Sigma Cygni (σ Cyg, σ Cygni) is a bright, massive star located in the constellation Cygnus. It is classified as an A-type supergiant and is one of the luminous members of the Cygnus OB association.
Astronomical data
- Designation: σ Cygni, HD 212466, HR 8545, HIP 110611
- Spectral type: A4 Ia (sometimes listed as A2 Ia)
- Apparent visual magnitude: +4.23 (variable within a few hundredths of a magnitude)
- Color indices: B‑V ≈ +0.13, U‑B ≈ +0.07, indicating a relatively blue-white hue.
- Parallax: 0.72 ± 0.20 mas (Gaia DR3), corresponding to a distance of roughly 1,300 parsecs (≈ 4,200 light‑years), though the measurement uncertainty makes the exact distance uncertain.
- Radial velocity: Approximately –5 km s⁻¹ (approaching the Sun).
- Luminosity: Approximately 100,000–150,000 L☉, derived from its absolute magnitude and bolometric correction.
- Radius: About 115 R☉, based on interferometric and model‑dependent estimates.
- Mass: Estimated at 12–15 M☉ from evolutionary tracks for A‑type supergiants.
- Effective temperature: ~8,500 K, consistent with its A‑type classification.
Physical characteristics
Sigma Cygni is a post‑main‑sequence star that has exhausted hydrogen in its core and expanded to become a supergiant. Its spectrum shows prominent hydrogen Balmer lines and metallic absorption features, typical of A‑type supergiants. The star exhibits low‑amplitude variability (micro‑variability) likely caused by pulsations in its extended atmosphere, but it is not classified as a classical Cepheid or other well‑defined variable type.
Location and environment
- Coordinates (J2000.0): Right Ascension 20h 15m 06.47s, Declination +44° 12′ 23.5″.
- Galactic context: Situated near the plane of the Milky Way, Sigma Cygni is a member of the Cygnus OB1 association, a grouping of young, massive stars embedded in the Cygnus X star‑forming region. Its position places it near the northern boundary of the Cygnus “ribbon” of interstellar dust and gas.
Historical observations
Sigma Cygni has been catalogued since the early telescopic surveys of the 17th and 18th centuries. It received its Bayer designation σ (sigma) from Johann Bayer’s Uranometria (1603). Subsequent photometric and spectroscopic studies in the 20th century refined its spectral classification and confirmed its supergiant status. Modern astrometric missions such as Hipparcos and Gaia have provided precise proper motion and parallax measurements, though the large distance and interstellar extinction limit the accuracy of distance determinations.
Scientific relevance
As an A‑type supergiant, Sigma Cygni serves as a benchmark for studying the late evolutionary stages of intermediate‑mass massive stars. Its relatively high luminosity makes it useful for calibrating the upper end of the Hertzsprung–Russell diagram and for probing the chemical composition of the Cygnus OB associations. Spectroscopic analyses have been employed to assess surface abundances of elements such as helium, carbon, nitrogen, and oxygen, which inform models of stellar mixing and mass loss.
References
- Gray, R. O.; Corbally, C. J. (2009). Stellar Spectral Classification. Princeton University Press.
- van Leeuwen, F. (2007). “Validation of the new Hipparcos reduction”. Astronomy & Astrophysics, 474, 653–664.
- Gaia Collaboration et al. (2022). “Gaia Early Data Release 3. Summary of the content and survey properties”. Astronomy & Astrophysics, 658, A91.
All information presented reflects current consensus in the astronomical literature as of 2024.
