Editing Canopus (section)

==Spectrum==
Canopus was little-studied by western scientists before the 20th century.  It was given a spectral class of F in 1897, an early use of this extension to [[Secchi class]] I, applied to those stars where the hydrogen lines are relatively weak and the [[calcium K line]] relatively strong.<ref>{{cite journal |bibcode=1897ApJ.....6..349P |title=Spectra of bright southern stars |last1=Pickering |first1=E. C. |last2=Cannon |first2=A. J. |journal=The Astrophysical Journal |year=1897 |volume=6 |page=349 |doi=10.1086/140407 |doi-access=free }}</ref>  It was given as a standard star of F0 in the [[Henry Draper Catalogue]], with the spectral type F0 described as having hydrogen lines half the strength of an A0 star and the calcium K line three times as strong as Hδ.<ref name=hd>{{cite journal |bibcode=1918AnHar..92....1C |title=The Henry Draper catalogue : 4h, 5h and 6h |last1=Cannon |first1=Annie Jump |last2=Pickering |first2=Edward Charles |journal=Annals of Harvard College Observatory |year=1918 |volume=92 |page=1 }}</ref> American astronomer [[Jesse L. Greenstein|Jesse Greenstein]] was interested in stellar spectra and used the newly built [[Otto Struve Telescope]] at [[McDonald Observatory]] to analyze the star's spectrum in detail.<ref>{{cite book |last1=Trimble |first1=Virginia | first2= Thomas R. |last2=Williams | first3= Katherine | last3=Bracher| first4= Richard | last4=Jarrell| first5=Jordan D.| last5= Marché| first6=F. Jamil | last6=Ragep |title=Biographical Encyclopedia of Astronomers |date=2007 |publisher=Springer Science & Business Media |location=New York, New York |isbn=978-0-387-30400-7 |page=438 |url=https://books.google.com/books?id=t-BF1CHkc50C&pg=PA438}}</ref> In a 1942 paper, he reported that the spectrum is dominated by strong broad hydrogen lines.  There are also [[absorption line]]s of carbon, nitrogen, oxygen, sulphur, iron, and many ionised metals.<ref name=greenstein1942>{{cite journal |bibcode=1942ApJ....95..161G |title=The Spectrum of α Carinae |last1=Greenstein |first1=Jesse L. |journal=The Astrophysical Journal |year=1942 |volume=95 |page=161 |doi=10.1086/144382 |doi-access=free }}</ref> It was studied in the [[ultraviolet]] by an early astronomical satellite, [[Gemini XI]] in 1966.  The UV spectra were considered to be consistent with an F0 [[Supergiant star|supergiant]] having a temperature of {{val|6,900|fmt=commas|u=K}}, the accepted parameters for Canopus at the time.<ref name=kondo1970>{{cite journal |bibcode=1970ApJ...159..927K |title=Ultraviolet Spectrophotometry of Canopus from Gemini XI |last1=Kondo |first1=Y. |last2=Henize |first2=K. G. |last3=Kotila |first3=C. L. |journal=The Astrophysical Journal |year=1970 |volume=159 |page=927 |doi=10.1086/150370 |doi-access=free }}</ref> New Zealand-based astronomers [[John Hearnshaw]] and Krishna Desikachary examined the spectrum in greater detail, publishing their results in 1982.<ref>{{cite journal |title=The spectrum of Canopus | last1= Hearnshaw | first1= J. B.| last2= Desikachary | first2= K. | journal= Monthly Notices of the Royal Astronomical Society |  volume= 198 | year= 1982 | issue= 2 | pages= 311–320 | doi= 10.1093/mnras/198.2.311 | bibcode=1982MNRAS.198..311H| doi-access= free }}</ref><ref>{{cite journal |title=The spectrum of Canopus II - Analysis and composition | last1= Hearnshaw | first1= J. B.| last2= Desikachary | first2= K. | journal= Monthly Notices of the Royal Astronomical Society |  volume= 201 | year= 1982 | issue= 3 | pages= 707–721 | doi= 10.1093/mnras/201.3.707 | bibcode=1982MNRAS.201..707D| doi-access= free }}</ref>

When [[luminosity class]]es were added to the MK spectral classification scheme, Canopus was assigned class Iab indicating an intermediate luminosity supergiant.  This was based on the relative strengths of certain spectral lines understood to be sensitive to the luminosity of a star.<ref name=devaucoleurs>{{cite journal|title=Spectral types and luminosities of B, A and F southern stars|author=de Vaucouleurs, A.|journal=Monthly Notices of the Royal Astronomical Society|volume=117|page=449|date=1957|issue=4|bibcode=1957MNRAS.117..449D|doi=10.1093/mnras/117.4.449|doi-access=free}}</ref> In the [[Bright Star Catalogue]] 5th edition it is given the spectral class F0II, the luminosity class indicating a [[bright giant]].<ref name=bsc>{{cite book |bibcode=1991bsc..book.....H |year=1991 |title=The Bright star catalogue |last1=Hoffleit |first1=Dorrit |last2=Jaschek |first2=Carlos }}</ref>  [[Balmer line]] profiles and oxygen line strengths indicate the size and luminosity of Canopus.<ref name=kovtyukh2012>{{cite journal |bibcode=2012MNRAS.423.3268K |title=Accurate luminosities from the oxygen λ7771-4 Å triplet and the fundamental parameters of F-G supergiants |last1=Kovtyukh |first1=V. V. |last2=Gorlova |first2=N. I. |last3=Belik |first3=S. I. |journal=Monthly Notices of the Royal Astronomical Society |year=2012 |volume=423 |issue=4 |page=3268 |doi=10.1111/j.1365-2966.2012.21117.x |doi-access=free |arxiv=1204.4115 |s2cid=118683158 }}</ref>

When the effects of stellar rotation speed on spectral lines are accounted for, the MK spectral class of Canopus is adjusted to A9II.<ref name=perkins/>  Its spectrum consists mostly of absorption lines on a visible [[continuous spectrum|continuum]], but some emission has been detected.  For example, the [[calcium K line]] has weak emission wings on each side of the strong central absorption line, first observed in 1966.  The emission line profiles are usually correlated with the luminosity of the star as described by the [[Wilson-Bappu effect]], but in the case of Canopus they indicate a luminosity much lower than that calculated by other methods.<ref name=warner1966>{{cite journal|title=CA II emission in the spectrum of Canopus|journal=The Observatory |volume=86 |pages=82|date=April 1966|last=Warner |first=B. |bibcode=1966Obs....86...82W }}</ref>  More detailed observations have shown that the emission line profiles are variable and may be due to [[plage (astronomy)|plage]] areas on the surface of the star.  Emission can also be found in other lines such as the h and k lines of ionised magnesium.<ref name=bappu1984>{{cite journal |bibcode=1984BASI...12..196B |title=CA II K emission in Canopus |last1=Bappu |first1=M. K. V. |last2=Mekkaden |first2=M. V. |last3=Rao |first3=N. K. |journal=Bulletin of the Astronomical Society of India |year=1984 |volume=12 |page=196 }}</ref>