Capella Ab
Star
Capella Ab - the hotter, slightly smaller yellow giant of the Capella pair, circling its companion Aa every 104 days at a distance of less than one AU. The two are among the closest, best-measured giant stars, benchmarks for how Sun-like stars swell as they age.
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Key facts
Mutual orbit
104 Earth days
Mass
4.93 × 10³⁰ kg
2.48 M☉
Radius
6,143,031 km
8.83× wider than the Sun
Surface temperature
5,730 K (5,457 °C)
Luminosity
2.78 × 10²⁸ W
Spectral type
G0III (yellow giant)
Science
Physical
- Mass
- 4.93 × 10³⁰ kg · 2.48 M☉
- Radius
- 6,143,031 km · 8.83× wider than the Sun
- Density
- 5.08 kg/m³ · 0.0051× water
- Computed from mass and radius
- Spectral type
- G0III (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
- 0.377 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
- Rotates faster than its companion Aa; the two orbit every 104 days.
Composition
- By mass
- A yellow giant of about 2.5 solar masses, hotter than its companion and caught in a rapid phase of stellar evolution (the Hertzsprung gap). Its precisely known mass and size make it a valuable check on models of evolving stars.
Temperature
- Surface
- 5,730 K (5,457 °C)
Radiation & temperature
- Luminosity
- 2.78 × 10²⁸ W · 72.7 L☉
- Surface temperature
- 5,730 K (5,457 °C)
- Peak wavelength
- 506 nm · green-yellow visible light
- Computed (Wien's law) from the surface temperature
Discovery & history
- Method
- Naked-eye star
- Name
- The secondary (Ab) of the Capella A giant pair; 'Capella' names the whole bright system in Auriga.
Resolved from its companion only by spectroscopy (1899) and later by interferometry, Capella Ab's mass and size are known to high precision from the pair's orbit - a rare direct check on models of evolved stars.
Images
Images from open archives · credit shown per image