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Gamma Cygni

Gamma Cygni, also known as Sadr, is a yellow supergiant star located in the northern constellation of Cygnus, where it marks the central intersection of the prominent Northern Cross asterism.^[1] With an apparent visual magnitude of 2.23, it ranks as the second-brightest star in Cygnus after Deneb and is visible to the naked eye as a steady, yellowish point of light high in the summer sky for northern observers.^[2] Positioned approximately 1,830 light-years from Earth, Gamma Cygni lies along the Milky Way's plane, embedded in a rich star-forming region that includes the Gamma Cygni Nebula (IC 1318), a complex of diffuse emission nebulae surrounding the star.^[3] Classified as an F8 supergiant of spectral type F8 Iab, Gamma Cygni exhibits the physical characteristics of a massive, evolved star with an original mass estimated at 14 to 16 times that of the Sun, a current radius of about 183 to 235 solar radii, and a bolometric luminosity roughly 33,000 to 60,000 times greater than the Sun's, corresponding to a surface temperature of around 5,870 K.^[2]^[4] The star shows spectroscopic variations, including radial velocity changes of up to 2 km/s on timescales of about 100 days or less, possibly indicating non-radial pulsations, and it has likely exhausted its core hydrogen fuel, placing it in a late evolutionary stage where it may soon undergo helium fusion or proceed toward a supernova explosion.^[2] Its coordinates in the J2000 epoch are right ascension 20h 22m 13.7s and declination +40° 15′ 24″, with a proper motion of 2.39 mas/year in right ascension and -0.91 mas/year in declination, and a radial velocity of -5.9 km/s relative to the Sun.^[5] Gamma Cygni's position near the Cygnus X molecular cloud complex highlights its association with active star formation, though the star itself is a foreground object relative to much of the surrounding nebulosity, which includes open clusters like NGC 6910 and the distant gamma Cygni supernova remnant (G78.2+2.1).^[3] As a key navigational and observational target, it serves as a reference for locating deep-sky objects in Cygnus and exemplifies the diversity of stellar evolution in our galaxy.^[6]

Visibility and Location

Celestial Coordinates

Gamma Cygni is located at equatorial coordinates of right ascension 20ʰ 22ᵐ 13.⁷⁰¹⁸⁴ and declination +40° 15′ 24.⁰⁴⁵⁰ for the J2000.0 epoch.^[5] In Galactic coordinates, the star lies at longitude 78.15° and latitude +1.87°.^[5] The annual proper motion of Gamma Cygni is +2.39 mas in right ascension and -0.91 mas in declination.^[5] Within the constellation Cygnus, Gamma Cygni marks the central intersection point of the Northern Cross asterism, formed by the bright stars Alpha Cygni (Deneb) at the top, Beta Cygni (Albireo) at the base, Epsilon Cygni (Gienah) to the east, and Delta Cygni to the west.^[7] The distance to Gamma Cygni is approximately 1,800 light-years (560 parsecs), derived from a Hipparcos parallax of 1.78 ± 0.27 mas.^[5]

Observational Characteristics

Gamma Cygni, with an apparent visual magnitude of 2.23, is readily visible to the naked eye under dark skies, ranking it among the brighter stars in the northern celestial hemisphere.^[8]^[9] As a supergiant of spectral class F8Ib, it exhibits a pale yellow hue characteristic of F-type stars, appearing as a steady, point-like source without any optically resolvable companions to unaided observers.^[8]^[3] The star is best observed during the northern summer months of July through September, when the constellation Cygnus is prominently positioned overhead for latitudes between 0° and 90° N.^[10] It culminates at midnight in August, providing optimal viewing conditions during evening hours. For observers north of 50° N latitude, Gamma Cygni is circumpolar, remaining visible throughout the year without setting below the horizon.^[11] Located at the heart of Cygnus, Gamma Cygni forms part of the constellation's extension from the prominent Summer Triangle asterism, aiding in its location by tracing southward from Deneb along the swan's body.^[12] Basic observing tips include seeking dark sites away from light pollution, as the star's brightness allows easy identification even in moderately rural areas during its seasonal peak.^[13]

