Editing Stellar classification (section)

===White dwarf classifications{{anchor|Class D}}===
The class&nbsp;D (for [[Electron-degenerate matter|Degenerate]]) is the modern classification used for white dwarfs—low-mass stars that are no longer undergoing [[nuclear fusion]] and have shrunk to planetary size, slowly cooling down. Class&nbsp;D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere.

The white dwarf types are as follows:<ref name="sionspectra">{{cite journal |bibcode=1983ApJ...269..253S |title=A proposed new white dwarf spectral classification system |journal=Astrophysical Journal |volume=269 |pages=253 |last1=Sion |first1=E. M. |last2=Greenstein |first2=J. L. |last3=Landstreet |first3=J. D. |last4=Liebert |first4=James |last5=Shipman |first5=H. L. |last6=Wegner |first6=G. A. |year=1983 |doi=10.1086/161036|doi-access=free }}</ref><ref>{{Cite journal |doi=10.1051/0004-6361:20041372 |title=The rate of period change in pulsating DB-white dwarf stars |year=2004 |last1=Córsico |first1=A. H. |last2=Althaus |first2=L. G. |journal=Astronomy and Astrophysics |volume=428 |pages=159–170 |arxiv=astro-ph/0408237 |bibcode=2004A&A...428..159C|s2cid=14653913 }}</ref>
* DA – a [[hydrogen]]-rich atmosphere or outer layer, indicated by strong Balmer hydrogen [[spectral line]]s.
* DB – a [[helium]]-rich atmosphere, indicated by neutral helium, [[spectroscopic notation|He&nbsp;I]], spectral lines.
* DO – a helium-rich atmosphere, indicated by ionized helium, [[spectroscopic notation|He&nbsp;II]], spectral lines.
* DQ – a [[carbon]]-rich atmosphere, indicated by atomic or molecular carbon lines.
* DZ – a [[metal (astronomy)|metal]]-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK, and DM).
* DC – no strong spectral lines indicating one of the above categories.
* DX – spectral lines are insufficiently clear to classify into one of the above categories.

The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/''T''<sub>eff</sub>, where ''T''<sub>eff</sub> is the [[effective temperature|effective surface temperature]], measured in [[kelvin]]s. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.(For example DA1.5 for IK Pegasi B)<ref name="sionspectra"/><ref name="villanovar4">{{cite journal |bibcode=1999ApJS..121....1M |title=A Catalog of Spectroscopically Identified White Dwarfs |journal=The Astrophysical Journal Supplement Series |volume=121 |issue=1 |pages=1–130 |last1=McCook |first1=George P. |last2=Sion |first2=Edward M. |year=1999 |doi=10.1086/313186 |citeseerx=10.1.1.565.5507|s2cid=122286998 }}</ref>

Two or more of the type letters may be used to indicate a white dwarf that displays more than one of the spectral features above.<ref name="sionspectra"/>

==== Extended white dwarf spectral types ====
[[File:Sirius A and B Hubble photo.jpg|thumb|[[Sirius]] A and B (a [[white dwarf]] of type DA2) resolved by [[Hubble Space Telescope|Hubble]]]]
* DAB – a hydrogen- and helium-rich white dwarf displaying neutral helium lines
* DAO – a hydrogen- and helium-rich white dwarf displaying ionized helium lines
* DAZ – a hydrogen-rich metallic white dwarf
* DBZ – a helium-rich metallic white dwarf

A different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars:<ref name="sionspectra"/>

{| class="wikitable"
|-
! Code
! Spectral peculiarities for stars
|-
! P
| Magnetic white dwarf with detectable polarization
|-
! E
| Emission lines present
|-
! H
| Magnetic white dwarf without detectable polarization
|-
! V
| Variable
|-
! PEC
| Spectral peculiarities exist
|}