Editing Arcturus (section)

===Spectrum===
Arcturus has [[stellar evolution|evolved]] off the main sequence to the [[red giant branch]], reaching an [[early-type star|early]] [[k-type star|K-type]] [[stellar classification]]. It is frequently assigned the spectral type of K0III,<ref name=gray/> but in 1989 was used as the spectral standard for type K1.5III Fe−0.5,<ref name=perkins/> with the suffix notation indicating a mild underabundance of iron compared to typical stars of its type. As the brightest K-type [[Giant star#Red giants|giant]] in the sky, it has been the subject of multiple [[spectral atlas|atlases]] with coverage from the [[ultraviolet]] to [[infrared]].<ref name=griffin1968/><ref name=hinckle2005/>

The spectrum shows a dramatic transition from [[emission line]]s in the ultraviolet to atomic [[absorption line]]s in the visible range and molecular absorption lines in the infrared. This is due to the optical depth of the atmosphere varying with wavelength.<ref name=hinckle2005/> The spectrum shows very strong absorption in some molecular lines that are not produced in the [[photosphere]] but in a surrounding shell.<ref name=tsuji2009/> Examination of [[carbon monoxide]] lines show the molecular component of the atmosphere extending outward to 2–3 times the radius of the star, with the [[stellar wind|chromospheric wind]] steeply accelerating to 35–40&nbsp;km/s in this region.<ref name="Ohnaka_Morales_Marín_2018"/>

Astronomers term "metals" those elements with higher [[atomic number]]s than [[helium]]. The atmosphere of Arcturus has an enrichment of [[alpha elements]] relative to [[iron]] but only about a third of solar [[metallicity]]. Arcturus is possibly a [[Population II star]].<ref name=schaaf/>