Editing Stellar classification (section)

===Late giant carbon-star classes===
Carbon-stars are stars whose spectra indicate production of carbon – a byproduct of [[Triple-alpha process|triple-alpha]] helium fusion. With increased carbon abundance, and some parallel [[s-process]] heavy element production, the spectra of these stars become increasingly deviant from the usual late spectral classes&nbsp;G, K, and M. Equivalent classes for carbon-rich stars are S and C.

The giants among those stars are presumed to produce this carbon themselves, but some stars in this class are double stars, whose odd atmosphere is suspected of having been transferred from a companion that is now a white dwarf, when the companion was a carbon-star.

====Class C====
{{Main|Carbon star}}
[[File:Curious spiral spotted by ALMA around red giant star R Sculptoris (data visualisation).jpg|thumb|Image of the carbon star [[R Sculptoris]] and its striking spiral structure]]
Originally classified as R and N&nbsp;stars, these are also known as ''carbon stars''. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid-G to late M. These have more recently been remapped into a unified carbon classifier C with N0 starting at roughly C6. Another subset of cool carbon stars are the C–J-type stars, which are characterized by the strong presence of molecules of [[Carbon-13|<sup>13</sup>]] [[cyanide|CN]] in addition to those of [[Carbon-12|<sup>12</sup>]] [[cyanide|CN]].<ref>Bouigue, R. (1954). Annales d'Astrophysique, Vol.&nbsp;17, p.&nbsp;104</ref> A few main-sequence carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants. There are several subclasses:
* C-R – Formerly its own class (''R'') representing the carbon star equivalent of late G- to early K-type stars.
* C-N – Formerly its own class representing the carbon star equivalent of late K- to M-type stars.
* C-J – A subtype of cool C stars with a high content of [[Carbon-13|<sup>13</sup>C]].
* C-H – [[Population II]] analogues of the C-R stars.
* C-Hd – Hydrogen-deficient carbon stars, similar to late G supergiants with [[Methylidyne radical|CH]] and [[diatomic carbon|C<sub>2</sub>]] bands added.

====Class S====
{{Main|S-type star}}
Class&nbsp;S stars form a continuum between class&nbsp;M stars and carbon stars. Those most similar to class&nbsp;M stars have strong [[Zirconium monoxide|ZrO]] [[spectral bands|absorption bands]] analogous to the [[Titanium monoxide|TiO]] bands of class&nbsp;M stars, whereas those most similar to carbon stars have strong [[sodium]] D lines and weak [[Diatomic carbon|C<sub>2</sub>]] bands.<ref>{{cite journal |last=Keenan |first=P. C. |year=1954 |title=Classification of the S-Type Stars |journal=Astrophysical Journal |volume=120 |page=484 |doi=10.1086/145937 |bibcode=1954ApJ...120..484K}}</ref> Class&nbsp;S stars have excess amounts of [[zirconium]] and other elements produced by the [[s-process]], and have more similar carbon and oxygen abundances to class&nbsp;M or carbon stars. Like carbon stars, nearly all known class&nbsp;S stars are [[asymptotic giant branch|asymptotic-giant-branch]] stars.

The spectral type is formed by the letter&nbsp;S and a number between zero and ten. This number corresponds to the temperature of the star and approximately follows the temperature scale used for class&nbsp;M giants. The most common types are S3 to S5. The non-standard designation S10 has only been used for the star [[Chi Cygni]] when at an extreme minimum.

The basic classification is usually followed by an abundance indication, following one of several schemes: S2,5; S2/5; S2 Zr4 Ti2; or S2*5. A number following a comma is a scale between 1 and 9 based on the ratio of ZrO and TiO. A number following a slash is a more-recent but less-common scheme designed to represent the ratio of carbon to oxygen on a scale of 1 to 10, where a 0 would be an MS star. Intensities of zirconium and [[titanium]] may be indicated explicitly. Also occasionally seen is a number following an asterisk, which represents the strength of the ZrO bands on a scale from 1 to 5.

====Classes MS and SC: Intermediate carbon-related classes====
In between the M and S classes, border cases are named MS stars. In a similar way, border cases between the S and C-N classes are named SC or CS. The sequence M → MS → S → SC → C-N is hypothesized to be a sequence of increased carbon abundance with age for [[carbon star]]s in the [[asymptotic giant branch]].