Capella (Alpha Aurigae)
Coordinates, Hipparcos distance (42.9 ly), twin G-giant 104-day interferometric binary orbit, and observing guide for Capella in Auriga.
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 |
|---|---|---|---|---|---|
| Combined apparent visual magnitude (V) | 0.08 mag | ± 0.01 mag | Johnson UBV photometry (Aa = 0.76 mag, Ab = 0.91 mag) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric & orbital parallax | 76.2 mas | ± 0.46 mas | Hipparcos New Reduction + CHARA/NPOI interferometric orbital parallax (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 42.92 ly | ± 0.05 ly | 3D spectroscopic + interferometric orbital parallax (Torres et al. 2015: 13.159 ± 0.013 pc) | definitive | SIMBAD Astronomical Database (CDS) |
| Binary orbital period (Aa–Ab) | 104.0213 d | ± 0.0002 d | Combined radial-velocity and optical interferometry | definitive | SIMBAD Astronomical Database (CDS) |
| Primary giant mass (Capella Aa) | 2.5687 M☉ | ± 0.0074 M☉ | 3D dynamical orbit (Ab = 2.4828 ± 0.0067 M☉) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Capella (Alpha Aurigae Giant Binary) at RA 05h 16m 41.36s, Dec +45° 59′ 52.8″ within Auriga, 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
Capella (Alpha Aurigae) is the sixth-brightest star in the night sky and the third-brightest in the Northern Celestial Hemisphere at combined magnitude 0.08. Located 13.16 ± 0.01 parsecs (42.92 light-years) away in Auriga, what appears to the naked eye and conventional telescopes as a single warm yellow star is actually a remarkable quadruple system led by two evolved G-type giant stars (Capella Aa and Ab) orbiting their common center of mass every 104.02 days at a separation of 0.74 AU (roughly the distance from Venus to the Sun).
In 1919 J. A. Anderson and Francis Pease used Michelson's stellar interferometer on the 100-inch Hooker Telescope at Mount Wilson to resolve Capella's 55-milliarcsecond binary orbit—the very first binary star ever resolved by optical interferometry. Because both components have been tracked both spectroscopically and astrometrically for over a century, Capella provides the most precise model-independent mass measurements of any giant stars in astrophysics.
Capella Aa (2.57 solar masses, 11.98 solar radii, 4,970 K) has already settled into stable core helium fusion in the red clump, while slightly less massive Capella Ab (2.48 solar masses, 8.83 solar radii, 5,730 K) is still crossing the Hertzsprung gap and spinning rapidly, generating a hot X-ray corona. Twelve arcminutes (~10,000 AU) away orbits a co-moving pair of red dwarfs, Capella H and L.
Practical Visual Observing Guide
- At declination +46°00′, Capella is circumpolar for observers north of +44°N (including northern Europe, Canada, and the northern United States), shining golden-yellow high overhead on winter evenings.
- While the 0.055-arcsecond Aa–Ab pair can only be separated by long-baseline interferometers, a 150 mm telescope at 100× can spot the 10.2-magnitude red dwarf companion Capella H (GJ 195 A) 12 arcminutes southeast of Capella.
- Just 3°–5° southwest of Capella lies the 'Kids' asterism—three 3rd- and 4th-magnitude stars (Epsilon, Zeta, and Eta Aurigae) that include two famous long-period eclipsing supergiant binaries.
Catalog Cross-Identifications & Aliases
Common Aliases: α Aurigae · HD 34029 · HIP 24608 · 13 Aurigae
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 24608 |
| HD | HD 34029 |
| SIMBAD | * alf Aur |
| WIKIDATA | Q12975 |
Related Objects, Comparisons & Guides
Aldebaran (Alpha Tauri Orange Giant)
Coordinates, Hipparcos distance (66.6 ly), 45.1 R☉ radius, Hyades foreground alignment, and observing guide for Aldebaran in Taurus.
Pleiades (Messier 45 Open Cluster)
Coordinates, Gaia DR3 distance (136.2 pc), magnitude 1.6, reflection nebulosity, and observing guide for the Pleiades (M45) in Taurus.
Castor (Alpha Geminorum Sextuple System)
Coordinates, Hipparcos distance (50.8 ly), 5.6″ A–B binary orbit, YY Geminorum eclipsing pair, and observing guide for Castor.
Arcturus (Alpha Bootis Red Giant)
Coordinates, Hipparcos distance (36.7 ly), magnitude −0.05, halo-stream kinematics, and observing guide for Arcturus in Boötes.
Winter Circle, Orion & Taurus Observing Guide
Winter sky observing tour of Orion, Taurus, Canis Major, Auriga, and Perseus: bright OB/M supergiants, open clusters, M42, and the Crab Nebula (M1).
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Bright, Multiple & Variable Stars Catalog
Cross-reference catalog of solar-neighborhood and navigation stars across Bayer, Henry Draper (HD), Hipparcos (HIP), WDS, and Gaia DR3 designations.
Scientific Sources & Dataset Provenance
- 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)
- IAU Working Group on Star Names (WGSN) (2026 IAU WGSN Bulletin): Mamajek, E., et al. (IAU WGSN) 2016–2026, IAU Catalog of Star Names — International Astronomical Union
- 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