Editing Stellar corona (section)

==Stellar coronae==
Coronal stars are ubiquitous among the stars in the cool half of the [[Hertzsprung–Russell diagram]].<ref name=Gudel>{{cite journal |author=Güdel M |title=X-ray astronomy of stellar coronae |journal=The Astronomy and Astrophysics Review |year=2004 |volume=12 |issue=2–3 |pages=71–237 |doi=10.1007/s00159-004-0023-2 |url=http://astronomy.sci.ege.edu.tr/~rpekunlu/GBDG/papers/XRayfromStellarCoronae.pdf |arxiv=astro-ph/0406661 |bibcode=2004A&ARv..12...71G |s2cid=119509015 |archive-url=https://web.archive.org/web/20110811230835/http://astronomy.sci.ege.edu.tr/~rpekunlu/GBDG/papers/XRayfromStellarCoronae.pdf |archive-date=2011-08-11 }}</ref> These coronae can be detected using [[X-ray telescope]]s. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. For example, [[FK Comae Berenices]] is the prototype for the FK Com class of [[variable star]]. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (''L''<sub>x</sub> ≥ 10<sup>32</sup> erg·s<sup>−1</sup> or 10<sup>25</sup>W) and the hottest known with dominant temperatures up to 40 MK.<ref name=Gudel/>

The astronomical observations planned with the [[Einstein Observatory]] by Giuseppe Vaiana and his group<ref name=Vaianaetal81>{{cite journal |doi= 10.1086/158797 |last=Vaiana |first= G.S. |title= Results from an extensive Einstein stellar survey |journal= The Astrophysical Journal|year=1981 |volume=245 |page=163 |bibcode=1981ApJ...245..163V|display-authors=etal}}</ref> showed that F-, G-, K- and M-stars have chromospheres and often coronae much like the Sun.
The ''O-B stars'', which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronae, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs.
Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae.