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NML Cygni
NML Cygni
NML Cygni
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NML Cygni

NML Cygni, seen as the deep red star at the center, from the Sloan Digital Sky Survey DR9. Note the green circumstellar nebula surrounding the star.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cygnus
Right ascension 20h 46m 25.54s[1]
Declination +40° 06′ 59.5″[1]
Apparent magnitude (V) 16.60 (variable)[2]
Characteristics
Evolutionary stage OH/IR[3] RHG[4]
Spectral type M4.5–M7.9 Ia–III[5]
Apparent magnitude (K) 0.791±0.204[1]
Apparent magnitude (G) 11.148[1]
Apparent magnitude (J) 4.877±0.037[1]
Apparent magnitude (H) 2.389±0.2[1]
B−V color index +2.04[2]
Variable type SR[6]
Astrometry
Proper motion (μ) RA: −1.55[4] mas/yr
Dec.: −4.59[4] mas/yr
Parallax (π)0.620±0.047 mas[4]
Distance5,250+420
−360 ly
(1,610+130
−110[4] pc)
Details
Mass25 (initial)[4] M☉
Radius<1,350+195
−229[7][a] R☉
Luminosity229,000+40,000
−41,000,[9] 270,000+50,000
−50,000[4] L☉
Temperature3,375[8] K
Age8[4] Myr
Other designations
NML Cyg, V1489 Cyg, AAVSO 2042+39, 2MASS J20462554+4006594
Database references
SIMBADdata

NML Cygni or V1489 Cygni (abbreviated to NML Cyg or V1489 Cyg) is a red hypergiant[4] or red supergiant (RHG or RSG) in the constellation Cygnus. It is one of the largest known stars currently known, and is also one of the most luminous and massive cool hypergiants, as well as one of the most luminous stars in the Milky Way.

The distance of NML Cygni from Earth is estimated to be around 1.6 kpc, about 5,300 light-years.[10] It 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] Based on the estimated distance and an upper limit of its angular diameter of 7.8±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 the Solar System, its surface would potentially extend past the orbit of Jupiter.

Observational history

[edit]
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 astronomers Neugebauer, Martz, and Leighton who described two extremely red luminous stars, their colour being consistent with a black body temperature of 1,000 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 as IK Tauri,[16] an M9 Mira 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 OH masers (1612 MHz) in 1968.[18] H
2O, SiO, CO, HCN, CS, SO, SO
2, and H
2S molecules have also been detected.[19]

Physical characteristics

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

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-losing OH/IR supergiant. It is also a semiregular variable star with a period of either 1,280 or 940 days.[11][5] It occupies the upper-right hand corner of the Hertzsprung–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 massive AGB stars, it may have once evolved blueward into a post-RSG warm hypergiant and then redward into an extreme red supergiant in a very short and final high mass loss state following a blue loop, before eventually exploding into a supernova or directly collapsing to a black hole.[20]

Size, luminosity, and temperature

[edit]
NML Cygni compared to the Sun and Earth's orbit.

The bolometric luminosity (Lbol) for NML Cygni was originally calculated to be 500,000 L☉ at an assumed distance of 2 kpc and the radius was calculated to be 3,700 R☉ based on an 8.6 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 normal red supergiant with consequently much lower luminosity and radius values.[24] More modern and accurate measurements give a distance around 1.6 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 the effective temperature of the star, giving a radius of 1,640 R☉ for a temperature of 3,250 K or possibly 2,770 R☉ for a temperature of 2,500 K.[b][4] However, another paper gives a much lower radius of 1,183 R☉ based on an assumed effective temperature of 3,834 K and a lower distance of 1.22 kpc.[6] There is a Gaia Data Release 2 parallax for NML Cygni of 1.5259±0.5677 mas, but the underlying measurements show a considerable level of noise and the parallax is considered unreliable.[25]

NML Cygni's uniform disk angular diameter was measured by interferometry, leading to an apparent size of 7.8±0.64 milliarcseconds.[8] Assuming the distance measured by Zhang et al. (2012) (1610+130
−110 parsecs),[4] it leads to a physical radius of 1,350 R☉.[7] If placed in the center of the Solar System, its photosphere 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]

Mass and mass loss

[edit]

NML Cygni lies close to the expected position that a 25 M☉ star would evolve to after eight million years.[4]

