SS Cygni (HD 206697)
Coordinates, VLBI/Gaia distance (114 pc), 6.60-hour white-dwarf + K-dwarf orbit, 49-day accretion disk outbursts, and guide for SS Cygni.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Visual magnitude range (quiescence to outburst) | 12.1 mag | ± 2 mag | AAVSO monitoring (quiescent at V ≈ 12.1; erupts to V ≈ 8.2–8.5 every ~49 days) | definitive | SIMBAD Astronomical Database (CDS) |
| Trigonometric distance (VLBI + Gaia DR3) | 114.6 pc | ± 0.6 pc | VLBA/EVN radio parallax (Miller-Jones et al. 2013: 114 ± 2 pc) + Gaia DR3 (114.6 ± 0.6 pc / 373.8 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Close binary orbital period | 0.2751297 d | ± 1e-7 d | Double-lined radial velocity (6 hours 36 minutes 11 seconds) | definitive | SIMBAD Astronomical Database (CDS) |
| Accreting white dwarf mass | 0.94 M☉ | ± 0.1 M☉ | Spectroscopic orbit & gravitational redshift (K-dwarf donor = 0.59 ± 0.06 M☉) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots SS Cygni (Cataclysmic Dwarf Nova Binary) at RA 21h 42m 42.80s, Dec +43° 35′ 09.9″ within Cygnus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
SS Cygni (HD 206697), discovered on Harvard photographic plates by Louisa D. Wells in 1896 in northeastern Cygnus, is the brightest and most intensively observed dwarf nova (U Geminorum-type cataclysmic variable) in the northern sky. Since its discovery, amateur and professional astronomers with the American Association of Variable Star Observers (AAVSO) have recorded over half a million visual and CCD observations of SS Cygni, never missing a single outburst in more than 125 years.
Located 114.6 ± 0.6 parsecs (373.8 light-years) from Earth, SS Cygni is an ultra-compact interacting binary in which a ~0.59-solar-mass K4–K5 V orange-red dwarf and a dense ~0.94-solar-mass white dwarf whip around their common center of mass every 6 hours and 36 minutes separated by only ~2.0 solar radii (1.4 million kilometers). Because the K-dwarf fills its gravitational Roche lobe, a stream of hydrogen gas pours continuously through the inner Lagrangian point into a swirling accretion disk surrounding the white dwarf.
Unlike a classical or recurrent nova (which is a thermonuclear fusion explosion on the white dwarf's surface), SS Cygni's outbursts are powered purely by gravitational potential energy inside the accretion disk: for ~35 to 45 days the disk accumulates gas in a cool, low-viscosity neutral state (magnitude 12.1) until it hits a critical density threshold (~10,000 K) where hydrogen ionizes, triggering a thermal-viscous instability that dumps matter onto the white dwarf and causes the disk to blaze 40 times brighter (up to magnitude 8.2) within 24 hours!
Practical Visual Observing Guide
- Located in northeastern Cygnus ~3° northeast of 3.8-magnitude Rho Cygni (ρ Cyg) and 1.2° northeast of the 5.1-magnitude red carbon star V460 Cygni (75 Cygni).
- During quiescence ( ~75% of the time), SS Cygni sits near magnitude 12.1 and requires a 100–150 mm (4–6 inch) telescope at 100×–150×; during an outburst (every 7 to 8 weeks, lasting 1 to 2 weeks), it jumps to magnitude 8.2–8.5 and is easily visible in a 50 mm finder scope or 15×70 binoculars.
Catalog Cross-Identifications & Aliases
Common Aliases: HD 206697 · BD+42 4189a · AAVSO 2138+43
| Catalog Authority | Identifier |
|---|---|
| GAIADR3 | 1966823426230804864 |
| HD | HD 206697 |
| SIMBAD | V* SS Cyg |
| WIKIDATA | Q2207885 |
Related Objects, Comparisons & Guides
T Coronae Borealis (The Blaze Star Nova)
Coordinates, Gaia DR3 distance (916 pc), 227.57-day symbiotic M4 III + 1.37-M☉ white dwarf orbit, 80-year recurrent nova cycle, and guide for T CrB.
Albireo (Beta Cygni Color Double Star)
Coordinates, Gaia DR3 parallaxes of Beta Cygni A & B, 34.6″ angular separation, color contrast, and observing guide for Albireo in Cygnus.
Cygnus X-1 (HDE 226868 X-Ray Black Hole)
Coordinates, VLBA/Gaia distance (2.22 kpc), 21.2-M☉ stellar black hole mass, 5.60-day O-supergiant binary, and observing guide for Cygnus X-1.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Trigonometric Parallax, Parsecs & Gaia DR3
How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.
Scientific Sources & Dataset Provenance
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg