The distance of NML Cygni from Earth is estimated to be around 1.6kpc, about5,300 light-years.[10] It is a part of theCygnus 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 of1.74±0.2 kpc.[11] Based on the estimated distance and an upper limit of itsangular diameter of7.8±0.64 milliarcseconds,[8] NML Cygni's physical radius is estimated to be no more than 1,350 R☉. If placed at the center of theSolar System, its surface would potentially extend past the orbit ofJupiter.
A near infrared (3.5 micron)light curve for V1489 Cygni, plotted from data published by Strecker (1975)[12]
NML Cygni was discovered in 1965 by American astronomersNeugebauer, Martz, andLeighton who described two extremely red luminous stars, their colour being consistent with a black body temperature of1,000 K.[13] The name NML comes from the names of these three discoverers.[14] The second star was briefly referred to as NML Tauri[15] but is now known asIK Tauri,[16] an M9Mira variable. NML Cygni has since also been given the designation V1489 Cygni on account of the small semi-regular brightness variations,[17] but is still most commonly referred to as NML Cygni. Its composition began to be revealed with the discovery of OHmasers (1612 MHz) in 1968.[18]H 2O,SiO,CO,HCN,CS,SO,SO 2, andH 2S molecules have also been detected.[19]
NML Cygni is an extremely large and luminous cool supergiant with parameters similar to that of another notable but more extreme cool hypergiant star,VY Canis Majoris, and is also known as a heavily mass-losingOH/IR supergiant. It is also asemiregular variable star with a period of either 1,280 or 940 days.[11][5] It occupies the upper-right hand corner of theHertzsprung–Russell diagram although most of the properties of the star depend directly on its distance.
Due to its similarity to VY CMa, NML Cygni has been suggested in 2025 to be a possible candidate for a star in a second red supergiant phase; similar to less massiveAGB stars, it may have once evolved blueward into a post-RSGwarm hypergiant and then redward into an extreme red supergiant in a very short and final high mass loss state following ablue loop, before eventually exploding into asupernova or directly collapsing to ablack hole.[20]
Thebolometric luminosity (Lbol) for NML Cygni was originally calculated to be 500,000 L☉ at an assumed distance of2 kpc and the radius was calculated to be 3,700 R☉ based on an8.6 mas angular diameter and distance.[21][22][23] A 2006 study, similar to those conducted on VY Canis Majoris, suggests that NML Cygni is a normalred supergiant with consequently much lower luminosity and radius values.[24] More modern and accurate measurements give a distance around1.6 kpc, which gives a luminosity around 200,000 L☉. A radio angular diameter of 44 mas was given based on the distance, suggesting the optical angular diameter may be around 22 mas.[4] This distance and a luminosity of 270,000 L☉ were combined with assumptions of theeffective temperature of the star, giving a radius of 1,640 R☉ for a temperature of3,250 K or possibly 2,770 R☉ for a temperature of2,500 K.[b][4] However, another paper gives a much lower radius of 1,183 R☉ based on an assumed effective temperature of3,834 K and a lower distance of1.22 kpc.[6] There is aGaia Data Release 2 parallax for NML Cygni of1.5259±0.5677 mas, but the underlying measurements show a considerable level of noise and the parallax is considered unreliable.[25]
NML Cygni's uniform diskangular diameter was measured byinterferometry, leading to an apparent size of7.8±0.64 milliarcseconds.[8] Assuming the distance measured by Zhanget al. (2012) (1610+130 −110parsecs),[4] it leads to a physical radius of 1,350 R☉.[7] If placed in the center of the Solar System, itsphotosphere would past the orbit of Jupiter. NML Cygni is covered by a complex dust shell, which is causing interference in the angular diameter, therefore this radius is only an upper limit.[8]
NML Cygni lies close to the expected position that a 25 M☉ star would evolve to after eight million years.[4]
NML Cygni isevolved and a number of heavy elements and molecules have been detected in its atmosphere, particularlyoxygen,hydroxyl, andwater. It is surrounded by dusty material[4][11] and it exhibits a bean-shaped asymmetricnebula that is coincident with the distribution of its H2O vapormasers.[26]
NML Cygni has an estimated mass loss rate of 4.2 to4.8×10−4M☉ per year,[3] one of the highest known for any star. Theannual parallax of NML Cygni is measured to be around 0.62 milliarcseconds.[4] 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.[27] These dust envelopes are formed due to the strong post-main-sequence wind, which has a velocity23 km/s.[11]
Because of the star's 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]
^abDe 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.S2CID16131273.
^abcdeMonnier, J. D.; Millan‐Gabet, R.; Tuthill, P. G.; Traub, W. A.; Carleton, N. P.; Coude du Foresto, V.; Danchi, W. C.; Lacasse, M. G.; Morel, S.; Perrin, G.; Porro, I. L.; Schloerb, F. P.; Townes, C. H. (2004-04-10). "High‐Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer".The Astrophysical Journal.605 (1):436–461.arXiv:astro-ph/0401363.doi:10.1086/382218.ISSN0004-637X.
^abcdeSchuster, M. T.; Marengo, M.; Hora, J. L.; Fazio, G. G.; Humphreys, R. M.; Gehrz, R. D.; Hinz, P. M.; Kenworthy, M. A.; Hoffmann, W. F. (2009). "Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics".The Astrophysical Journal.699 (2):1423–1432.arXiv:0904.4690.Bibcode:2009ApJ...699.1423S.doi:10.1088/0004-637X/699/2/1423.S2CID17699562.
^Monnier, J. D; Bester, M; Danchi, W. C; Johnson, M. A; Lipman, E. A; Townes, C. H; Tuthill, P. G; Geballe, T. R; Nishimoto, D; Kervin, P. W (1997). "Nonuniform Dust Outflow Observed around Infrared Object NML Cygni".The Astrophysical Journal.481 (1): 420.arXiv:astro-ph/9702103.Bibcode:1997ApJ...481..420M.doi:10.1086/304050.S2CID9503967.
^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.; Morel, S.; Perrin, G.; Porro, I. L.; Schloerb, F. P.; Townes, C. H. (2004). "High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer".The Astrophysical Journal.605 (1):436–461.arXiv:astro-ph/0401363.Bibcode:2004ApJ...605..436M.doi:10.1086/382218.S2CID7851916.
