Achird (Eta Cassiopeiae A & B)
Coordinates, Gaia DR3 parallax (168.8 mas, 19.32 ly), 480-year G0V + K7V binary orbit (13.5″ separation), and observing guide for Achird.
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 |
|---|---|---|---|---|---|
| Apparent visual magnitude (Achird A) | 3.44 mag | ± 0.01 mag | Johnson UBV photometry (companion Achird B = 7.51 mag) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax (Gaia DR3) | 168.77 mas | ± 0.09 mas | Gaia DR3 five-parameter astrometry (5.925 ± 0.003 pc / 19.32 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Distance in light-years | 19.32 ly | ± 0.01 ly | Derived from Gaia DR3 parallax 168.77 ± 0.09 mas (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Binary orbital period (A–B) | 175320 d | ± 1800 d | Visual binary orbit since Herschel 1779 (~480 ± 5 yr, a = 11.99″ / 71 AU, e = 0.497) | definitive | SIMBAD Astronomical Database (CDS) |
| Primary solar-analog mass (Achird A) | 0.972 M☉ | ± 0.015 M☉ | Keplerian dynamical orbit + CHARA radius (1.039 R☉; Achird B = 0.57 ± 0.01 M☉) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Achird (Eta Cassiopeiae Nearby Solar Binary) at RA 00h 49m 06.29s, Dec +57° 48′ 54.7″ within Cassiopeia, 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
Achird (Eta Cassiopeiae, 24 Cassiopeiae) is a remarkable nearby visual binary star located just 5.925 ± 0.003 parsecs (19.32 light-years) from Earth in Cassiopeia, making it one of the closest Sun-like star systems in our galactic neighborhood. Discovered as a double star by William Herschel in August 1779, the two stars have been tracked through more than half of their 480-year eccentric orbit (semi-major axis 71 AU, ranging from 36 AU at periastron to 107 AU at apastron) while racing across the sky with a high proper motion of 1.22 arcseconds per year.
The primary star, Achird A (magnitude 3.44), is a near-twin of our own Sun: a G0 V main-sequence dwarf with 0.972 solar masses, 1.039 solar radii, an effective temperature of 5,973 K, and 1.23 solar luminosities, though slightly metal-poor ([Fe/H] = −0.28 dex). If you stood on a planet orbiting Achird A at 1.11 AU, it would look almost identical to the Sun in Earth's sky.
Orbiting Achird A at a current angular separation of ~13.5 arcseconds (position angle ~329°) is Achird B, a 7.51-magnitude K7 V orange-red dwarf (0.57 solar masses, 0.66 solar radii, 4,036 K) shining with only 6% of the Sun's bolometric luminosity. Because the two stars differ by nearly 2,000 K in temperature and 4.1 magnitudes in brightness, they display one of the most vivid color contrasts of any nearby dwarf binary.
Practical Visual Observing Guide
- Easy to locate with the naked eye inside the W of Cassiopeia, sitting almost exactly halfway between Schedar (α Cas) and Gamma Cassiopeiae (γ Cas).
- At 13.5 arcseconds separation, any 50–80 mm telescope at 60×–100× cleanly resolves Achird, revealing a warm buttercup-yellow primary (Achird A) accompanied by a striking garnet, copper-orange, or dusky purple-rose secondary (Achird B).
Catalog Cross-Identifications & Aliases
Common Aliases: η Cassiopeiae · HD 4614 · HIP 3821 · 24 Cassiopeiae
| Catalog Authority | Identifier |
|---|---|
| GAIADR3 | 425040000962559616 |
| HIP | HIP 3821 |
| HD | HD 4614 |
| SIMBAD | * eta Cas |
| WIKIDATA | Q13060 |
Related Objects, Comparisons & Guides
Tau Ceti (HD 10700 Nearby Solar Analog)
Coordinates, Gaia DR3 parallax (273.81 mas, 11.91 ly), magnitude 3.50, debris disk, and observing guide for G8V star Tau Ceti.
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.
Delta Cephei (Cepheid Distance Ladder Anchor)
Coordinates, Gaia/HST distance (290 pc), 5.366-day pulsation period, Leavitt law calibration, and observing guide for Delta Cephei.
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.
Proper Motion, Radial Velocity & Redshift
How astronomers combine tangential proper motion (mas/yr), Doppler radial velocity (km/s), and cosmological redshift (z) to reconstruct 3D space motion.
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
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- 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