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68 Cygni is a massive [[blue giant]] of [[stellar classification|spectral type]] O7.5IIIn((f)).[ Such massive stars only remain in the [[main sequence]] phase for a few million years, less than a thousandth of the expected main sequence lifetime of the [[sun]]. 68 Cygni has already expanded off the main sequence to become a blue giant, and is likely expanding to become a [[blue supergiant]], although for very hot stars such as 68 Cygni evolution to the giant and supergiant classes does not indicate exhaustion of core hydrogen, nor is it accompanied by a large increase in luminosity. Instead the products of fusion burning are distributed throughout the star by convection and rotational mixing, producing helium and nitrogen lines in the [[stellar spectrum|spectrum]], leading to an increase in the thickness of the stellar wind, and accompanied by a modest expansion and cooling of the star. The "n" in the spectral type indicates the presence of broad absorption lines due to how rapidly the star is spinning. The "((f))" indicates strong absorption in the He II [[spectral line]]s and weak N III [[emission line]]s.][{{Cite journal|title=Early-type stars in the core of the young open cluster Westerlund 2|journal=Astronomy & Astrophysics |volume=463 |issue=3 |pages=981–991 | doi = 10.1051/0004-6361:20066495|year=2007 |last1=Rauw |first1=G. |last2=Manfroid |first2=J. |last3=Gosset |first3=E. |last4=Nazé |first4=Y. |last5=Sana |first5=H. |last6=De Becker |first6=M. |last7=Foellmi |first7=C. |last8=Moffat |first8=A. F. J. |bibcode=2007A&A...463..981R |arxiv = astro-ph/0612622 |s2cid=17776145 }}] |
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68 Cygni is a massive [[blue giant]] of [[stellar classification|spectral type]] O7.5IIIn((f)).[ Such massive stars only remain in the [[main sequence]] phase for a few million years, less than a thousandth of the expected main sequence lifetime of the [[sun]]. 68 Cygni has already expanded off the main sequence to become a blue giant, and is likely expanding to become a [[blue supergiant]], although for very hot stars such as 68 Cygni evolution to the giant and supergiant classes does not indicate exhaustion of core hydrogen, nor is it accompanied by a large increase in luminosity. Instead the products of fusion burning are distributed throughout the star by convection and rotational mixing, producing helium and nitrogen lines in the [[stellar spectrum|spectrum]], leading to an increase in the thickness of the stellar wind, and accompanied by a modest expansion and cooling of the star. The "n" in the spectral type indicates the presence of broad absorption lines due to how rapidly the star is spinning. The "((f))" indicates strong absorption in the He II [[spectral line]]s and weak N III [[emission line]]s.][{{Cite journal|title=Early-type stars in the core of the young open cluster Westerlund 2|journal=Astronomy & Astrophysics |volume=463 |issue=3 |pages=981–991 | doi = 10.1051/0004-6361:20066495|year=2007 |last1=Rauw |first1=G. |last2=Manfroid |first2=J. |last3=Gosset |first3=E. |last4=Nazé |first4=Y. |last5=Sana |first5=H. |last6=De Becker |first6=M. |last7=Foellmi |first7=C. |last8=Moffat |first8=A. F. J. |bibcode=2007A&A...463..981R |arxiv = astro-ph/0612622 |s2cid=17776145 }}] |
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The star is has been tentatively classified as a [[rotating ellipsoidal variable]], varying between magnitudes 4.98 and 5.09.[{{cite web|url=http://www.aavso.org/vsx/index.php?view=about.vartypes|title=Variable Star Type Designations in the VSX|last1=Otero|first1=S. A. |last2=Watson |first2=C. |last3=Wils |first3=P. |work=AAVSO Website|publisher=[[American Association of Variable Star Observers]]|access-date=9 August 2014}}] However, the actual cause of the variability, and even whether or not there is a companion, was uncertain. No companion has ever been directly detected, but its existence has been inferred from spectral line variations (radial velocity variations) in the primary O-type star. A period of 5.1 days has been assigned for the orbit of the binary, which is approximately the same as the rotation period of the star. Unlike typical rotating ellipsoidal variables, there is no clearly defined period in the variations in brightness.[{{Cite journal|year=2009|title=A systematic study of variability among OB-stars based on HIPPARCOS photometry|journal=Astronomy and Astrophysics|volume=507|issue=2|pages= 1141–1201 | doi = 10.1051/0004-6361/200912304|last1=Lefèvre|first1=L.|last2=Marchenko|first2=S. V.|last3=Moffat|first3=A. F. J.|last4=Acker|first4=A.|bibcode=2009A&A...507.1141L|doi-access=free}}] Data derived from the assumption of a binary system, for example the mass, are highly uncertain due to the lack of information about the inclination or eccentricity of the orbit,[{{Cite journal|arxiv=1010.2204 |last1=Weidner |first1=Carsten |title=The masses, and the mass discrepancy of O-type stars |journal=Astronomy & Astrophysics |volume=524 |pages=A98 |last2=Vink |first2=Jorick |year=2010 |doi=10.1051/0004-6361/201014491 |bibcode = 2010A&A...524A..98W |s2cid=118836634 }}] |
