Capella is the sixth brightest star in the night sky and the third brightest in the northern celestial hemisphere. It's also the northernmost first magnitude star with a declination of nearly +46 degrees and therefore circumpolar (never sets) from many northern temperate locations. However, despite this it's visible at one time or another from almost all inhabited countries including all of Australia, almost all of New Zealand and Argentina. It's not visible from Antarctica and the Falkland Islands.
Capella has an apparent magnitude of +0.08, which mean it's marginal fainter than Vega. To the naked eye the brightness difference is difficult to notice but what's obvious is the colour contrast, deep yellow tinged Capella against stark blue-white Vega. Capella is the standout bright member of the relatively large constellation of Auriga, the Charioteer. The Milky Way passes through the heart of Auriga and therefore it contains numerous bright open clusters, nebulae and interesting stars.
The name Capella is Latin and means "small female goat". The star marks the left shoulder of the Charioteer or according to Ptolemy's 2nd century Almagest, the goat that the charioteer is carrying. The three faint stars that form a triangle close beside it are 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 it was one of the stars making up Woo Chay, the Five Chariots (the others Beta, Theta, Kappa and Gamma Aurigae). To the Arabs it was known as Al Rakib, the Driver as it was often visible in the early evening sky before other stars came into view.
Some ancient astronomers including Ptolemy referred to Capella as reddish but this has to due to atmospheric affects, there isn't the slightest chance the star has changed colour in the past 2000 years. In 1899, William W. Campbell of the Lick Observatory, Mount Hamilton, California and Hugh Newall at Cambridge in England measured the spectrum of Capella and found that the star was not a single star but a binary. The components are so close together they are incredibly difficult to separate visually.
Both components are similar. The brighter Aa yellow star is a type G1 with a diameter of 17 million kilometers (10.6 million miles) or 12 times that of the Sun. The Ab star has a spectral type K0 and therefore orange with a diameter of 12.5 million kilometers (7.8 million miles). This is equivalent to 9 times the size of the Sun. They have masses of 2.7 and 2.6 times and are 90 and 70 more times luminous than our star respectively.
Once it was realized Capella was a binary system immediate efforts were made to visually resolve the components. A partial success was achieved in 1901, when astronomers at Greenwich noted the star as "elongated" when using the 28-inch refractor. Confirmation was achieved in 1919 by John Anderson and Francis Pease using the 100-inch Hooker telescope at Mount Wilson, Los Angeles. However, the star remains as a single point of light in amateur scopes.
Astronomers recently discovered that the Capella system contains a second pair of red dwarfs stars. They are designated Capella H and Capella L and are located around 10,000 astronomical units from the main Aa / Ab pair.
The Capella star system is currently located 42.8 light-years distant from Earth. For 50,000 years from 210,000 BC to 160,000 BC, Capella was the brightest star in the night sky. At best it shone at magnitude -0.82 and was 27.9 light-years distant.
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).||4940 (Aa), 5700 (Ab)|
|Luminosity (Sol)||90 (Aa), 70 (Ab)|
|Age (million years)||570|
|Other designations||HR 1708, HD 34029, HIP 24608|
|Notable features||System also contains two red dwarf stars|