Capella Aa
Star
Capella Aa - the cooler and larger of the two yellow giants that make up brilliant Capella, 42.8 light-years away in Auriga. It and its near-twin Ab circle each other every 104 days, so close that no telescope splits them by eye - only their blended spectrum reveals the pair. Together they shine as the sixth-brightest star in the night sky.
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Key facts
Mutual orbit
104 Earth days
Mass
5.11 × 10³⁰ kg
2.57 M☉
Radius
8,334,486 km
12× wider than the Sun
Surface temperature
4,970 K (4,697 °C)
Luminosity
3.01 × 10²⁸ W
Spectral type
G8III (yellow giant)
Science
Physical
- Mass
- 5.11 × 10³⁰ kg · 2.57 M☉
- Radius
- 8,334,486 km · 12× wider than the Sun
- Density
- 2.11 kg/m³ · 0.0021× water
- Computed from mass and radius
- Spectral type
- G8III (yellow giant)
Motion through space
- Note
- Catalogued for the system as a whole: the motion of its stars' common centre of mass.
- Proper motion
- 0.433 arcseconds per year · its drift across our sky
- SIMBAD — 2007A&A...474..653V
- Radial velocity
- +29.2 km/s · moving away from the Sun
- SIMBAD — 2007A&A...474..653V
- Speed across our line of sight
- 27 km/s
- Computed from proper motion and parallax
- Speed through space, relative to the Sun
- 39.7 km/s
- Computed from proper motion, parallax and radial velocity
Orbit (around the system barycenter)
- Semi-major axis
- 0.363 AU
- From the spectroscopic-binary orbit of Torres et al. 2015 (P = 104.02 d, a = 0.74 AU, e = 0.00)
- Eccentricity
- 0
- From the spectroscopic-binary orbit of Torres et al. 2015 (P = 104.02 d, a = 0.74 AU, e = 0.00)
- Orbital period
- 104.0 days
- From the spectroscopic-binary orbit of Torres et al. 2015 (P = 104.02 d, a = 0.74 AU, e = 0.00)
Rotation
- Note
- The cooler giant rotates slowly; the pair circle each other every 104 days.
Composition
- By mass
- A yellow giant of about 2.6 solar masses that has evolved off the main sequence, swelling to ~12 times the Sun's width. With its near-twin Ab it forms one of the best-measured giant binaries, a key test of how Sun-like stars age.
Temperature
- Surface
- 4,970 K (4,697 °C)
Radiation & temperature
- Luminosity
- 3.01 × 10²⁸ W · 78.7 L☉
- Surface temperature
- 4,970 K (4,697 °C)
- Peak wavelength
- 583 nm · yellow light
- Computed (Wien's law) from the surface temperature
Discovery & history
- Method
- Naked-eye star
- Name
- Its IAU name Capella is Latin for 'little she-goat'; it is Alpha Aurigae, the brightest star of the Charioteer, anciently the goat he carries.
Capella has been a bright marker of the northern sky since antiquity. In 1899 it became one of the first stars found to be a spectroscopic binary, its two giant components betrayed by the rhythmic Doppler shift of their light; in 1919 it was among the first stars resolved by an interferometer.
Images
Images from open archives · credit shown per image