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[4][11] and it exhibits a bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers.[26]

NML Cygni has an estimated mass loss rate of 4.2 to 4.8×10−4 M☉ per year,[3] one of the highest known for any star. The annual 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 velocity 23 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]

See also

[edit]

Notes

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

NML Cygni

View on Grokipedia
from Grokipedia
NML Cygni is a red hypergiant star and one of the most extreme examples of massive , characterized by its enormous size, high , and intense loss that envelops it in a thick circumstellar shell of dust and gas. Located in the constellation Cygnus near the Cygnus OB2 association, NML Cygni lies at a distance of 1.61^{+0.13}{-0.11} kpc from , as determined by trigonometric measurements of its H₂O and SiO masers using the Very Long Baseline Array (VLBA). Its coordinates are 20ʰ 46ᵐ 25.⁵³⁹ˢ and +40° 06′ 59.⁴⁶⁴″ (J2000). Classified as a spectral type M6–M8 with dust obscuration, the star has an estimated initial of approximately 25 M⊙. NML Cygni's bolometric is (2.7 ± 0.5) × 10⁵ L_⊙, derived from integration accounting for its distance and circumstellar extinction. The star's is around 2500 K, yielding a stellar radius of approximately 2600 R_⊙ (or about 12 AU), based on its of 15 mas measured via radio and rescaled to the distance. This makes NML Cygni one of the largest known stars, with its radio extending to 44 ± 16 mas due to the extended molecular layers. It exhibits significant variability as a (V1489 Cyg), with optical magnitudes obscured by dust (G ≈ 11.1), but it appears bright in the (J ≈ 4.9, K ≈ 0.8). NML Cygni is renowned for its extreme mass-loss rate of ≈10^{-4} M_⊙ yr^{-1}, among the highest for red supergiants, driving powerful outflows that form an asymmetric, bean-shaped enriched with silicates, , and OH masers, as confirmed by recent ALMA and observations. This high mass loss, influenced by ultraviolet radiation from nearby hot stars in Cygnus OB2, creates a complex environment with ionized hydrogen regions and episodic ejections extending hundreds of AU. Its strong 1612 MHz OH radio emission—the strongest detected at the time—was discovered in 1968. NML Cygni exemplifies the late evolutionary stages of massive stars, potentially transitioning toward a Wolf–Rayet phase or .

Observational history

Discovery

NML Cygni was first identified in 1965 as part of the Caltech Two Micron Sky Survey conducted using the 62-inch telescope at Mount Wilson Observatory. The discovery was made by astronomers Gerry Neugebauer, D. E. Martz, and Robert B. Leighton, who detected it as an extremely bright and red near-infrared source with photometry at wavelengths of 1.6, 2.2, 3.4, and 4.8 microns showing magnitudes of approximately I ≈ 8 and K ≈ 1. No optical counterpart was detected in initial plates down to a limiting visual magnitude of about 19, indicating heavy obscuration by circumstellar dust. The object's provisional name, NML Cygni, follows the convention of prefixing the initials of the discoverers' surnames (Neugebauer, Martz, Leighton) to the genitive form of the constellation Cygnus. It received formal catalog designations shortly thereafter, including IRC +40448 from the Caltech Infrared Catalog based on follow-up surveys in the late 1960s, and RAFGL 2650 (also known as AFGL 2650) from the U.S. Air Force Geophysics Laboratory infrared survey in the early 1970s. Early classification proved difficult owing to the source's dominant infrared emission and lack of prominent optical features, leading to speculation that it might represent a highly obscured cool or even an exotic stellar remnant. Subsequent in 1966–1967 confirmed its extreme redness and hinted at an M-type spectral classification, while deeper optical imaging in the revealed a faint counterpart at visual magnitude V ≈ 16.6, consistent with a dust-enshrouded late-type giant or .

