Capella (Alpha Aurigae) is the brightest star in the constellation Auriga, about 46 light years from Earth. Although it appears as a single point to the naked eye, Capella is actually a bright close binary pair of yellow giant stars alongside a second, fainter binary.

In the Star Trek original series episode "Friday's Child", Capella IV is the fourth planet in the Capella system located near Federation space and the Klingon Empire, and is known for its rich deposits of topaline. A series of events involving a Klingon agent, Kras, and Captain Kirk led to the signing of a mining treaty between the Federation and the Capellans.



Capella, as depicted in "Friday's Child"
Capella is in reality a yellow star and it's interest lies in the fact that it is an easily-studied non-eclipsing spectroscopic binary star. These two giant G-class stars have luminosities of around 50 and 80 times that of the Sun and lie less than 100 million km apart. The stars were probably of spectral class A during their main sequence, similar to Sirius, and are in the process of becoming red giants in a few million more years as they continue to expand, cool, and brighten. Capella is a source of X-rays, probably due to surface magnetic activity on one of the pair. The central stars also have a faint companion that is itself a double star, consisting of two M-class red dwarf stars, that orbit at around a light year away from the main pair.