Capella vs Altair First-Magnitude Comparison
Compare spectroscopic G-giant binary Capella (Alpha Aurigae) against rapidly rotating A7 V dwarf Altair (Alpha Aquilae) in rotation, distance, and age.
Compared Celestial Objects
Capella (Alpha Aurigae Giant Binary)
Coordinates, Hipparcos distance (42.9 ly), twin G-giant 104-day interferometric binary orbit, and observing guide for Capella in Auriga.
Constellation: Auriga · Coords: 05h 16m 41.36s, +45° 59′ 52.8″
Altair (Alpha Aquilae Rapid Rotator)
Coordinates, Hipparcos distance (16.73 ly), magnitude 0.76, 8.9-hour oblate spin imaged by CHARA, and observing guide for Altair in Aquila.
Constellation: Aquila · Coords: 19h 50m 47.00s, +08° 52′ 06.0″
Normalized Shared Parameter Table
| Parameter | Unit / Scale | Capella (Alpha Aurigae Giant Binary) | Altair (Alpha Aquilae Rapid Rotator) |
|---|---|---|---|
| Apparent Visual Magnitude (V) | mag | +0.08 ± 0.01 (combined Aa+Ab) | +0.76 ± 0.01 |
| Heliocentric Distance | pc | 13.123 ± 0.009 pc (42.80 ly) | 5.130 ± 0.005 pc (16.73 ly) |
| Spectral Classification | type | G8 III + G0 III binary | A7 Vn rapid rotator |
| Combined Bolometric Luminosity | L☉ | 151.2 ± 2.5 L☉ (78.7 + 72.7 L☉) | 10.6 ± 0.3 L☉ |
| Equatorial Rotational Velocity (v sin i) | km/s | 4.1 km/s (Aa) / 35 km/s (Ab) | 240 ± 5 km/s (8.9 hr period) |
Astrophysical & Observational Synthesis
When you look at yellow-gold Capella (V = +0.08) high in the winter sky of Auriga, your eye integrates two nearly equal 2.5-solar-mass yellow giant stars (Capella Aa at 11.98 R☉, 4,970 K and Capella Ab at 8.83 R☉, 5,730 K) orbiting each other in a perfectly circular 104.02-day orbit separated by just 0.74 AU (56 milliarcseconds). Accompanied by a distant red-dwarf pair (Capella H and L, 12 arcminutes away), Capella is a canonical quadruple system.
Altair (V = +0.76) in the summer constellation Aquila is a single 1.86-solar-mass A7 V main-sequence star sitting 2.56 times closer to the Sun (5.13 pc vs 13.12 pc). CHARA optical interferometry directly resolved Altair's photosphere, proving it spins once every 8.9 hours at an equatorial velocity of 286 km/s (~90% of breakup speed).
That centrifugal spin flattens Altair's equatorial radius (2.008 R☉) to 23% wider than its polar radius (1.634 R☉), triggering von Zeipel gravity darkening that cools Altair's equator to 6,860 K while its poles blaze at 8,450 K.
Related Reference Guides
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.
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.
Primary Catalogs & Scientific Sources
- 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
- Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC