NML Cygni

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NML Cygni
Cygnus constellation map.svg
Red circle.svg
Location of NML Cygni (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cygnus
Right ascension 20h 46m 25.54s[1]
Declination +40° 06′ 59.4″[1]
Apparent magnitude (V) 16.60[2]
Spectral type M6I[3]
Apparent magnitude (K) 12.3[4]
B−V color index +2.04[2]
Variable type SR[5]
Proper motion (μ) RA: −1.55[6] mas/yr
Dec.: −4.59[6] mas/yr
Parallax (π)0.620 ± 0.047[6] mas
Distance1,610[6] pc
Mass50[7] M
Radius1,183[4] (1,640[8]-2,770[9])[6] R
Diameter1,183[4] (1,640[8]-2,770[9])[6] D
Luminosity272,000[4][6] L
Temperature3,834[4] (2,500-3,250)[6] K
Age8[6] Myr
Other designations
V1489 Cyg, RAFGL 2650, IRC+40448, 2MASS J20462554+4006594, AAVSO 2042+39
Database references

NML Cygni or V1489 Cygni is a red hypergiant[6] and one of the largest stars currently known with a radius of 1,640 solar radii (1.14×109 kilometres; 7.6 astronomical units) or between 1,183–2,770 solar radii (823,000,000–1.927×109 kilometres; 5.50–12.88 astronomical units). Its distance from Earth is estimated to be around 1.6 kpc, about 5,300 light-years.[10]

NML Cygni is a part of the Cygnus OB2 association, one of the closest massive associations to the Sun, spanning nearly 2° on the sky or ∼30 pc in radius at the distance of 1.74±0.2 kpc.[11]

Observational history

H-alpha light image of Cygnus OB2, the stellar association in which NML Cygni is located.

NML Cygni was discovered in 1965 by Neugebauer, Martz, and Leighton who described two extremely red luminous stars, their colour being consistent with a black body temperature of 1,000 K.[12] The name NML comes from the names of these three discoverers.[13] The second star was briefly referred to as NML Tauri[14] but is now known as IK Tauri,[15] an M9 Mira variable. NML Cygni has since also been given the designation V1489 Cygni on account of the small semi-regular brightness variations,[16] but is still most commonly referred to as NML Cygni. Its composition began to be revealed with the discovery of OH masers (1612 MHz) in 1968.[17] H
, SiO, CO, HCN, CS, SO, SO
, and H
molecules have also been detected.[18]


The radius of NML Cygni has recently been calculated to be 1,183 D with an effective temperature of 3,834 K.[4] An accurate measure of its distance and luminosity combined with assumptions of its effective temperature give a radius of 1,640 D[19] for a temperature of 3,250 K or 2,770 D[20] for a temperature of 2,500 K.[6] In 2004, Zubko et al estimated the radius to be much larger around 3,740 D,[21][22] based on an assumed distance of 2,000 pc and an 8.6 mas angular diameter.[23] More modern measurements give a radio angular diameter of 44 mas at 1,600 pc, suggesting the optical angular diameter may be around 22 mas.[6]

If placed at the center of the Solar System, its surface would extend past the orbit of Jupiter or Saturn. It contains a volume between 1.6 and 21.4 billion times that of the Sun. The bolometric luminosity (Lbol) for NML Cygni is near 3×105 L. Its bolometric magnitude (Mbol) is around −9.0. It is one of the most luminous cool hypergiants, as well as one of the most luminous stars in the Milky Way. An earlier calculation of the luminosity gave a luminosity of 5×105 L.[21] Massey, Levesque, and Plez's study, like VY CMa, think that NML Cygni is a normal red supergiant with a much smaller luminosity, and consequently a smaller value for the radius.[24] NML Cyg is also a semiregular variable star with a period of around 940 days.[11]

NML Cygni lies close to the expected position that a 25 M star would evolve to after eight million years.[6] Estimates of its current mass are difficult. One published measure is 50 M.[7]

NML Cygni is evolved and a number of heavy elements and molecules have been detected in its atmosphere, particularly oxygen, hydroxyl, and water. It is surrounded by dusty material[6][11] and it exhibits a bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers.[25]

NML Cygni has an estimated mass loss rate of 2×10−4 M per year,[18] one of the highest known for any star. The annual parallax of NML Cygni is measured to be around 0.62 milliarcseconds.[6] From the observations, it is estimated that NML Cygni has two discrete optically thick envelopes of dust and molecules. The optical depth of the inner shell is found to be 1.9, whereas that of the outer one is 0.33.[26] These dust envelopes are formed due to the strong post-main-sequence wind, which has a velocity 23 km/s.[11]

Because of its position on the outskirts of the massive Cygnus OB2 association, the detectable effects of NML Cygni's radiation on the surrounding dust and gas are limited to the region away from the central hot stars of the association.[11]


