Capella is the brightest star in the constellation of Auriga. It is the sixth brightest star in the night sky and the third brightest in the northern celestial hemisphere after Arcturus and Vega.
Key Facts & Summary
- Capella is relatively close to us, at a distance of 42.9 light-years / 13.2 parsecs.
- To the naked eye, Capella appears a single star but it is actually a quadruple star system. This system is organized by two binary pairs made up of the stars Capella Aa, Capella Ab, Cappella H, and Capella L.
- The primary pair of binary stars Capella Aa, and Capella Ab, are two bright yellow giant stars, both of which are 2.5 times as massive as the Sun.
- The secondary pair of binary stars Capella H, and Capella L, are two faint and relatively cool red dwarfs that are 10.000 AU away from the primary pair.
- Both Capella Aa and Capella Ab have exhausted their core hydrogen, and thus have cooled and expanded, moving off the main sequence.
- Capella Aa and Capella Ab are in a very tight circular orbit about 0.74 AU apart. They orbit each other once every 104 days.
- Capella Aa is the cooler and more luminous of the two with spectral class K0III. It is 78.7 ± 4.2 times the Sun’s luminosity and 11.98 ± 0.57 times its radius.
- Capella Ab is slightly smaller than Capella Aa, and also hotter. Its spectral class is G1III. It is 72.7 ± 3.6 times as luminous as the Sun and 8.83 ± 0.33 times its radius.
- Many other stars in the same visual field have been cataloged as companions but they are physically unrelated.
- Capella is one of the brightest X-ray sources in the sky, thought to come primarily from the corona of Capella Aa.
- Both Capella A and Capella B will evolve into red giants and expand many hundreds of times their original size.
Often referred to as the Goat Star, Capella has been associated with many myths and legends throughout the ages. The name Capella is Latin for “female goat”, though the alternative “Capra” was more used in classical times.
The Greeks and Romans associated the constellation of Auriga with chariots and horses with Auriga being a horse trainer and the inventor of the four-horse chariot. To the more ancient civilizations, Auriga was, in fact, a goatherd and the guiding light for shepherds according to skywatching columnist Joe Rao. This seems to be the most plausible explanation as to why Capella is often called/referred to as the Goat Star.
The Greek mythology presents Capella as Amalthea, the Cretan goat that suckled Zeus when he was young. As Zeus grew more powerful, he broke Amalthea’s horn, the horn was then transformed into the Cornucopia, the horn of plenty, which would be filled with anything its owner desired.
In the Middle Ages, Capella was part of the fifteen Behenian fixed stars. They were considered stars of great astrological energy and they were often used in magic rituals.
When it comes to Capella, it was associated with the plant’s mint, horehound, mandrake root, and the sapphire stone.
Capella is estimated to have formed around 590 – 650 million years ago. Capella formed from a cloud or nebula of dust and gas. As the gravitation pulled the gas and dust together, it resulted in the brightest yellow star that we today now see. It is unclear whether the binary star system formed at exactly the same time or separately and then somehow ended up being a binary pair.
Distance, Size, and Mass
Capella is only at a distance of 42.9 light-years / 13.2 parsecs away from the sun. Because of this, it is the brightest yellow star visible in our sky.
The primary pair of binary stars Capella Aa, and Capella Ab, are 2.5 times as massive as the Sun. Capella Aa has 11.98 ± 0.57 times the Sun’s radius and thus around twenty-two times its diameter. Capella Ab is slightly smaller than Capella Aa, having 8.83 ± 0.33 times the Sun’s radius and thus around seventeen times its diameter.
Star System – Characteristics
The primary pair Capella Aa and Capella Ab, have exhausted their core hydrogen, and thus have cooled and expanded, moving off the main sequence. They have bounded in a very tight circular orbit about 0.74 AU apart, orbiting each other once every 104 days.
Capella Aa is the cooler and more luminous of the two being around 78.7 ± 4.2 times brighter than the sun. It has an average surface temperature of around 4.970 ± 50 K. Capella Ab is hotter but dimmer, being 72.7 ± 3.6 times brighter than the sun, and having a surface temperature of around 5.730 ± 60 K.
The second pair Capella H and Capella L, are two faint and relatively cool red dwarfs that are 10.000 AU away from the primary pair. Both stars have almost two times less mass than our sun. Capella H has an estimated 0.57 solar masses while Capella L has 0.53
They are very dim, Capella H has 0.05 the sun’s luminosity and around 0.54 solar radii. To be more accurate, both stars have a combined visual luminosity around 1% that of our sun. Capella H has average temperatures are around 3.700 K. The stars complete one orbit around each other in around 300 years and have a semi-major axis of around 40 AU. Capella is actually a good example of an RS Canum Venaticorum variable binary star.
Its components have active chromospheres which cause large starspots and variations in magnitude. However, neither of the two stars pass in front of each other while observed from Earth. Interactions between the magnetic fields of the two stars are believed to energize the corona of Capella Aa, and thus it creates an X-ray luminosity about 10.000 times that of the Sun’s corona.
Capella is located in the constellation of Auriga, marking the left shoulder of the celestial charioteer. It is the most luminous object and thus designated Alpha Aurigae.
In some other works, Capella actually marks the goat that the charioteer is carrying. This was written as such by Ptolemy in his 2nd century Almagest. Bayer, on the other hand, in his work called Uranometria, Capella marks the charioteer’s back.
Capella is also the brightest star in the five-sided star pattern. It is also part of the little triangle of stars asterism named The Kids.
Since both Capella A and Capella B are significantly more massive than our sun, they will quickly burn their remaining supply of fuel. They will then evolve into red giants and expand many hundreds of times their original size.
Did you know?
- Capella held the title as the brightest star in the night sky around 210.000 years ago to 160.000 years ago. It is the same spectral class and color as the Sun.
- Capella is the closest first-magnitude star to the celestial north pole. It lies on the opposite side of the pole from Vega.
- Capella is visible throughout the year in the northern hemisphere.
- The star is traveling through the Milky Way at a speed of 39.7 km / 24.6 mi per second, relative to the Sun. Its projected Galactic orbit takes it between 21.900 to 27.100 light-years from the Milky Way’s center.
- The binary nature of Capella was first discovered in 1899 by professor William Wallace Campbell, after analyzing photographic plates taken in a seventh month period of observation.
- Capella is the first object to be imaged by a separate element optical interferometer. This happened in 1995 and was conducted by the Cambridge Optical Aperture Synthesis Telescope.
- The American astronomer O.J. Eggen discovered in 1960 that Capella was a member of the Hyades group. This group of stars are moving in the same direction and have a similar age.
- In English literature, Capella is sometimes referred to as the Shepherd’s Star.
- One of the earliest mentions of Capella is in an Akkadian inscription that dates back to the 20th century B.C.
- There are Roman authors such as Pliny the Elder and Manilius who both called the star Capra long before Ptolemy, in the 1st century A.D.