Nomenclature and Designations

Bayer Designation

The Bayer designation for this star is γ Cygni (Latinized as Gamma Cygni), assigned by the German lawyer and amateur astronomer Johann Bayer in his influential 1603 star atlas Uranometria, the first to systematically label stars within constellations using Greek letters in approximate order of brightness.^[14] This gamma label indicates it as nominally the third-brightest in Cygnus under Bayer's scheme, following α Cygni (Deneb) and β Cygni (Albireo); however, with an apparent visual magnitude of 2.23, γ Cygni is actually the second-brightest star in the constellation, surpassing β Cygni (magnitude 2.90).^[2] γ Cygni appears in key historical and modern catalogs, including the Henry Draper Catalogue as HD 194093, the Hipparcos Catalogue as HIP 100453, and Gaia Data Release 3 as source ID 2067518817314952576.^[15] In current astronomical practice, the Bayer designation γ Cygni functions as the core identifier for data integration and queries in resources like the SIMBAD astronomical database.

Traditional Names

The primary traditional name for Gamma Cygni is Sadr, derived from the Arabic word ṣadr, meaning "chest," which refers to the star's position marking the breast or chest of the Swan in the constellation's figure.^[16] This name, sometimes rendered as Sadir or Sador, was historically part of Arabic astronomical nomenclature and reflects the star's role in the asterism Al Fawāris, or "the Riders," associated with the Swan's body.^[16] The International Astronomical Union (IAU) officially approved Sadr as the proper name for the primary component (Washington Double Star Catalog designation J20222+4015A) on August 21, 2016, through its Working Group on Star Names. In Ptolemaic astronomy, as adapted in medieval Arabic catalogs, Gamma Cygni represented the breast or heart of the Swan, a depiction carried forward in works like Abd al-Rahman al-Sufi's Book of Fixed Stars (964 CE), where it is listed as Sadr, emphasizing its central placement in the bird's torso.^[16] Some historical Arabic texts refer to it as Al Sadr al Dajājah, translating to "the Hen's Breast," linking it to avian imagery beyond the Swan and aligning with broader cultural associations of the constellation with celestial birds.^[16] In Chinese astronomy, the star is known as Tiān Jīn yī (天津一), or "First Star of the Celestial Ford," part of an asterism depicting a ford across the Milky Way.^[17] While Cygnus as a whole features in Greek mythology as the Swan transformed from Zeus or Ornithus, no unique mythological narratives are specifically tied to Gamma Cygni itself, with its naming focused on anatomical and positional descriptors across cultures.^[16]

Stellar Properties

Physical Parameters

Gamma Cygni, the primary star in this system, is a massive supergiant with an estimated original mass of 14.5 ± 1.1 solar masses (M⊙).^[2] This substantial mass places it among the more evolved high-mass stars, influencing its rapid evolutionary trajectory. Estimates of its radius vary: direct interferometric measurements suggest approximately 183 solar radii (R⊙), while calculations from luminosity and temperature yield about 235 R⊙, rendering it one of the largest known stars and extending its photosphere to a scale comparable to or exceeding the orbit of Earth around the Sun.^[2] The star's surface temperature is approximately 6000 K, consistent with its F8 spectral classification.^[18] Using the Stefan-Boltzmann law, its bolometric luminosity can be derived as follows:

L = 4\pi R^2 \sigma T^4

where R \approx 183-235 \, R_\odot, T \approx 6000 \, \mathrm{K}, and \sigma = 5.67 \times 10^{-8} \, \mathrm{W \, m^{-2} \, K^{-4}} is the Stefan-Boltzmann constant. Substituting the solar values (R_\odot = 6.96 \times 10^8 \, \mathrm{m}, L_\odot = 3.828 \times 10^{26} \, \mathrm{W}) yields a luminosity of approximately 33,000 to 60,000 solar luminosities (L⊙), highlighting its exceptional energy output.^[2] At an estimated age of about 12 million years, Gamma Cygni has evolved off the main sequence, transitioning through a post-red supergiant phase into the current yellow supergiant stage.^[2] Its high mass suggests it will ultimately culminate as a core-collapse supernova progenitor, potentially contributing to the enrichment of the interstellar medium in its vicinity. Observations indicate suspected semi-regular variability with a small photometric amplitude of ~0.1 magnitude, accompanied by radial velocity variations of ~2 km/s over timescales exceeding 100 days, indicative of pulsational instability in its envelope.^[2]

