SU Cygni
Variable star system in the constellation Cygnus
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SU Cygni is a triple star system in the northern constellation of Cygnus, abbreviated SU Cyg. The primary component of the system is a classical Cepheid variable with a period of 3.84559 days. The changing luminosity of this star causes the system to vary in brightness from a peak apparent visual magnitude of 6.44 down to magnitude 7.22 over the course of its cycle.[3] The distance to this system is approximately 3,000 light years based on parallax measurements, and is the most accurate distance among Cepheids.[7] It is a member of the Turner 9 open cluster of stars.[11]
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cygnus |
| Right ascension | 19h 44m 48.734s[2] |
| Declination | +29° 15′ 52.90″[2] |
| Apparent magnitude (V) | 6.44 to 7.22[3] |
| Characteristics | |
| Spectral type | F2Iab: + B8.0V + A0V:[4][5] |
| Variable type | δ Cep[3] |
| Astrometry | |
| Radial velocity (Rv) | −21.5±2.4[6] km/s |
| Proper motion (μ) | RA: 0.107 mas/yr[2] Dec.: −3.247 mas/yr[2] |
| Parallax (π) | 1.080±0.006 mas[7] |
| Distance | 3,021 ± 16 ly (926.3±5.0 pc)[7] |
| Orbit[7] | |
| Primary | SU Cyg A |
| Name | SU Cyg B |
| Period (P) | 549.077±0.013 d |
| Semi-major axis (a) | 2.827±0.009 AU |
| Eccentricity (e) | 0.339±0.002 |
| Inclination (i) | 81.28±0.27° |
| Longitude of the node (Ω) | 266.24±0.24° |
| Periastron epoch (T) | 2,443,765.94±0.63 HJD |
| Argument of periastron (ω) (secondary) | 223.18±2.12° |
| Semi-amplitude (K1) (primary) | 30.25±0.05 km/s |
| Semi-amplitude (K2) (secondary) | 28.59±0.15 km/s |
| Orbit[7] | |
| Primary | SU Cyg Ba |
| Name | SU Cyg Bb |
| Period (P) | 4.67529(1) d |
| Semi-major axis (a) | ≥0.0287±0.0001 |
| Periastron epoch (T) | 2,456,977.994±0.003 HJD |
| Semi-amplitude (K1) (primary) | 66.89±0.21 km/s |
| Details[7] | |
| Aa | |
| Mass | 4.859±0.058 M☉ |
| Radius | 31.9±6.0 R☉ |
| Luminosity | 2,138±109 L☉ |
| Surface gravity (log g) | 2.1–2.4[8] cgs |
| Temperature | 5,956 to 6,314[8] K |
| Metallicity [Fe/H] | −0.03±0.04[9] dex |
| Ba | |
| Mass | 3.595±0.033 M☉ |
| Bb | |
| Mass | 1.546±0.009 M☉ |
| Other designations | |
| SU Cyg, BD+28°3460, GC 27336, HD 186688, HIP 97150, HR 7518, SAO 87659, PPM 109630, WDS J19448+2916Aa,Ab[10] | |
| Database references | |
| SIMBAD | data |
The variable luminosity of this star was announced by G. Müller and P. Kempf in 1898.[12] The following year, M. Luizet determined a period of 3.846 days.[13] In 1906, the radial velocity of this star was found to be variable by J. D. Maddrill, with its cycle matching the luminosity period but trailing in phase by half a day.[14] By 1916 it was classified as a Cepheid variable, with spectrographic studies showing that the spectral type varied over the course of each cycle. It ranged from a class of A6 near peak luminosity down to F7 at minimum.[15]
A companion star, designated component B, was detected photometrically by B. F. Madore in 1977,[16] with the colors suggesting a B-type main-sequence star with a class of B6–7V. This finding was supported by observations of J. D. Fernie in 1979, who determined a class of B6V. In 1984, M. Imbert measured an orbital period of 549.2 days for the pair, with an orbital eccentricity (ovalness) of 0.35.[17] Ultraviolet observations with the IUE spacecraft in 1985 showed that the companion is a close binary system with a period of 4.7 days. The members of this pair have classes of B8 and A0.[18] In 1998, the B-type companion was found to be a chemically peculiar HgMn star.[19]
There is a candidate comoving companion star located at an angular separation of 24″ to the west of SU Cyg. It is an A-type main-sequence star with a class of A2V. This star does not appear to be gravitationally bound to SU Cyg.[11]