Early detections

Following its initial identification as an infrared anomaly, NML Cygni was confirmed as a bright variable source through early radio and surveys in the late and , including inclusion in the Geophysics Laboratory (AFGL) catalog of point sources. The object was optically matched to a faint sixteenth-magnitude (V ≈ 16.6) red star in the constellation Cygnus by Becklin and Neugebauer during their near- imaging and spectroscopic campaigns between 1968 and 1970, which provided the first positional association between the optical and emissions. Early spectroscopic observations in the 1970s classified NML Cygni as an M5–M8 supergiant, with features consistent with a highly reddened late-type giant or supergiant suffering significant extinction. Spectra revealed prominent emission lines, including Ba II and [Fe II], suggestive of ongoing mass ejection and circumstellar activity. These classifications built on preliminary assessments from the mid-1960s, refining the object's nature as an extreme red supergiant embedded in dust. A key early detection was the strong OH maser emission at 1612 MHz reported in 1968, the strongest such line observed at the time, which confirmed the presence of extensive circumstellar material and variability in the envelope. This radio detection, conducted with the MIT 120-ft telescope, highlighted NML Cygni as one of the first OH/IR stars, linking infrared excess to molecular outflows. Subsequent detailed profiles in 1970 further characterized the maser distribution. Initial polarimetric observations in the early 1980s revealed linear and varying with , indicating an asymmetric envelope and non-spherical mass loss geometry around the star. These measurements, using optical and near-infrared , provided the first evidence of structured circumstellar scattering, consistent with episodic ejection events.

Recent studies

In the , high-resolution of NML Cygni advanced through mid-infrared observations, resolving the star's extended atmosphere and inner shell with sub-arcsecond precision. These studies measured a modeled of approximately 16.2 mas for the central star based on earlier estimates, while direct yielded a core envelope size of σ ≈ 91–124 mas and an inner rim of the circumstellar shell at approximately 105 mas (FWHM) at wavelengths of 8.8, 9.8, 11.3, and 18.5 μm, confirming the object's extended beyond earlier unresolved detections. Spectroscopic surveys in the millimeter regime have provided detailed insights into the molecular content and dynamics of NML Cygni's envelope. A 1 mm survey using the Arizona Radio Observatory's Submillimeter Telescope, with results published in 2022, detected over 100 emission lines from 17 molecules, including CO, SiO, H₂O, HCN, HCO⁺, CN, and HNC, revealing a complex oxygen-rich chemistry with evidence of outflows. Building on this, 2025 observations at 1.3 mm (230 GHz) with ~0.4″ resolution identified asymmetric continuum emission from two dust components and extended CO J=2–1 line emission, showing a pronounced northwest-southeast in the envelope extent (up to 11″.4 southeast versus 6″.1 northwest) and arc-like structures indicative of episodic mass loss. Radio has illuminated the clumpy structure and magnetic properties of the . interferometric observations in 2004 at 1612 and 1665 MHz produced the first polarimetric images of the OH shell, revealing a clumpy, asymmetric distribution spanning ~1″ and a toroidal magnetic field morphology with strengths estimated at 1–4 mG, suggesting magneto-centrifugal launching of the outflow. Recent 2025 analyses, informed by these radio data and millimeter , reinforce the role of strong (up to several mG) in shaping large dust clumps and supporting asymmetric envelope structures, drawing parallels to similar fields in hypergiants like . Key updates from 2025 studies emphasize NML Cygni's extreme nature, with its dense, dusty, and asymmetric envelope—total dust mass ~2 × 10⁻³ M⊙—mirroring that of VY Canis Majoris, prompting suggestions of a comparable evolutionary phase potentially involving a second red supergiant stage characterized by enhanced instability and mass ejection. These observations, set within the broader Cygnus OB2 association, also contextualize NML Cygni amid regional high-energy phenomena, though direct gamma-ray associations remain unconfirmed.

Physical characteristics

Position and visibility

NML Cygni is situated in the constellation Cygnus, within the Cygnus OB2 association that forms part of the broader Cygnus X star-forming complex. Its equatorial coordinates for the J2000 epoch are right ascension 20ʰ 46ᵐ 25.⁵³⁹ˢ and declination +40° 06′ 59.⁴⁶″. The distance to NML Cygni is estimated at 1.61^{+0.13}_{-0.11} kpc, derived from very long baseline interferometry (VLBI) astrometry yielding a parallax of 0.620 ± 0.047 mas of its H₂O and SiO masers. Gaia measurements are unreliable due to heavy obscuration. Due to significant dust obscuration from both interstellar material (A_V ≈ 3.7 mag) and its dense (contributing an additional ≈ 10 mag), NML Cygni appears faint at optical wavelengths with an apparent visual magnitude of approximately 16.5, rendering it invisible to the and challenging for small telescopes. It is considerably brighter in the , with a K-band magnitude of about 0.8, and observations are optimally conducted in the mid- where absorption is minimized. The star's positive of +40° makes it accessible for observation from latitudes northward of approximately 50° S, particularly favoring sites at mid-latitudes. Seasonal visibility peaks during to when Cygnus transits the meridian in the evening sky.

