Star Facts: Capella

Star Facts: Capella
Auriga by Jimmy Westlake

Capella is a yellow star located in the constellation of Auriga, and is also the sixth most luminous star in the whole night sky. Meaning “small female goat” in Latin, Capella may appear as a single star to the naked eye, but it is in fact a binary system in which the components are both binary stars in themselves, one pair being giant stars, and the other a pair of diminutive red dwarfs.

Quick Facts

Coordinates: RA: 5h 16m 41.4s/Dec: +45° 59′ 53
Star Type: Yellow Type-G Giant Stars
Constellation: Auriga
Distance: 42.8 light years
Apparent Mag: 0.08
Luminosity: 79 solar luminosity
Surface Temperature: Capella A (8,900F)
Diameter: 12 solar radii
Mass: 2.69 solar masses
Age: 590 to 650 million years
Other Names: Alhajoth, Capella, Hokulei, Alpha Aurigae,


Capella is the most luminous star in Auriga, a beautiful pentagonal constellation representing “the charioteer” located just north of the constellations Orion and Taurus. Another way to find Capella is to draw an imaginary line through the Pole Star to Vega, which is located opposite Capella, almost at the same distance from Polaris as Capella. The star Capella is located just a few degrees northward of “The Kids”, a triangle formed by the neighboring stars Epsilon, Zeta, and Eta Aurigae. As such, it is the closest first-magnitude star to the celestial north pole.

Best Seen

Look for Capella almost at the zenith during winter in mid-northern latitudes; however, for observers in much of the northern hemisphere, Capella is visible right through the year, depending on time and location. In fact, Capella is not visible to observers south of latitude 44 degrees N, which area includes much of southern New Zealand, Chile, Argentina, and the Falkland Islands. On the other hand, Capella is circumpolar and never sets for observers north of latitude 44 degrees N, which area includes all of the United Kingdom, the Scandinavian countries, much, if not most of France, Canada, and the far northern Unites States.

Star Facts: Capella

Physical Properties

Without optical aid, Capella appears as a single star, but it is in fact a binary system in which the components are both binary stars themselves. Other stars around the Capella system(s) are designated Capella C to G, and I to K, but they do not form part of the main system, being unrelated stars that fall within the same visual field as the Capella system proper.

– Primary Binary System is comprised of two yellow G-class stars, one being Capella Aa, a G8III giant, with 3.05 solar masses; and the other Capella Ab, a G0III giant that is slightly less massive at 2.57 solar masses. The primary pair are separated by 0.76 AU, and have an orbital period of 104 days. The primary pair is a non-eclipsing binary, meaning that from our point of view, neither star passes in front of the other during their 104-day orbit around each other at a distance of 100 million kilometres. Both stars in the primary system have burnt all of their hydrogen, and as a result have morphed into giants, although their exact location on the main sequence is uncertain. The stars are now both cooling and expanding into the red giant phase, in a process that will take another several million years.

– Companion Binary System; the companion system consists of two faint red dwarfs, Capella H and Capella L, located about 10,000 AU away from the primary system. However, the orbital period of the secondary pair is somewhat uncertain, since only about 300 of their orbit has been computed, but based on available data, it is thought that it would take about 400 years for the secondary pair to complete one orbit around the primary pair.

Capella in History

Traditionally, Capella marks the left shoulder of the charioteer that is depicted by the constellation Auriga, but according to Ptolemy’s Almagest, the star more properly represents the small goat carried by the charioteer, hence the stars’ other common name, “the Goat Star”. However, before Ptolemy merged the “Kids” and the Charioteer in the Almagest, the “Kids” asterism was a constellation on its own, but prior to the merge, Capella was often seen as a constellation in itself, especially by Manilius and Pliny the Elder, both of whom referred to the one-star-constellation as Capra, Caper, or Hircus, names that all refer to the name “Goat Star” via various interpretations.

During the Middle Ages, Capella was seen as one of the Behenian fixed stars, fifteen stars that were considered a a source of astrological energy, and particularly useful for magical spells. In this context, it was associated with the plants mint, horehound, and mandrake root, as well as the stone sapphire.

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