68 Cygni

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68 Cygni
Diagram showing star positions and boundaries of the Cygnus constellation and its surroundings
Cercle rouge 100%.svg

Location of 68 Cygni (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cygnus
Right ascension 21h 18m 27.18561s[1]
Declination +43° 56′ 45.4070″[1]
Apparent magnitude (V) 5.00[2]
Characteristics
Spectral type O7.5IIIn((f))[3][4]
U−B color index −0.94[2]
B−V color index −0.01[2]
Variable type ELL[5][n 1]
Astrometry
Radial velocity (Rv) 1 ± 5[6] km/s
Proper motion (μ) RA: 4.85 ± 0.22[1] mas/yr
Dec.: −8.40 ± 0.20[1] mas/yr
Parallax (π) 0.70 ± 0.23[1] mas
Distance approx. 5,000 ly
(approx. 1,400 pc)
Absolute magnitude (MV) −6.70
Details
Mass 23[7]–51[8] M
Surface gravity (log g) 3.6[9] cgs
Temperature 34,000[9] K
Rotational velocity (v sin i) 399[4] km/s
Other designations
V1809 Cygni, A Cygni, HD 203064, HR 8154, BD+43 3877, HIP 105186, WDS J21185+4357
Database references
SIMBAD data

68 Cygni is the Flamsteed designation for a star in the constellation Cygnus. Located approximately 1,400 parsecs (4,600 ly) distant, the star is a hot blue giant of spectral type O7.5IIIn((f)), a massive star that is likely currently expanding to become a supergiant. The star is surrounded by a ring-shaped nebula (likely a Strömgren sphere) named S 119.

68 Cygni is currently classified as a rotating ellipsoidal variable, varying between apparent magnitudes 4.98 and 5.09, although the classification as a rotating ellipsoidal variable is subject to controversy. Barely visible to the naked eye, the star is likely to have a mass of around 26 solar masses and a temperature of approximately 34,000 Kelvin, although many of the star's physical parameters are subject to uncertainties due to the unclear nature of the system.

History[edit]

Being visible to the naked eye, 68 Cygni would have been observable since ancient times, but the first known cataloging and designation of the star was by Johann Bayer, who assigned it the Bayer designation A Cygni.[10] Bayer grouped stars into six magnitude groupings (1st- through 6th-magnitude) and assigned Greek letters to the brightest 24 stars in these groupings, sorted by right ascension within groups. When he ran out of Greek letters, Bayer then moved on to Latin letters; thus, 68 Cygni was the 25th star Bayer designated in Cygnus.[11]

The star was later observed by John Flamsteed, who cataloged naked-eye stars by constellation. Contrary to popular belief, Flamsteed did not assign the stars Flamsteed designations; rather, the French astronomer Joseph Jérôme de Lalande assigned Flamsteed numbers to stars in each constellation in order of right ascension. Since this star was the 68th star by right ascention in Cygnus, it was numbered 68 Cygni,[12] the designation by which this star is most commonly-known by today, as most Latin-letter Bayer designations are no longer in common use.

Observation[edit]

68 Cygni has an apparent magnitude of approximately 5.0,[2] making the star a fairly inconspicuous star in the night sky, and nearly invisible when the moon is full. With a new moon, according to the Bortle scale, the star cannot be easily seen with the naked eye from polluted areas or with a full moon, but can be seen from suburban areas when overhead.[13]

In the night sky, 68 Cygni is located west of the main asterism of Cygnus, approximately seven degrees east-southeast of the first-magnitude star Deneb.[14] This is near the midpoint of the imaginary line between the 3rd-magnitude stars Xi Cygni and Rho Cygni. The star can also be found by continuing north along the line between the stars Zeta Cygni, Upsilon Cygni, Tau Cygni, and Sigma Cygni, as 68 Cygni is the first easily-visible naked-eye star one will come to.

Properties[edit]

According to the star's measured parallax of 0.70 milliarcseconds, it is located approximately 1,400 parsecs (4,600 ly) distant, although such low parallax values are subject to low precision. With taking into account the error estimate of 0.23 milliarcseconds, the star's distance could be anywhere between 1,080 parsecs (3,500 ly) and 2,130 parsecs (6,900 ly) distant,[1] although values close to the mean value are more likely.

68 Cygni is a massive blue giant of spectral type O7.5IIIn((f)).[3][4] 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 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 lines and weak N III emission lines.[15]

The star is currently tentatively classified as a rotating ellipsoidal variable, varying between magnitudes 4.98 and 5.09.[5] Rotating ellipsoidal variables are binary systems in which the constituent components are distorted into ellipses due to fast rotation rates, and as the two stars orbit each other, the surface area of the star facing Earth changes, causing variations in brightness.[16] As the 1809th star without a Bayer designation discovered to be variable in Cygnus, 68 Cygni was assigned the variable star designation V1809 Cygni in 1984.[17] However, the actual cause of the variability, and even whether or not there is a companion, is uncertain. No companion has ever been directly detected, but its existence has been inferred from spectral line 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.[18] 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.[7]