Size, luminosity, and temperature

NML Cygni is classified as a red hypergiant with an exceptionally large , estimated at approximately 2600 R_⊙ (or about 12 AU), based on its of 15 mas measured via radio and rescaled to the distance of 1.61 kpc. The radio photosphere extends to 44 ± 16 mas due to extended molecular layers. The star's bolometric is (2.7 ± 0.5) × 10⁵ L_⊙, derived from integration accounting for its distance and circumstellar extinction; this corresponds to an absolute bolometric magnitude M_bol ≈ -9.5. The of the is around 2500 K, inferred from fitting and blackbody modeling to its cool continuum. This temperature reflects the extended, dusty atmosphere of the . The relationship between these parameters follows the Stefan-Boltzmann law for the photospheric emission: L=4πR2σT4,L = 4\pi R^2 \sigma T^4, where σ\sigma is the Stefan-Boltzmann constant, allowing derivation of RR once LL and TT are specified. For example, using T=2500T = 2500 K and L=2.7×105L⊙L = 2.7 \times 10^5 L_\odot, the radius computes to ≈2600R⊙\approx 2600 R_\odot. The further contextualizes the via m−M=5log⁡10(d)−5m - M = 5\log_{10}(d) - 5, with dd in parsecs supporting the estimate. In scale, NML Cygni exceeds Betelgeuse (≈800R⊙\approx 800 R_\odot) and is comparable to upper estimates for other hypergiants, positioning it among the largest known stars and highlighting its extreme evolutionary state as a massive progenitor nearing instability.

Mass and composition

NML Cygni's initial mass is estimated at 25–40 M_⊙ based on its luminosity and location in the Hertzsprung-Russell diagram, aligning with evolutionary models for massive stars in the core helium-burning phase. Earlier estimates placed the initial mass as high as 50 M_⊙, reflecting uncertainties in distance and luminosity determinations at the time. Due to extensive mass loss throughout its evolution, particularly during the red supergiant phase, the current stellar mass is thought to be lower, in the range of 20–30 M_⊙ according to comparisons with stellar evolution tracks. These mass estimates rely primarily on theoretical models rather than direct dynamical methods, as no binary companion has been detected to enable orbital analysis. The star's core is inferred to be in the helium-burning stage, consistent with its spectral classification as an M6–M8 and its position on the upper branch. The atmosphere shows evidence of products from the , mixed to the surface through convective processes during earlier evolutionary phases. The of the Cygnus OB2 association, in which NML Cygni resides, is nearly solar, with [Fe/H] ≈ -0.01 for member dwarf stars. The surface composition is oxygen-rich, characteristic of M-type supergiants, with a carbon-to-oxygen ratio (C/O) of about 0.5, indicating a sub-solar carbon abundance relative to oxygen. This oxygen dominance arises from the lack of significant carbon enhancement in massive stars compared to lower-mass asymptotic giant branch stars. Detailed elemental abundances beyond C, O, and Fe remain poorly constrained due to the star's heavy obscuration by circumstellar material, which complicates high-resolution spectroscopy. Uncertainties in mass and composition stem from the interplay between evolutionary modeling, high mass-loss rates (∼10^{-4} M_⊙ yr^{-1}), and the absence of resolved companions or precise dynamical probes.

