Capella: The Goat Star

Capella is the sixth brightest star in the night sky and the third brightest in the northern celestial hemisphere. This golden star in the constellation Auriga, the Charioteer, shines high overhead during winter evenings and is a cornerstone of the famous Winter Hexagon asterism. What appears as a single star to the naked eye is actually a remarkable quadruple star system. Spot it with StarGlobe.

How to Find Capella

During winter, Capella is nearly overhead for observers at mid-northern latitudes, making it easy to find. A useful guide uses the Big Dipper in Ursa Major: draw a line from the handle through the bowl and extend it, and you will reach Capella. Its golden-yellow color and isolation from other equally bright stars help confirm its identity.

Capella forms part of the Winter Hexagon (or Winter Circle), a large asterism connecting Sirius, Rigel, Aldebaran, Capella, the Gemini stars Castor and Pollux, and Procyon. Capella sits at the northernmost point of this hexagon.

Capella is circumpolar from latitudes above about 44 degrees north, meaning it never sets from locations like northern Europe and much of Canada. Even from more southern locations, it remains above the horizon for most of the night during winter.

A Quadruple Star System

Capella consists of two pairs of binary stars. The primary pair, Capella Aa and Ab, are both evolved giant stars orbiting each other every 104 days at a separation of about 100 million kilometers. Capella Aa is a yellow giant (spectral type G8III) about 79 times more luminous than the Sun, while Capella Ab is a slightly cooler giant (G0III) about 73 times the Sun's luminosity.

The two giants have nearly depleted the hydrogen in their cores and are in different stages of stellar evolution, despite having formed together. Their combined light makes Capella shine at magnitude 0.08. The pair is too close to separate with any amateur telescope, but their binary nature is revealed through spectroscopy.

A second pair of faint red dwarf stars, Capella H and L, orbits the main pair at a much greater distance. These dim companions contribute negligible light to the system's total output and are only visible in large telescopes.

Physical Characteristics

The Capella system lies about 43 light-years from Earth. The combined luminosity of the two main components is roughly 152 times that of the Sun. The primary star has expanded to about 12 times the Sun's diameter, while the secondary is about 9 times the Sun's diameter. Both stars have surface temperatures in the range of 4,900 to 5,700 Kelvin, giving the system its characteristic warm yellow color, similar to the Sun's but amplified by the giants' greater size.

Capella is one of the brightest X-ray sources among nearby stars. The intense X-ray emission comes from the hot coronas of the two giant components, heated by their strong magnetic activity. This makes Capella an important target for X-ray astronomy studies of stellar coronas.

The Haedi: The Kids

Two small stars near Capella, Epsilon and Zeta Aurigae, are traditionally known as the Haedi, or the Kids, representing baby goats carried by the charioteer alongside the she-goat Capella. Epsilon Aurigae is a remarkable eclipsing binary with one of the longest known periods: every 27 years, a mysterious dark companion passes in front of the primary, causing an eclipse that lasts nearly two years. The nature of the eclipsing body has been debated for decades, with current models suggesting a large disk of dust around a companion star.

Cultural Significance

The name Capella comes from the Latin diminutive of "capra," meaning she-goat. The star has been associated with the goat Amalthea, who nursed the infant Zeus in Greek mythology. In gratitude, Zeus placed the goat among the stars. The horn of Amalthea became the cornucopia, the horn of plenty.

In Hindu astronomy, Capella is associated with Brahma, the creator. Aboriginal Australians incorporated the star into their astronomical traditions, and it was significant in Polynesian navigation as well. Capella's prominence in the winter sky has made it a important seasonal marker across many cultures.

Best Time to Observe

Capella is best observed from November through March, reaching its highest point during January evenings. Because of its far northern declination (+46 degrees), it is visible for much of the year from northern latitudes, rising in the northeast during autumn evenings and remaining visible through spring. From the Southern Hemisphere, Capella is visible but very low on the northern horizon and only from tropical or subtropical latitudes.

Neighboring Stars and Constellations

Taurus lies to the south, with Aldebaran and the Pleiades nearby. Gemini is to the southeast, and Perseus sits to the west. The Milky Way passes through Auriga, enriching the constellation with star clusters including M36, M37, and M38, all of which are visible in binoculars. Explore Capella's winter setting with StarGlobe.

Quick Facts

Capella has an apparent magnitude of 0.08, a distance of about 43 light-years, and a combined luminosity of approximately 152 times the Sun. The system consists of four stars in two pairs. Its right ascension is 5h 17m, and its declination is +46 degrees 00 minutes. Capella is the closest first-magnitude star to the north celestial pole.