Delta Cygni
Third-magnitude star in the constellation Cygnus
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Delta Cygni is a binary star of a combined third-magnitude in the constellation of Cygnus. It is also part of the Northern Cross asterism whose brightest star is Deneb. Its name is a Bayer designation that is Latinized from δ Cygni, and abbreviated Delta Cyg or δ Cyg. Based upon parallax measurements obtained during the Hipparcos mission, Delta Cygni is located roughly 165 light-years (51 parsecs) distant from the Sun.[1]
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cygnus |
| Right ascension | 19h 44m 58.47854s[1] |
| Declination | +45° 07′ 50.9161″[1] |
| Apparent magnitude (V) | 2.87[2] |
| Characteristics | |
| Spectral type | A0 IV[3] (B9 III + F1 V[4]) |
| U−B color index | −0.10[5] |
| B−V color index | −0.02[5] |
| Variable type | Suspected[6] |
| Astrometry | |
| Radial velocity (Rv) | −20.1[7] km/s |
| Proper motion (μ) | RA: +44.07[1] mas/yr Dec.: +48.66[1] mas/yr |
| Parallax (π) | 19.77±0.48 mas[1] |
| Distance | 165 ± 4 ly (51 ± 1 pc) |
| Absolute magnitude (MV) | −0.74[8] |
| Orbit[9] | |
| Primary | A |
| Name | B |
| Period (P) | 780.27 yr |
| Semi-major axis (a) | 3.0″ |
| Eccentricity (e) | 0.4670 |
| Details[10] | |
| Mass | 2.93 M☉ |
| Radius | 4.81±0.36[11] R☉ |
| Luminosity | 155 L☉ |
| Surface gravity (log g) | 3.4±0.25[11] cgs |
| Temperature | 10400±400[11] K |
| Rotational velocity (v sin i) | 142[11] km/s |
| Other designations | |
| Fawaris, δ Cyg, 18 Cygni, 18 Cyg, BD+44°3234, HD 186882, HIP 97165, HR 7528, SAO 48796, WDS J19450+4508AB | |
| Database references | |
| SIMBAD | data |
Delta Cygni's two components are designated Delta Cygni A (officially named Fawaris /fəˈwɛərɪs/)[12] and B. More widely separated is a faint third component, a 12th magnitude star that is moving along with the others. Together they form a triple star system.[13]
Nomenclature
δ Cygni (Latinised to Delta Cygni) is the binary's Bayer designation. The designations of the two components as Delta Cygni A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[14]
Traditionally, Delta Cygni had no proper name.[13] It belonged to the Arabic asterism al-Fawāris (الفوارس), meaning "the Riders" in indigenous Arabic,[15] together with Zeta, Epsilon, and Gamma Cygni, the transverse of the Northern Cross. In 2016, the IAU organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[17] It approved the name Fawaris for the component Delta Cygni A on 1 June 2018 and it is now so included in the List of IAU-approved Star Names.[12]
In Chinese, 天津 (Tiān Jīn), meaning Celestial Ford, refers to an asterism consisting of Delta Cygni, Gamma Cygni, 30 Cygni, Alpha Cygni (Deneb) and Nu, Tau, Upsilon, Zeta and Epsilon Cygni.[18] Consequently, the Chinese name for Delta Cygni itself is 天津二 (Tiān Jīn èr, English: the Second Star of Celestial Ford).[19]
Properties
The primary, Delta Cygni A, is a blue-white giant star of spectral class B9,[4] with a temperature of 10,400 K.[11] It is nearing the end of its main-sequence life stage with a luminosity 155 times that of the Sun,[10] a radius of 4.81 solar radii,[11] and a mass approximately 2.93 solar masses. Like many hot stars, it spins rapidly, at least 135 kilometers per second at the equator, about 60 times that of the Sun.[10]
The close companion Delta Cygni B is a yellow-white F-type main-sequence star of the sixth magnitude (6.33) with a luminosity about 6 times that of the Sun, and a mass about 1.5 times the Sun's. The two stars orbit each other at an average distance of 157 AU and a period of 780 years.[13]
The much more distant third companion is an orange (class K) twelfth magnitude star, and only two thirds as massive.[13]
The two main stars together appear with a spectral type of A0 IV.[3] As seen from Earth, the entire triple star system of Delta Cygni shines at a combined apparent magnitude of 2.87.[2] Both δ Cygni A and B have been suspected to vary in brightness. δ Cygni A was reported in 1951 as varying between magnitudes 2.85 and 2.89, and δ Cygni B was reported in 1837 to vary between magnitudes 6.3 and 8.5. The variability of the stars has not been confirmed.[6]
Pole Star
Delta Cygni is a visible star located within 3° of the precessional path traced across the celestial sphere by the Earth's North pole. For at least four centuries around 11,250 AD it will probably be considered a pole star, a title currently held by Polaris which is just 0.5° off of the precessional path.