Editing Stellar classification (section)

===Class O===
{{Main|O-type star}}
{{See also|O-type main-sequence star|Blue giant|Blue supergiant}}
[[File:O5V star spectrum.png|thumb|Spectrum of a hypothetical O5V star]]
O-type stars are very hot and extremely luminous, with most of their radiated output in the [[ultraviolet]] range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the [[solar neighborhood]] are O-type stars.<ref group=lower-alpha name="proportions">These proportions are fractions of stars brighter than absolute magnitude&nbsp;16; lowering this limit will render earlier types even rarer, whereas generally adding only to the M class.  The proportions are calculated ignoring the value of 800 in the total column since the actual numbers add up to 824.</ref><ref name="LeDrew2001">{{cite journal |title=The Real Starry Sky |journal=Journal of the Royal Astronomical Society of Canada |last=Ledrew |first=Glenn |volume=95 |page=32 |date=February 2001 |bibcode=2001JRASC..95...32L}}</ref> Some of the [[List of most massive stars|most massive stars]] lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult.

O-type spectra formerly were defined by the ratio of the strength of the [[Helium|He]]&nbsp;II λ4541 relative to that of He&nbsp;I λ4471, where λ is the radiation [[wavelength]]. Spectral type O7 was defined to be the point at which the two intensities are equal, with the He&nbsp;I line weakening towards earlier types. Type&nbsp;O3 was, by definition, the point at which said line disappears altogether, although it can be seen very faintly with modern technology. Due to this, the modern definition uses the ratio of the [[nitrogen]] line N&nbsp;IV λ4058 to N&nbsp;III λλ4634-40-42.<ref name="Walborn"/>

O-type stars have dominant lines of absorption and sometimes emission for [[helium|He]]&nbsp;II lines, prominent ionized ([[silicon|Si]]&nbsp;IV, [[oxygen|O]]&nbsp;III, [[nitrogen|N]]&nbsp;III, and [[carbon|C]]&nbsp;III) and neutral [[helium]] lines, strengthening from O5 to O9, and prominent hydrogen [[Balmer lines]], although not as strong as in later types. Higher-mass O-type stars do not retain extensive atmospheres due to the extreme velocity of their [[stellar wind]], which may reach 2,000&nbsp;km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the [[main sequence]].

When the MKK classification scheme was first described in 1943, the only subtypes of class&nbsp;O used were O5 to O9.5.<ref>[http://nedwww.ipac.caltech.edu/level5/ASS_Atlas/frames.html ''An atlas of stellar spectra, with an outline of spectral classification''], W. W. Morgan, P. C. Keenan and E. Kellman, Chicago: The University of Chicago Press, 1943.</ref> The MKK scheme was extended to O9.7 in 1971<ref name=walborn>{{Cite journal |last1=Walborn |first1=N. R. |doi=10.1086/190239 |title=Some Spectroscopic Characteristics of the OB Stars: An Investigation of the Space Distribution of Certain OB Stars and the Reference Frame of the Classification |journal=The Astrophysical Journal Supplement Series |volume=23 |pages=257 |year=1971 |bibcode=1971ApJS...23..257W|doi-access=free }}</ref> and O4 in 1978,<ref>{{cite journal |bibcode=1978rmsa.book.....M |title=Revised MK Spectral Atlas for stars earlier than the sun |journal=Williams Bay: Yerkes Observatory |last1=Morgan |first1=W. W. |last2=Abt |first2=Helmut A. |last3=Tapscott |first3=J. W. |year=1978}}</ref> and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.<ref>{{cite journal |doi=10.1086/339831 |bibcode=2002AJ....123.2754W |title=A New Spectral Classification System for the Earliest O Stars: Definition of Type O2 |journal=The Astronomical Journal |volume=123 |issue=5 |pages=2754–2771 |year=2002 |last1=Walborn |first1=Nolan R. |last2=Howarth |first2=Ian D. |last3=Lennon |first3=Daniel J. |last4=Massey |first4=Philip |last5=Oey |first5=M. S. |last6=Moffat |first6=Anthony F. J. |last7=Skalkowski |first7=Gwen |last8=Morrell |first8=Nidia I. |last9=Drissen |first9=Laurent|last10=Parker|first10=Joel Wm.|s2cid=122127697 |url=http://discovery.ucl.ac.uk/9632/1/9632.pdf }}</ref>

'''Example spectral standards:'''<ref name="Garrison">{{cite book |last=Garrison |first=R. F. |author-link=Robert F. Garrison |editor-last=Corbally |editor-first=C. J. |editor-last2=Gray |editor-first2=R. O. |editor-last3=Garrison |editor-first3=R. F. |chapter=A Hierarchy of Standards for the MK Process |chapter-url=http://aspbooks.org/publications/60/3.pdf |title=The MK Process at 50 Years: A Powerful Tool for Astrophysical Insight |url=http://aspbooks.org/a/volumes/table_of_contents/?book_id=172 |url-access=subscription |series=Astronomical Society of the Pacific conference series |volume=60 |publisher=Astronomical Society of the Pacific |publication-place=San Francisco |year=1994 |isbn=978-1-58381-396-6 |oclc=680222523 |pages=3–14}}</ref>
* O7V – [[S Monocerotis]]
* O9V – [[10 Lacertae]]