| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cygnus |
| Right ascension | 21h 42m 42.8034s[2] |
| Declination | +43° 35′ 09.864″[2] |
| Apparent magnitude (V) | 7.7–12.4[3] |
| Characteristics | |
| Spectral type | K5V[4] |
| Variable type | Dwarf nova[3] |
| Astrometry | |
| Radial velocity (Rv) | −6.3±2.3[5] km/s |
| Proper motion (μ) | RA: 112.373±0.113[2]mas/yr Dec.: 33.589±0.094[2]mas/yr |
| Parallax (π) | 8.7242±0.0491 mas[2] |
| Distance | 372±7 ly (114±2[6] pc) |
| Orbit[5] | |
| Period (P) | 0.27512973[7] days |
| Semi-major axis (a) | 1.51–1.77 R☉ |
| Inclination (i) | 45–56° |
| Semi-amplitude (K1) (primary) | 71.4±1.6 km/s |
| Semi-amplitude (K2) (secondary) | 158.6±0.7 km/s |
| Details[5] | |
| Red dwarf | |
| Mass | 0.19–0.30 M☉ |
| Temperature | 4,560[7] K |
| White dwarf | |
| Mass | 0.42–0.67 M☉ |
| Other designations | |
| SS Cyg,HD 206697,BD+42°4189a,TYC 3196-723-1 | |
| Database references | |
| SIMBAD | data |
SS Cygni is a variable star in the northern constellationCygnus (the Swan). It was discovered in 1896 by Louisa D. Wells, a computer working underEdward Pickering atHarvard College Observatory.[8][9] It is the prototype of the subclass ofdwarf novae that show only normal eruptions. It typically rises from 12th magnitude to 8th magnitude for 1–2 days every 7 or 8 weeks. The northerlydeclination of SS Cygni (about 44° N) makes the star almostcircumpolar from European and North American latitudes, allowing a large proportion of the world'samateur astronomers to monitor its behavior. Furthermore, since the star lies against the rich backdrop of theMilky Way band, the telescope field of view around SS Cygni contains an abundance of useful brightness comparison stars.
SS Cygni, like all othercataclysmic variables, consists of a closebinary system. One of the components is ared dwarf-type star, cooler than the Sun, while the other is awhite dwarf. The stars in the SS Cygni system are separated by a distance "only" 1.51 times larger than the Sun,[7] completing their orbital revolution in slightly over six hours and 36 minutes. The inclination of the system is estimated to be 45-56 degrees, yielding masses in the range of0.42–0.67 M☉ for the white dwarf primary star and0.19–0.30 M☉ for the red dwarf secondary star.[5]
Astronomically speaking, SS Cygni is also fairly close by. Originally thought to be at 90 to 100 light years,[10] its distance was revised in 1952 to about 400 light years. In 2007Hubble Space Telescope data indicated a distance of about 540 light years, though this value caused difficulties with the theory of dwarf novae;[11] this was checked during 2010–2012 usingradio astrometry withVLBI, which yielded a smaller distance of 114 ± 2 parsecs (371.8 ± 6.5 ly).[6] This value is much more in accord with the old (≈400 light-year) value, and it removes completely the difficulties the larger HST distance made for the theory of dwarf novae.
