=Kilic |first2=Mukremin |last3=Bergeron |first3=Pierre |last4=Jewett |first4=Gracyn |last5=Brown |first5=Warren R. |date=2025-04-01 |title=The Emerging Class of Double-faced White Dwarfs |journal=The Astrophysical Journal |volume=983 |issue=1 |pages=14 |doi=10.3847/1538-4357/adbd3a |doi-access=free |issn=0004-637X|arxiv=2501.05649 }}</ref>

==== Metal-rich white dwarfs ====
[[File:Periodic Table White Dwarfs.png|thumb|Elements discovered in the atmosphere of white dwarfs colder than {{val|25000|u=K}}.]]
Around 25–33% of white dwarfs have metal lines in their spectra, which is notable because any heavy elements in a white dwarf should sink into the star's interior in just a small fraction of the star's lifetime.<ref name=":0">{{cite journal |title=Extrasolar Cosmochemistry |journal=Annual Review of Earth and Planetary Sciences |date=2014-01-01|pages=45–67 |volume=42 |issue=1 |doi=10.1146/annurev-earth-060313-054740 |first1=M. |last1=Jura |first2=E.D. |last2=Young |bibcode=2014AREPS..42...45J|doi-access=free }}</ref> The prevailing explanation for metal-rich white dwarfs is that they have recently accreted rocky [[planetesimal]]s.<ref name=":0" /> The bulk composition of the accreted object can be measured from the strengths of the metal lines. For example, a 2015 study of the white dwarf Ton 345 concluded that its metal abundances were consistent with those of a [[Planetary differentiation|differentiated]], rocky planet whose mantle had been eroded by the host star's wind during its [[asymptotic giant branch]] phase.<ref>{{cite journal |title=The composition of a disrupted extrasolar planetesimal at SDSS J0845+2257 (Ton 345) |journal = Monthly Notices of the Royal Astronomical Society |date=2015-08-11|pages=3237–3248 |volume=451 |issue=3 |doi=10.1093/mnras/stv1201 |language=en |first1=D.J. |last1=Wilson |first2=B.T. |last2=Gänsicke |first3=D. |last3=Koester |first4=O. |last4=Toloza |first5=A. F. |last5=Pala |first6=E. |last6=Breedt |first7=S.G. |last7=Parsons |doi-access = free |arxiv=1505.07466 |bibcode=2015MNRAS.451.3237W|s2cid=54049842 }}</ref>