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The star is has been tentatively classified as a [[rotating ellipsoidal variable]], varying between magnitudes 4.98 and 5.09.[{{cite web|url=http://www.aavso.org/vsx/index.php?view=about.vartypes|title=Variable Star Type Designations in the VSX|last1=Otero|first1=S. A. |last2=Watson |first2=C. |last3=Wils |first3=P. |work=AAVSO Website|publisher=[[American Association of Variable Star Observers]]|access-date=9 August 2014}}] However, the actual cause of the variability, and even whether or not there is a companion, was uncertain. No companion has ever been directly detected, but its existence has been inferred from spectral line variations (radial velocity variations) in the primary O-type star. A period of 5.1 days has been assigned for the orbit of the binary, which is approximately the same as the rotation period of the star. Unlike typical rotating ellipsoidal variables, there is no clearly defined period in the variations in brightness.[{{Cite journal|year=2009|title=A systematic study of variability among OB-stars based on HIPPARCOS photometry|journal=Astronomy and Astrophysics|volume=507|issue=2|pages= 1141–1201 | doi = 10.1051/0004-6361/200912304|last1=Lefèvre|first1=L.|last2=Marchenko|first2=S. V.|last3=Moffat|first3=A. F. J.|last4=Acker|first4=A.|bibcode=2009A&A...507.1141L|doi-access=free}}] Data derived from the assumption of a binary system, for example the mass, are highly uncertain due to the lack of information about the inclination or eccentricity of the orbit,[>{{Cite journal | doi = 10.1051/0004-6361/200913895|title= The mass ratio and initial mass functions in spectroscopic binaries|journal=Astronomy & Astrophysics|volume= 525|pages= A26|year= 2010|last1= Ducati|first1= J. R.|last2= Penteado|first2= E. M.|last3= Turcati|first3= R.|bibcode= 2011A&A...525A..26D|doi-access= free|hdl= 10183/99264|hdl-access= free}}> or even whether there is a companion.][{{Cite journal|arxiv=1010.2204 |last1=Weidner |first1=Carsten |title=The masses, and the mass discrepancy of O-type stars |journal=Astronomy & Astrophysics |volume=524 |pages=A98 |last2=Vink |first2=Jorick |year=2010 |doi=10.1051/0004-6361/201014491 |bibcode = 2010A&A...524A..98W |s2cid=118836634 }}] |
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A 2021 review of multiplicity of O-type stars rules out the previously suspected companion, finding the radial velocity variations to be intrinsic to the star. 68 Cygni is more likely a single star.[{{Cite journal |last=Trigueros Páez |first=E. |last2=Barbá |first2=R. H. |last3=Negueruela |first3=I. |last4=Maíz Apellániz |first4=J. |last5=Simón-Díaz |first5=S. |last6=Holgado |first6=G. |date=November 2021 |title=MONOS: Multiplicity Of Northern O-type Spectroscopic systems. II. Orbit review and analysis for 35 single-lined spectroscopic binary systems and candidates |bibcode=2021A&A...655A...4T |journal=Astronomy and Astrophysics |language=en |volume=655 |pages=A4 |doi=10.1051/0004-6361/202141018 |issn=0004-6361|arxiv=2106.08865 }}] |
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A 2021 review of multiplicity of O-type stars rules out the previously suspected companion, finding the radial velocity variations to be intrinsic to the star. 68 Cygni is more likely a single star.[{{Cite journal |last=Trigueros Páez |first=E. |last2=Barbá |first2=R. H. |last3=Negueruela |first3=I. |last4=Maíz Apellániz |first4=J. |last5=Simón-Díaz |first5=S. |last6=Holgado |first6=G. |date=November 2021 |title=MONOS: Multiplicity Of Northern O-type Spectroscopic systems. II. Orbit review and analysis for 35 single-lined spectroscopic binary systems and candidates |bibcode=2021A&A...655A...4T |journal=Astronomy and Astrophysics |language=en |volume=655 |pages=A4 |doi=10.1051/0004-6361/202141018 |issn=0004-6361|arxiv=2106.08865 }}] |
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