One of the brightest stars in the northern sky, forming one third of the summer triangle with its sister stars Arcturus and Vega, Capella is actually a binary system composed of two giant yellow stars orbiting each other at a distance roughly two-thirds of that between the Earth and the Sun. Both stars have a surface temperature comparable to that of the sun, but a diameter ten times as great, indicating that they have finished the hydrogen-fusion phase of their life cycle. One of the stars has begun its helium-fusion phase, while the other, slightly dimmer, seems to have a helium core that is still contracting prior to the beginning of fusion.
Capella's two stars cannot be separated by the naked eye, nor even by the most powerful telescopes, but by using a process called interferometry to analyze the light signals cast off by the star. In fact, the Capella binary star itself has another, dimmer companion - at a distance of less than one light year away, it is orbited by another binary system composed of two red dwarf stars.
At 42 light years distance from the Earth, Capella's size, proximity and golden yellow hue make it one of the most important stars in the northern sky. As an easily viewed, large binary star, it can be weighed very accurately by astronomers who can compare the gravitational interaction of its components as well as the interference patterns of their spectra. Each of the twin stars weighs approximately 2.5 times as much as our sun, their enormous size offset by the greater density of Sol, which is a younger star. Capella was also discovered in the 1970's to be an x-ray source 1000 times more powerful than our sun.
Capella, the she-goat from ancient Greek mythology, is found in the constellation of Auriga, the charioteer, who is carrying the goat on his shoulder. Capella has also been called Amalthea, the goat that suckled Zeus as a baby. The Vedas describe Capella as Brahma Ridaya, the Heart of Brahma. It has been regarded as the herald of spring in many cultures, for example that of ancient Egypt, which called it by the name of the god Ptah, the Opener, and it was known as a sign of plenty and renewal in the myths and folklore of the ancient Indo-European people as long ago as 4500 B.C.
References and further reading: