Bright Capella plays it close to the vest when it comes to companions, but with a good map and steady skies you can track down its dwarf binary.
'Tis the season of Capella's rise in the northeastern sky. This brilliant star, the alpha luminary of Auriga, announces the coming of winter as loudly as any of Orion's stars; soon enough it will sparkle overhead like a glittering shard of ice.
Located 42 light-years from Earth, Capella is really a pair of first-magnitude yellow giants so close to one another only a telescope array such as COAST (Cambridge Optical Aperture Synthesis Telescope), which uses interferometry techniques, can reveal them.
Pictures obtained with COAST show the brighter star, Capella Aa, a G8 giant, separated from its G0 giant companion, Ab, by only 0.76 AU — almost the identical distance of Venus from the Sun. They revolve about their common center of mass every 104 days, never eclipsing one another from our perspective. Every time you look at Capella, both stars are always in view, even if you can't split them!
But there's more going on with Capella to make it a tempting observing target. It's accompanied by a pair of red dwarf companions with the unusual name of Capella HL. Of the many putative companions of Capella, this compact pair appears to be the real thing; it's the only star in the neighborhood that shares Capella's proper motion.
The star's original name was Capella H (the letters B through G were assigned to faint field stars), but in 1936, American astronomer Carl Stearns discovered "a small but distinct protuberance" on Capella H, with a magnitude of +12 and a separation of 1.8″. Named Capella L, it forms a tight red dwarf binary with H and travels through space alongside the equally "tight" pair of giants that comprise Capella. That makes our featured star at least a quadruple.
Capella HL is easy to spot even in a 6-inch scope. Start at Capella and shift your gaze just 12′ (1/5°) to the southeast to a small arc of stars anchored on either end by magnitude +8.4 and +9.5 field stars. H, at magnitude +10.2, sits almost midway between them and looks like an orange-red spark. In a 10-inch reflector, it was a snap to see. A fainter ~11.5-magnitude star lies almost due east of H — this is not the companion. Capella L snuggles approximately 3″ south of H and shines weakly at magnitude +13.7.
(A note of caution when making your own charts to find the binary. The Hubble Guide Star Catalog integrated into many planetarium-style sky programs flips the position of Capella H with the neighboring star to the east. The map and photo in this article show it correctly.)
Separating the two dwarfs was too mighty a task for my 15-inch under average seeing conditions. I tried and suspected the dimmer companion, but didn't feel certain enough to call the observation a success. I'll be trying again at the next dark-sky opportunity. You may have better luck, especially when the seeing cooperates. Use high power in excess of 200x for best results.
Capella HL's tiny red dwarfs stand in dramatic contrast to Capella's dual giants. The brighter (H) packs just 30-40% of the Sun's mass and shines with 1.2-1.4% of its luminosity. L is even shrimpier. It's not quite a third as large, and has only 10% the mass of the Sun and 0.05% its luminosity. 48 A.U. separate the duo, and they orbit about each other every 388 years at the fantastic distance of 10,000 A.U. (932 billion miles / 1.5 trillion km) from Capella proper.
Robert Burnham, author of the venerable guide, Burnham's Celestial Handbook, paints a wonderful picture of the entire Capella system:
" ... a scale model would show Capella A and B as two globes 13 inches and 7 inches in diameter and 10 feet apart; the components of Capella H would then be each 0.7 inch in diameter, 420 feet apart, and 21 miles from the main pair A & B!" (Vol. 1, page 264)