Capella is the brightest star we see at night of the same spectral type, or color, as our daytime star, the sun. Like our sun, Capella is a yellow or golden star. It is the brightest yellow star visible in our sky – much bigger and brighter than our sun in absolute terms, but of course much farther away, about 42 light-years in contrast to the sun’s 8 light-minutes.
Like so many stars that appear single to the eye, Capella actually consists of two stars. Capella A and Capella B, as they’re called, are similar to each other, both roughly 10 times the sun’s diameter. They emit about 80 and 50 times more overall light than the sun, respectively. Another component to this system, a binary of small red stars, orbits about a light-year away.
Capella A and B are both yellow giant stars at the end of their normal lifetimes. Each being more than two and a half times more massive than the sun, the two components of Capella likely are also younger. This is because more massive stars have higher internal pressures, which causes them to burn their nuclear fuel faster and to have shorter lifespans. The stars of Capella are in a transitional period from the smaller, hotter stars they once were, to the cooler and larger red giants they must ultimately become in their final phase. However, for now, their surface temperatures are similar to that of the sun, and so they share its spectral type of G.
The two stars we see as Capella mark the 6th brightest star in our sky. Astronomers measure the combined magnitude of this system as 0.08.
How to see it
From mid-latitudes of the U.S. and Europe, Capella is far enough to the north to be seen at some time of the night all year round. For us in the Northern Hemisphere, it’s best in winter, when you’ll find golden Capella high overhead before bedtime.
Capella is the brightest star in a five-sided star pattern, which we know as the constellation Auriga the Charioteer. Capella is the Latin word for ‘nanny goat,’ and this lovely star is often called the Goat Star.
Here is the key to knowing you’ve found Capella. Near it, you’ll find a tiny asterism – a noticeable pattern on the sky’s dome – consisting of three fainter stars. This little triangle of stars is approriately called the Kids, and it makes Capella instantly recognizable.
The star El Nath originally was part of the constellation Auriga, too. Now El Nath marks the tip of one of horns of the constellation Taurus the Bull. Still, including El Nath, you can see a large misshapen pentagon of stars in Capella’s part of the sky. The shape is difficult to reconcile with the idea of a man driving a chariot, but it is a noticeable pattern and easy to find.
Some people use the Big Dipper to locate Capella. The two bowl stars – Megrez and Dubhe – more or less point toward Capella. It’s a long hop, though, in the direction opposite of the Big Dipper handle.
History and myth
For such a large constellation with such a bright star, the mythology of Auriga and Capella is sparse. The constellation has been associated with the Greek sea god Posideon (the Roman god Neptune). Other stories say Auriga represents Erichthonius, the ancient lame king of Athens who invented the horse-drawn chariot.
Auriga seems to have been associated with shepherds and flocks, so the title of nanny goat — or ‘she goat’ — for Capella is reasonable. However, neither Capella nor its constellation Auriga figures prominently in any major mythological stories from Greek or Roman culture.
Richard Hinkley Allen, in his famed Star Names, says that the ancient Arabs called the star Capella by a name that meant ‘The Driver’ and implies that this star was seen as a shepherd driving a flock across the sky. The flock might have been the nearby star cluster Pleiades, although – instead of sheep or goats – early Arabian stargazers saw this pattern composed of camels. Capella was also apparently important in ancient Egypt. It appears on the Denderah Zodiac as a mummified cat.
Interestingly, in China, Capella and four other stars of Auriga were known as the Five Chariots. The other four stars are Beta, Theta, Kappa and Gamma (El Nath, which is now Beta Tauri).
Capella’s position is RA: 5h 16m 41.4s, Dec: +45° 59′ 53″.