Spectral Classification

Gamma Cygni is classified as an F8 Ib supergiant in the Morgan-Keenan (MK) system of spectral classification, a designation that reflects its intermediate temperature among F-type stars and its high luminosity as an evolved supergiant with broad absorption lines arising from low surface gravity and expanded atmosphere. This luminosity class Ib indicates less luminous supergiant status, distinguishing it from brighter Ia or intermediate Iab types through the width and shape of diagnostic lines. The classification highlights the star's position in the post-main-sequence evolution, where atmospheric expansion leads to pressure-broadened profiles in its optical spectrum. The spectrum of Gamma Cygni exhibits prominent features typical of F-type supergiants, including strong absorption lines from the calcium H and K doublet near 3933 Å and 3968 Å, which are enhanced due to the low density in the outer layers, the neutral hydrogen Balmer series with maximum strength around Hβ and Hγ, and numerous lines from iron-group metals such as Fe I and Ti II that dominate the blue and green regions. Metallicity is near solar, with [Fe/H] ≈ +0.02. These features provide a benchmark for understanding atmospheric composition in similar evolved stars, where metal lines help trace turbulence and velocity fields. Since its adoption in 1943 with the introduction of the MK system by Morgan, Keenan, and Kellman, the spectrum of Gamma Cygni has served as one of the stable anchor points for calibrating F supergiant classifications, ensuring consistency across observational datasets by defining reference line strengths and ratios. Spectral analyses reveal abundance patterns influenced by evolutionary dredge-up, with enhancements in carbon and oxygen relative to solar values due to mixing of processed material from deeper layers during the red giant phase. Diagnostic tools, such as the equivalent width of the G-band (a molecular feature of CH at ~4300 Å), further confirm the Ib luminosity class by showing reduced strength compared to lower-luminosity giants, reflecting dilution of carbon-bearing molecules in the extended envelope. Recent spectroscopic analyses have refined atmospheric parameters, yielding an effective temperature of approximately 6000 K and surface gravity log g ≈ 0.85, values that align with the supergiant status while improving precision on evolutionary models.^[18]

Multiple Star System

Primary Star

Gamma Cygni A, designated as WDS J20222+4015A in the Washington Double Star Catalog, is the dominant primary component of this optical multiple star system and is classified as an F8 Iab supergiant.^[18] This classification indicates a luminous, evolved star in the yellow supergiant phase, with a bolometric luminosity exceeding 30,000 times that of the Sun and a surface temperature around 5,700 K.^[18] The primary overwhelmingly dominates the system's visual output, accounting for approximately 99% of the total flux owing to the faint magnitudes of the accompanying components (around 8th to 11th magnitude). No evidence of a close binary orbit involving the primary has been confirmed through astrometric or spectroscopic observations. The primary is separated from the BCD companion triplet by 41 arcseconds, equivalent to a projected physical distance of roughly 0.37 light-years given the system's distance of approximately 1,830 light-years (based on a Hipparcos parallax of 1.78 mas).^[18] This substantial separation implies minimal dynamical influence between the primary and companions, with no detected orbital motion in long-term Washington Double Star Catalog monitoring. The wide configuration suggests the components may be physically unbound or bound in a very wide orbit with a period potentially spanning thousands of years, consistent with many optical multiples in the solar neighborhood. As an evolved supergiant, the primary exhibits significant mass loss, estimated at approximately $10^{-6} \, M_\odot \, \mathrm{yr}^{-1}, which drives the formation of circumstellar material and envelopes detectable in infrared observations.^[19] This ongoing mass ejection shapes the star's extended atmosphere but, due to the large separation from the companions, does not lead to direct interactions or shared circumstellar structures within the system. The mass loss contributes to the evolutionary trajectory of the primary toward potential instability as a yellow supergiant, influencing its position near the Cepheid instability strip.^[19]

Companion Components

The Gamma Cygni multiple star system includes a secondary subsystem comprising components B, C, and D, located approximately 41 arcseconds from the primary star A. Components B and C form a close pair separated by 1.8 arcseconds, with visual magnitudes of 7.9 and 10.1, respectively, while component D is fainter at magnitude 11.0 and separated from the BC pair by approximately 26 arcseconds. The combined magnitude of the BCD subsystem is approximately 7.8, rendering it significantly fainter than the primary.^[18]^[2] The close BC pair was discovered by the American astronomer Sherburne Wesley Burnham in 1878 during his systematic survey of double stars, cataloged as BU 665. Component D was later resolved in 1911. Observations of relative proper motions indicate that the BCD components share no common motion with the primary and are likely an unrelated line-of-sight alignment rather than a physically bound group with the supergiant A, though the tight BC separation suggests they may form a bound binary with a short orbital period of a few years. No orbital elements have been published for the subsystem as of 2025.^[2] Spectral classifications for components B, C, and D are not detailed in standard double star catalogs, though their magnitudes suggest lower-mass main-sequence stars. The companions contribute less than 1% to the system's total luminosity due to the large magnitude difference with the primary.^[2]

Surrounding Environment

Gamma Cygni Nebula

The Gamma Cygni Nebula, cataloged as IC 1318 and also known as the Sadr Region, is a complex of diffuse emission nebulae surrounding the supergiant star Gamma Cygni (Sadr) in the constellation Cygnus. This nebula forms part of the larger Cygnus X star-forming region and spans an angular extent of about 1 degree across the sky, encompassing multiple bright patches and dark clouds.^[20] IC 1318 exhibits a structured appearance with three prominent emission components labeled A, B, and C, featuring bright rims and intricate filaments of ionized gas, often resembling a butterfly or shell-like morphology in wide-field images. The dominant emission arises from H-alpha lines, produced as ultraviolet radiation ionizes hydrogen atoms in the interstellar medium, creating a glowing H II region. A dark nebula, LDN 889, approximately 20 light-years thick, bisects components B and C, casting prominent dust lanes that obscure background light and contribute to the nebula's textured appearance.^[21] The nebula's physical extent is estimated at around 50 light-years for its main patches, with the overall complex covering up to 100 light-years, situated at a distance of approximately 4,900 light-years from Earth—farther than the foreground star Gamma Cygni at about 1,800 light-years. This places IC 1318 within the Orion Arm of the Milky Way, where it is associated with active star formation. Embedded within the nebula are young stellar clusters, such as NGC 6910, containing hot, massive O- and B-type stars that drive the ionization and shaping of the gas through stellar winds and radiation pressure. A key ionizing source for parts of IC 1318, including components B and C, is the O9V star GSC 03156-00657, rather than Gamma Cygni itself, which lacks sufficient ultraviolet output due to its F8 spectral type.^[3]^[22] IC 1318 was discovered photographically in the late 19th century by astronomer Williamina Fleming as part of her cataloging efforts at Harvard Observatory, with earlier visual notes on related features by William Herschel in 1786. The nebula is best observed using H-alpha filters to highlight its red emissions against the Milky Way's backdrop, revealing fine details invisible to the naked eye. Recent imaging from ground-based telescopes continues to uncover its filamentary structures, though no dedicated James Webb Space Telescope observations of IC 1318 have been reported as of 2025.^[22]^[23]