  1. 1.0 1.1 Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T. et al. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally published in: 2003yCat.2246....0C 2246. Bibcode 2003yCat.2246....0C.
  2. 2.0 2.1 Johnson, Harold L.; Mendoza v., Eugenio E.; Wisniewski, Weislaw Z. (1965). "Observations of "Infrared Stars."". Astrophysical Journal 142: 1249. Bibcode 1965ApJ...142.1249J. doi:10.1086/148393.
  3. Monnier, J. D.; Millan‐Gabet, R.; Tuthill, P. G.; Traub, W. A.; Carleton, N. P.; Coude Du Foresto, V.; Danchi, W. C.; Lacasse, M. G. et al. (2004). "High‐Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer". The Astrophysical Journal 605: 436. arXiv:astro-ph/0401363. Bibcode 2004ApJ...605..436M. doi:10.1086/382218.
  4. 4.0 4.1 4.2 4.3 4.4 De Beck, E.; Decin, L.; De Koter, A.; Justtanont, K.; Verhoelst, T.; Kemper, F.; Menten, K. M. (2010). "Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: Derivation of mass-loss rate formulae". Astronomy and Astrophysics 523: A18. arXiv:1008.1083. Bibcode 2010A&A...523A..18D. doi:10.1051/0004-6361/200913771.
  5. Blöcker, T.; Balega, Y.; Hofmann, K.-H.; Weigelt, G. (2001). "Bispectrum speckle interferometry observations and radiative transfer modelling of the red supergiant NML Cyg. Multiple dust-shell structures evidencing previous superwind phases". Astronomy and Astrophysics 369: 142. arXiv:astro-ph/0102092. Bibcode 2001A&A...369..142B. doi:10.1051/0004-6361:20010095.
  6. 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 Zhang, B.; Reid, M. J.; Menten, K. M.; Zheng, X. W.; Brunthaler, A. (2012). "The distance and size of the red hypergiant NML Cygni from VLBA and VLA astrometry". Astronomy & Astrophysics 544: A42. arXiv:1207.1850. Bibcode 2012A&A...544A..42Z. doi:10.1051/0004-6361/201219587.
  7. 7.0 7.1 Morris, M.; Jura, M. (1983). "The nature of NML Cygnus". Astrophysical Journal 267: 179. Bibcode 1983ApJ...267..179M. doi:10.1086/160856.
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  11. 11.0 11.1 11.2 11.3 11.4 Schuster, M. T.; Marengo, M.; Hora, J. L.; Fazio, G. G.; Humphreys, R. M.; Gehrz, R. D.; Hinz, P. M.; Kenworthy, M. A. et al. (2009). "Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics". The Astrophysical Journal 699 (2): 1423. arXiv:0904.4690. Bibcode 2009ApJ...699.1423S. doi:10.1088/0004-637X/699/2/1423.
  12. Neugebauer, G.; Martz, D. E.; Leighton, R. B. (July 1965). "Observations of Extremely Cool Stars". Astrophysical Journal 142: 399–401. Bibcode 1965ApJ...142..399N. doi:10.1086/148300.
  13. Lua error in Module:Citation/CS1 at line 637: attempt to concatenate local 'chapter' (a table value).
  14. Pesch, P. (1967). "Objective-Prism Spectra of Some Very Red Stars". The Astrophysical Journal 147: 381. Bibcode 1967ApJ...147..381P. doi:10.1086/149015.
  15. Kukarkin, B. V.; Efremov, Yu. N.; Frolov, M. S.; Medvedeva, G. I. et al. (8 November 1968). "Identification List of the New Variable Stars Nominated in 1968". Information Bulletin on Variable Stars 311 (1): 1. Bibcode 1968IBVS..311....1K. http://www.konkoly.hu/cgi-bin/IBVS?0311.
  16. Kukarkin, B. V.; Kholopov, P. N.; Kukarkina, N. P. (27 November 1975). "61st Name-List of Variable Stars". Information Bulletin on Variable Stars 1068 (1): 1. Bibcode 1975IBVS.1068....1K. http://www.konkoly.hu/cgi-bin/IBVS?1068.
  17. Cohen, R. J.; Downs, G.; Emerson, R.; Grimm, M. et al. (1 April 1987). "Narrow polarized components in the OH 1612-MHz maser emission from supergiant OH-IR sources". Monthly Notices of the Royal Astronomical Society 225 (3): 491–498. Bibcode 1987MNRAS.225..491C. doi:10.1093/mnras/225.3.491.
  18. 18.0 18.1 Lua error in Module:Citation/CS1 at line 637: attempt to concatenate local 'chapter' (a table value).
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  21. 21.0 21.1 Zubko, Viktor; Li, Di; Lim, Tanya; Feuchtgruber, Helmut; Harwit, Martin (2004). "Observations of Water Vapor Outflow from NML Cygnus". The Astrophysical Journal 610: 427. arXiv:astro-ph/0405044. Bibcode 2004ApJ...610..427Z. doi:10.1086/421700.
  22. {{{1}}}
  23. Monnier, J. D; Bester, M; Danchi, W. C; Johnson, M. A; Lipman, E. A; Townes, C. H; Tuthill, P. G; Geballe, T. R et al. (1997). "Nonuniform Dust Outflow Observed around Infrared Object NML Cygni". The Astrophysical Journal 481: 420. arXiv:astro-ph/9702103. Bibcode 1997ApJ...481..420M. doi:10.1086/304050.
  24. Massey, Philip; Levesque, Emily M.; Plez, Bertrand (1 August 2006). "Bringing VY Canis Majoris down to size: an improved determination of its effective temperature". The Astrophysical Journal 646 (2): 1203–1208. arXiv:astro-ph/0604253. Bibcode 2006ApJ...646.1203M. doi:10.1086/505025.
  25. Schuster, M. T.; Humphreys, R. M.; Marengo, M. (2006). "The Circumstellar Environments of NML Cygni and the Cool Hypergiants". The Astronomical Journal 131: 603. arXiv:astro-ph/0510010. Bibcode 2006AJ....131..603S. doi:10.1086/498395.
  26. DanchiI, W. C.; Green, W. H.; Hale, D. D. S.; McEleroy, K. et al. (July 2001). "Proper Motions of Dust Shells Surrounding NML Cygni". The Astrophysical Journal 555: 405. Bibcode 2001ApJ...555..405D. doi:10.1086/322237.