Spot Capella, a quadruple star system
Capella (Alpha (a) Aurigae) is Auriga’s brightest star and the sixth brightest in the night sky. Its declination of 46˚ means it’s circumpolar from the UK, dipping towards the northern horizon late in the evening in summer, but riding high during late autumn and early winter.
This is the opposite situation to the bright star Vega (Alpha (a) Lyrae), an equally bright star of summer. If you extend a line from Capella through Polaris for the same distance again, it almost takes you to Vega. This means both bright stars act like a giant seasonal sky clock. In many ways Vega and Capella are rivals, the former, at mag. +0.03, being a bit brighter than Capella at mag. +0.08. In the northern half of the night sky, only Arcturus (Alpha (a) Boötis) is brighter.
Capella appears to have a yellowish colour and turns out to be a quadruple system organised as two binary pairs in mutual orbit around each other. The main pair are two giant stars, Capella Aa and Capella Ab. The former is an orange giant of spectral type K0III, while the latter is a yellow giant of type G1III.
Two red giants, Capella H and Capella L, form a second pair with spectral types of M2.5 and M4 respectively.
Capella Aa and Capella
Ab form a tight pair, being separated by 0.74 AU, in a 104-day circular orbit. Through amateur scopes they are too tight to split, but Capella H and Capella L, sitting 12 arcminutes to the southeast of Capella, can be split with high magnification. They appear separated by 3 arcseconds, the brighter component Capella H at mag. +10.2 and Capella L at mag. +13.7.