Burnham describes Capella as the sixth brightest star in the sky, and the nearest star to the pole of the stars that are first magnitude or brighter.
A Pair of Giants
Capella is a quadruple system consisting of two giants and two dwarves. The A and B components are a close binary consisting of a yellow G5IIIe giant having 7.2 times the diameter of the sun 2.67 times the mass and a yellow G0III giant having 5.6 times the diameter of the sun and 2.55 times the mass.
The two stars individually have 80 and 50 times the luminosity of the sun. The data of the The Bright Star Catalog seem to indicate that the G5 star is the brighter of the pair.
The spectral types imply temperatures near that of the sun, 5600 K for the G0 star and 5000 K for the G5.
The two stars are separated by about 65 million miles and revolve with a period of 104 days.
The Red Dwarves
The remaining pair of stars in the system are two red dwarves of spectral type M1 and M5 designated as Capella H. These stars appear at Apparent Magnitude 10.0 and 13.7 respectively. The separation between the stars is 2 sec of arc, corresponding to a projected distance of 26 A.
The pair of dwarves appear separated from the AB pair by 12 minutes of arc, a projected distance of at least 0.17 ly.
The dwarves are small stars having approximately 40% and 10% of the mass of the sun respectively with 60% and 30% of the sun's diameter. The stars have respective luminosities of about 1.4% of the sun and 0.05%.