Circumstellar environment

Envelope structure

The circumstellar envelope of NML Cygni is an extended, asymmetric structure surrounding the , characterized by a clumpy distribution of gas and that extends to an of approximately 7 arcseconds in optical and radio observations. This corresponds to a physical of roughly 5600 AU at a distance of about 1.6 kpc, encompassing multiple shells and arcs indicative of past mass-ejection events. Recent Northern Extended Millimeter Array () imaging from 2025 has resolved a complex inner structure, including a dense, elongated torus-like component (Component A) offset approximately 281 mas southwest of the at a position angle of -135°, connected to a more distant northwestern component (Component B) at about 985 mas with a position angle of -65°. These features form high-velocity lobes extending along a southeast-northwest axis, with outflow speeds reaching up to 54 km/s in CO emission and 55 km/s in SiO, highlighting the envelope's bipolar tendencies despite lacking perfect opposition between blue- and redshifted components. The OH shell, mapped via observations, appears irregular and incomplete, spanning about 5.3 arcseconds at 1612 MHz, further emphasizing the envelope's clumpy and asymmetric nature. CO emission extends to angular distances of up to 11.4 arcseconds in the southeast, corresponding to a physical extent of approximately 18,000 AU. Dynamically, the envelope exhibits radial expansion at velocities of 20-30 km/s, as evidenced by position-velocity diagrams showing outward motion that decreases with distance—such as 35 km/s at 1 arcsecond and 22 km/s at 7.5 arcseconds—consistent with a decelerating flow in an expanding shell. Spectral line surveys indicate multiple velocity components, including a systemic spherical at -1 to 3 km/s and collimated outflows extending to 4-5 arcseconds (500-650 stellar radii), pointing to episodic ejections over timescales of 1200-1300 years. These dynamics, observed from studies spanning the to the 2020s, reveal evidence of intermittent mass loss, with blue-shifted features in the southeast suggesting foreground material and red-shifted emission in the northwest indicating receding lobes. The pronounced asymmetry in the envelope's structure and kinematics implies non-spherical mass-loss processes, potentially influenced by , , or possibly a hidden binary companion, as inferred from the alignment of distributions and polarization patterns in the OH shell. This irregular geometry contributes to the and absorption of stellar , enhancing NML Cygni's appearance.

Dust and molecular content

The of NML Cygni is dominated by amorphous , characteristic of oxygen-rich evolved stars, with a total estimated at approximately 2 × 10^{-3} M_⊙. This exhibits a prominent 10 μm absorption feature, indicating an τ ≈ 10 in the mid-infrared, and consists of grains with typical sizes around 0.1–1 μm, including a modeled of 0.2 μm for warm components. The -to-gas ratio is approximately 1:100, consistent with expectations for such envelopes. Millimeter-wave surveys have identified a rich molecular inventory in the , including CO, SiO, SO, SO₂, HCN, HNC, CN, H₂O (via emission), H₂S, SiS, NaCl, AlO, and the HCO⁺, with over 100 lines detected from 17 distinct species. Abundances relative to H₂ are elevated for SiO (∼8 × 10^{-7}) and SO₂ (∼6.5 × 10^{-7}), attributed to shock-induced release from dust grains, while H₂O is prominent in the inner regions as evidenced by strong emission. Carbon-chain molecules like CN and HNC suggest in the outer layers, and the presence of HCO⁺ points to ion-molecule reactions in denser regions. Isotopic studies reveal ratios such as ^{12}C/^{13}C ≈ 20, derived from CO and lines, indicating circumstellar processing distinct from solar values. The total gas in the envelope is estimated at 0.1–0.3 M_⊙, based on CO emission extents and mass-loss rates of several × 10^{-4} M_⊙ yr^{-1}. The molecular distribution shows influences from the envelope's asymmetric structure, with enhanced emission in certain outflows.

Mass-loss mechanisms

NML Cygni exhibits one of the highest mass-loss rates among red supergiants (RSGs), estimated at approximately 10−410^{-4} to 10−310^{-3} M⊙M_\odot yr−1^{-1}, derived from modeling of CO emission and production in its . This extreme rate surpasses typical RSG values by factors of 10 or more, positioning NML Cygni as an archetype for intense mass ejection in evolved massive stars. The primary mechanism driving this mass loss is radiation pressure exerted on dust grains formed in the cool stellar atmosphere, which couples to the surrounding gas and accelerates the outflow to escape velocities. Pulsation-enhanced winds contribute significantly, with stellar pulsations likely triggering episodic ejections that amplify the baseline dust-driven flow. Recent studies also suggest possible magnetic acceleration, potentially influencing outflow collimation and clump formation, as inferred from polarimetric observations and modeling around similar hypergiants. Theoretical models of the wind describe the terminal velocity as v≈2GM/Rv \approx \sqrt{2GM/R}
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