Can you separate bright Capella’s two true companion stars from the impostors?
Capella (Alpha (α) Aurigae) rides high across the winter sky, a yellow beacon at the top of the misshapen pentagon that represents the main part of the constellation of Auriga, the Charioteer. Look at Capella through a telescope and you’ll see numerous other stars nearby. These are listed in the Washington Double Star Catalogue as Capella A (the main star) through to Capella R. Of these, only two are known to be true physical companions, Capella H and Capella L. The others appear close simply because they occupy the same line of sight as Capella A. This month’s challenge is to locate Capella H and possibly Capella L.
The main star, Capella A, is a spectroscopic binary, Aa and Ab, two yellow giant suns in a 104-day mutual orbit around one another. Their mean orbital distance is around 0.74 AU, which is approximately the distance between Venus and the Sun. At a distance of 42.8 lightyears, amateurs can’t see these stars separated, and they appear as one.
Capella H and L are different and are viable targets for amateur telescopes. The stars are a binary pair of red dwarf stars gravitationally linked to Capella Aa and Ab, thus forming a quadruple system. While Capella A is easy to find with any size of telescope, Capella H needs at least 150mm of aperture and Capella L a 250mm scope. A large instrument over 300mm in aperture is recommended to separate the pair convincingly
Capella H and L appear very close to one another in the night sky, separated by just 3 arcseconds. They sit 12 arcminutes southeast of Capella A, between two field stars, mag. +10.0 TYC 3358-2785-1 and mag. +8.7 TYC 3358-3023-1. This makes Capella H easier to locate. Start with a low- to mid-power eyepiece giving you a field of view at least 30 arcminutes across; that’s roughly the apparent diameter of the Moon. Centre on bright Capella A, then identify mag. +10.2 TYC 3358-3142-1, 1.7 arcminutes southeast of Capella A. This is a key navigational star for two reasons. Its brightness is the same as mag. +10.2 Capella H and if you can see easy-to-find TYC 3358-3142-1, you should be able to see H. Secondly, a line from Capella A through TYC 3358-3142-1 points to TYC 3358-2785-1, mentioned earlier.
Once you’ve located TYC 3358-2785-1 you should be able to identify brighter TYC 3358-3023-1 quite easily. There are several faint stars between TYC 3358- 2785-1 and TYC 3358-3023-1, Capella H being the brightest one, roughly mid-way between them. Once identified, increase magnification to see whether you can see a southern ‘extension’ to Capella H. If you can, you’ve located Capella H’s close binary companion, mag. +13.7 Capella L. Imaging the pair really helps reveal the distinct orange hue of this red dwarf binary.