Capella is a bright yellow star located in the northern section of sky. With an apparent magnitude of +0.08, it's marginally fainter than another bright northern star, Vega. Capella is the standout star of the relatively large constellation of Auriga, the Charioteer. The Milky Way passes through the heart of Auriga and as a result it contains numerous bright open clusters, nebulae and interesting stars.
Capella is the northernmost first magnitude star and is circumpolar from latitudes greater than +44 degrees. The star can still be spotted from most southerly regions including Australia, New Zealand and Argentina. However, it's not visible from the Falkland Islands or Antarctica.
Capella means "small female goat" in Latin. This star marks the left shoulder of the Charioteer or according to Ptolemy's 2nd century Almagest, the goat carried by the charioteer. The three fainter stars on the south-western side of Capella are known as the Haedi, or the "Kids". In Greek Mythology, Capella represented the goat, Amalthea, whose horn was accidentally broken off by Zeus. The horn was transformed into the Cornucopia or the "horn of plenty", which would be filled with whatever its owner desired. To the Chinese, Capella was one of the stars making up Woo Chay or the Five Chariots (along with beta, theta, kappa and gamma Aurigae). To the Arabs, since it was often visible in the early evening sky before other stars, Capella was known as Al Rakib or the driver.
Some ancient astronomers, including Ptolemy, referred to Capella as reddish but this could only be due to atmospheric affects. There is not a remote chance that the star has changed colour in the past 2,000 years. In 1899, William W. Campbell of the Lick Observatory, Mount Hamilton, California and Hugh Newall at Cambridge, England measured the spectrum and determined a binary star. However, since the components are very close together they are incredibly difficult to separate visually. The brighter, Aa, yellow star is a type G1 with a diameter of 17 million kilometres (10.6 million miles) or 12 times that of the Sun. The fainter, Ab, orange star has a spectral type K0 with a diameter of 12.5 million kilometres (7.8 million miles). This is equivalent to 9 times that of the Sun. The stars have masses of 2.7 and 2.6 times and are 90 and 70 more times luminous than our star.
Once it was realised that Capella is a binary system, efforts were made to visually split it. In 1901, partial success was achieved by astronomers at Greenwich, who observed an "elongated star" when using the 28-inch refractor. In 1919, John Anderson and Francis Pease finally achieved visual separation with the 100-inch Hooker telescope at Mount Wilson, Los Angeles. Through amateur scopes the star always appears as a single point of light.
Recently, it was discovered that the Capella system also contains a pair of red dwarf stars. They are designated Capella H and Capella L and are located around 10,000 astronomical units from the main Aa/Ab pair.
Capella is 42.8 light-years distant from Earth. For 50,000 years, from 210,000 BC to 160,000 BC, it was the brightest star in the night sky. It peaked at mag. -0.82, when located 27.9 light-years from us.
Capella Data Table
|Bayer||alpha (α) Aur|
|RA (J2000)||05h 16m 41s|
|DEC (J2000)||+45d 59m 53s|
|Apparent Mag.||+0.08v (+0.91 Aa, +0.76 Ab)|
|Apparent Mag. Range||+0.03 -> +0.16|
|Absolute Mag.||-0.48 (+0.35 Aa, +0.20 Ab)|
|Spectral Type||K0III (Aa), G1III (Ab)|
|Radius (Sol)||12.2 (Aa), 9.2 (Ab)|
|Surface Temp (K).||4,940 (Aa), 5,700 (Ab)|
|Luminosity (Sol)||90 (Aa), 70 (Ab)|
|Age (years)||570 Million|
|Other designations||HR 1708, HD 34029, HIP 24608|
|Notable features||System also contains two red dwarf stars|