Supernova Remnant

The Gamma Cygni supernova remnant, designated SNR G78.2+2.1 or γ Cygni SNR, is a shell-type remnant spanning approximately 1–2 degrees in angular diameter, corresponding to a physical size of about 30–50 parsecs at its estimated distance.^[24] It is classified as a filled-shell structure with non-uniform brightness, featuring a prominent radio shell observed at 408 MHz by the Dominion Radio Astrophysical Observatory, where the flux density reaches around 480 Jy, indicating synchrotron emission from relativistic electrons in a magnetic field compressed by the shock wave. The remnant's age is estimated at 6,800–10,000 years based on Sedov-phase evolutionary models fitted to X-ray spectral data, placing it in the middle stages of supernova remnant expansion.^[25] Its distance is approximately 1.7–1.8 kpc, positioning it within the Cygnus star-forming complex but distinct from the nearer foreground star γ Cygni itself.^[24] Multi-wavelength observations reveal a complex structure driven by shock-heated plasma and particle acceleration. Chandra X-ray imaging from the 2000s detects bright clumps along the northern rim, with spectra showing thermal emission at temperatures of 0.6–1.2 keV, consistent with plasma heated to millions of degrees by the supernova shock.^[25] These X-ray features overlap partially with the radio shell, suggesting interactions between the blast wave and dense interstellar medium. Gamma-ray emission in the 100 MeV–1 TeV range has been detected by the Fermi Large Area Telescope, indicating diffusive shock acceleration of cosmic rays, with extended morphology that partially aligns with the radio boundary but shows offsets from the shell interior.^[26] The remnant overlies the γ Cygni region, encompassing the IC 1318 nebula and associated star-forming sites, where its shock expansion is believed to have triggered recent star formation by compressing molecular clouds, though the progenitor was a different massive star unrelated to the current γ Cygni supergiant.^[27] No direct causal link exists between the remnant and γ Cygni, as the star's evolutionary stage predates the supernova event by millions of years. A 2025 reanalysis of 15 years of Fermi-LAT data confirms distinct GeV and TeV morphologies, with a softer spectrum in the TeV band attributed to interactions of escaped cosmic rays with nearby molecular clouds, supporting a hadronic origin for the gamma rays and identifying the remnant as a candidate site for pulsar wind nebula-like features potentially linked to the nearby pulsar PSR J2021+4026.^[28]

References

[1]
Northern Cross: Stars, Constellation, Facts, Myth
Sep 30, 2022 · Sadr (Gamma Cygni), the star at the centre of the Northern Cross, is a yellow-white supergiant of the spectral type F8 Iab. It shines at ...
[2]
Sadr - JIM KALER
Nov 30, 2012 · Sadr (though second brightest) is the Gamma star of the constellation Cygnus, the Swan, and lies prominently at the center of Cygnus's famed asterism.
[3]
Gamma Cygni Nebula (Sadr Region) - Constellation Guide
Dec 31, 2018 · With 12 solar masses and 150 times the Sun's radius, the star is about 33,000 times more luminous than the Sun. The Gamma Cygni Nebula has ...
[4]
Sadr (Gamma Cygni): Star Type, Name, Location, Constellation
Oct 20, 2019 · Mass, 12.11 ± 0.71 M ; Luminosity, 33,023 L ; Radius, 150 ± 80 R ; Temperature, 5,790 ± 100 K ; Metallicity, +0.02 dex.
[5]
Gamma Cygni
### Key Astronomical Data for Gamma Cygni
[6]
A guide to star Sadr - BBC Sky at Night Magazine
Aug 18, 2022 · Sadr is known as Gamma (γ) Cygni and despite the gamma label, at mag. +2.2 it is the second-brightest star in Cygnus after mag. +1.3 Deneb ...
[7]
2013 July 9 - Supergiant Star Gamma Cygni - APOD
Jul 9, 2013 · Supergiant star Gamma Cygni lies at the center of the Northern Cross, a famous asterism in the constellation of the Swan (Cygnus).
[8]
Gamma Cyg
**Summary of Gamma Cyg (SIMBAD Data):**
[9]
Sadr - γ Cygni (gamma Cygni) - Star in Cygnus | TheSkyLive
Sadr visual magnitude is 2.2, making it the 63th brightest star in the sky. Because of its brightness, Sadr is clearly visible to the naked eye when observed ...
[10]
Summer Constellations
For northern observers, these are the three constellations whose brightest stars (Altair, Deneb and Vega) form the Summer Triangle – Aquila, Cygnus and Lyra – ...
[11]
Summer stargazing in Cygnus - Society for Popular Astronomy
Sep 1, 2022 · The mild nights of late summer are great for stargazing, and one of the most popular constellations, Cygnus, the Swan, is almost overhead.<|control11|><|separator|>
[12]
Summer Triangle: Vega, Deneb and Altair - Constellation Guide
Aug 3, 2021 · Sadr appears next to a large, diffuse emission nebula known as the Sadr Region or Gamma Cygni Nebula (IC 1318). The nebula's distance is ...
[13]
Two subtle wonders near Gamma Cygni | Astronomy.com
Jul 28, 2014 · Sadr (Gamma [γ] Cygni) holds a key location in the Cygnus part of the Milky Way, which stretches high overhead after sunset this month.Missing: facts - | Show results with:facts -
[14]
Bayer letters - Star Tales - Ian Ridpath
Johann Bayer's Uranometria, published in 1603 at Augsburg, Germany, was the first major printed star atlas and one of the most influential celestial atlases ...Missing: Cygni | Show results with:Cygni
[15]
Gamma Cygni
- **Bayer Designation**: Gamma Cygni (γ Cyg)
[16]
LacusCurtius • Allen's Star Names — Cygnus
### Summary of Gamma Cygni Names and Arabic Traditions
[17]
Cygnus Constellation (the Swan): Stars, Myth, Facts, Location
The brightest star in the constellation is Deneb, Alpha Cygni, which is also the 19th brightest star in the sky, with an apparent magnitude of 1.25.
[18]
The ultraviolet spectrum of Gamma Cygni. - NASA ADS
The ultraviolet spectrum of Gamma Cygni. Johnson, H. M.. Abstract ... Abundance;; Magnesium;; Rare Earth Elements;; Stellar Mass Ejection;; Stellar ...
[19]
Gamma Cygni
### Summary of Gamma Cygni Data
[20]
https://arxiv.org/pdf/astro-ph/0001214
[21]
Sadr
Taken from the Henry Draper Catalogue or no spectrum in source catalog. catalogues. source catalogue, FK4, catalogue number: 765. Durchmusterung, BD+39 4159.
[22]
[PDF] arXiv:astro-ph/0001214v1 12 Jan 2000
Together with NGC 6910, NGC 6914, the. BD+41◦3731 region and IC 1318, it is part of the giant star forming region 2 Cyg, at a distance of about 1 kpc ( ...<|separator|>
[23]
IC1318 - The Gamma Cygni Nebula - An Atlas of The Universe
The Gamma Cygni nebula, IC 1318, has three bright patches (parts A, B and C,) and they each have a diameter of about 50 light years.
[24]
[PDF] arXiv:2004.10500v1 [astro-ph.SR] 22 Apr 2020
Apr 22, 2020 · A possible ionizing source of the IC 1318 b/c nebula is an O9V type star names as 'GSC 03156-00657' (Arkhipova & Lozinskaia 1978 ...
[25]
Gamma Cygni Nebula (IC 1318) & Open Cluster NGC 6910
The Gamma Cygni Nebula is the remnant of a supernova (SNR G78.2+2.1), which occurred here about 6'800-10'000 years ago and triggered the formation of new stars ...
[26]
Exploring the region encompassing γ Cygni SNR and MAGIC J2019 ...
The γ Cygni SNR (or G78.2+2.1) is a young (-7 × 103 yr) supernova remnant (SNR) located in the Cygnus region at a distance of ~1.8 kpc (Higgs et al. 1977 ...
[27]
X-ray and radio observations of the γ Cygni supernova remnant G78 ...
From SIMBAD, the apparent V magnitude and B − V colour are V = 10.88 and B − V = 0.57. The spectral type is B3 and its distance is 980 pc (Manoj et al ...
[28]
AGILE Study of the Gamma-Ray Emission from the SNR G78.2+2.1 ...
2+2.1 (Gamma Cygni). In order to investigate the possible presence of γ rays associated with the SNR below 1 GeV, it is necessary to analyze the γ-ray radiation ...
[29]
The true extent of the Gamma Cygni supernova remnant
From the known age of 3C 10 and from estimates of its diameter (`-`14 pc) given by Clark and Caswell (1976), we estimate an age for the `y Cygni SNR of i0"'~°~ ...Missing: luminosity | Show results with:luminosity
[30]
γ-Cygni supernova remnant in γ-rays: Signatures of trapped and ...
We conduct a comprehensive analysis of the GeV γ-ray emission toward the γ-Cygni area based on 15 years of data from the Fermi-LAT instrument. The photon events ...