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) | 1.0275 ± 0.0522 mas[2] |
Distance | 3,200 ± 200 ly (970 ± 50 pc) |
Orbit[7] | |
Primary | SU Cyg A |
Companion | SU Cyg B |
Period (P) | 549.24±0.02 d |
Eccentricity (e) | 0.350+0.004 −0.003 |
Periastron epoch (T) | 2,443,766.2±1.1 HJD |
Argument of periastron (ω) (secondary) | 223.85±0.85° |
Semi-amplitude (K1) (primary) | 29.83±0.15 km/s |
Details | |
Primary | |
Mass | 4.7[4] M☉ |
Radius | 43.9[8] R☉ |
Luminosity | 1,349[4] L☉ |
Surface gravity (log g) | 2.1–2.4[9] cgs |
Temperature | 5,956 to 6,314[9] K |
Metallicity [Fe/H] | −0.03±0.04[7] dex |
Secondary | |
Mass | 4.7±0.7[10] M☉ |
Other designations | |
Database references | |
SIMBAD | data |
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 3.8455473 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 3,200 light years based on parallax measurements.[2] It is a member of the Turner 9 open cluster of stars.[10]
The variable luminosity of this star was announced by G. Müller and G. 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.[8] 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.[17] In 1998, the B-type companion was found to be a chemically peculiar HgMn star.[18]
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.[10]
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