68 Cygni is known to be surrounded by a ring-shaped nebula, named S 119.[5] Although it was originally proposed that this nebula was formed from 68 Cygni's stellar wind, it now appears that the nebula is simply a Strömgren sphere that is being ionized by 68 Cygni. Instead, the stellar wind is likely, due to the star's high velocity through space, forming a bow shock.[19]

The mass of the star is likely to be around 26 solar masses, according to a 2011 study by Ducati, Penteado, and Turcati. However, due to the uncertain nature of the binary system hypothesis, the true mass could be much different than this.[7] If the star actually has a mass of 51 solar masses (the median mass reported by Hohle, Neuhäuser, and Schutz in 2010), the star's bolometric luminosity would be over 1 million solar luminosities,[8] making it among the most luminous stars known, although data to support this mass is tenuous at best.[7]

Notes[edit]

  1. ^ Unusual variations for such a variable

References[edit]

  1. ^ a b c d e f van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.  Vizier catalog entry
  2. ^ a b c d Ducati, J. R. (2002). "Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. Bibcode:2002yCat.2237....0D.  Vizier catalog entry
  3. ^ a b Sota, A.; Maíz Apellániz, J.; Walborn, N. R.; Alfaro, E. J.; Barbá, R. H.; Morrell, N. I.; Gamen, R. C.; Arias, J. I. (2011). "The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-Violet at R ∼ 2500". The Astrophysical Journal Supplement Series 193 (2): 24. arXiv:1101.4002. Bibcode:2011ApJS..193...24S. doi:10.1088/0067-0049/193/2/24.  edit
  4. ^ a b c Simón-Díaz, S.; Herrero, A. (2014). "The IACOB project: I. Rotational velocities in northern Galactic O- and early B-type stars revisited. The impact of other sources of line-broadening". Astronomy & Astrophysics 562. doi:10.1051/0004-6361/201322758.  edit
  5. ^ a b c Esipov, V. F.; Klementeva, A. Y.; Kovalenko, A. V.; Lozinskaya, T. A.; Lyntyj, V. M.; Sitnik, T. G.; Udalstov, V. A. (1982). "The Ring Nebulae around the O-Stars - Observations of S119 and its Central Star 68-CYGNI". Astronomicheskii Zhurnal 59 (September–October): 965–974. Bibcode:1982AZh....59..965E. 
  6. ^ Wilson, R. A. (1953). General catalogue of stellar radial velocities. Carnegie Institute of Washington. Bibcode:1953GCRV..C......0W.  Vizier catalog entry
  7. ^ a b c d "The mass ratio and initial mass functions in spectroscopic binaries". Astronomy & Astrophysics. doi:10.1051/0004-6361/200913895.  edit
  8. ^ a b Hohle, M. M.; Neuhäuser, R.; Schutz, B. F. (2010). "Masses and luminosities of O- and B-type stars and red supergiant". Astronomische Nachrichten 331 (4): 349–361. arXiv:1003.2335. Bibcode:2010AN....331..349H. doi:10.1002/asna.200911355.  Vizier catalog entry
  9. ^ a b Soubiran, C.; Le Campion, J. -F.; Cayrel De Strobel, G.; Caillo, A. (2010). "The PASTEL catalogue of stellar parameters". Astronomy and Astrophysics 515: A111. doi:10.1051/0004-6361/201014247.  edit
  10. ^ Easton, C. "A New Theory of the Milky way". Astrophysical Journal 12: 136–158. Bibcode:1900ApJ....12..136E. 
  11. ^ Ridpath, I. "Bayer's Uranometria and Bayer letters". Star Tales. self-published. Retrieved 22 August 2014. 
  12. ^ Ridpath, I. "Flamsteed numbers – where they really came from". Star Tales. self-published. Retrieved 22 August 2014. 
  13. ^ Bortle, John E. (February 2001). "The Bortle Dark-Sky Scale". Sky & Telescope. Sky Publishing Corporation. Retrieved 2013-02-20. 
  14. ^ Kaler, J. (22 November 2013). "68 Cygni". University of Illinois. Retrieved 22 August 2014. 
  15. ^ "Early-type stars in the core of the young open cluster Westerlund 2". Astronomy & Astrophysics. doi:10.1051/0004-6361:20066495.  edit
  16. ^ Otero, S. A.; Watson, C.; Wils, P. "Variable Star Type Designations in the VSX". AAVSO Website. American Association of Variable Star Observers. Retrieved 9 August 2014. 
  17. ^ Kholopov, P. N.; Samus, N. N.; Kazarovets, E. V.; Perova, N. B. (1985). "The 67th Name-List of Variable Stars". Information Bulletin on Variable Stars 2681: 1–32. Bibcode:1985IBVS.2681....1K. 
  18. ^ A systematic study of variability among OB-stars based on HIPPARCOS photometry 507. 2009. pp. 1141–1201. doi:10.1051/0004-6361/200912304.  edit
  19. ^ Wisotzki, L.; Wendker, H. J. (1989). "Is HS 420 an interstellar bubble?". Astronomy and Astrophysics. Bibcode:1989A&A...221..311W.