Capella is the bright golden-yellow star in the constellation Auriga, the Charioteer. Shining prominently in the northern sky, Capella appears as a single star to the naked eye, but there is considerably more to this familiar point of light than we can see from Earth.
Here are some interesting facts about Capella.
Capella Is the Sixth-Brightest Star in the Night Sky
Capella is the sixth-brightest star in the night sky, with an apparent magnitude of about 0.08. It is also by far the brightest star in Auriga, making it one of the constellation’s easiest features to recognize. Capella lies around 43 light-years from Earth, relatively nearby by stellar standards.
Find Capella Above Orion and Taurus
Capella is particularly easy to find during the Northern Hemisphere winter. Start with Orion, one of the most recognizable constellations in the sky, and look above it toward Taurus and Aldebaran. Continue farther north and you’ll reach Auriga, where Capella stands out as the constellation’s brightest star. A small triangle of fainter stars known as the Kids lies nearby. The names are connected: Capella means “little she-goat,” while the Kids represent her offspring.
The Capella We See Comes Mainly From Two Giant Stars
Although Capella appears as a single star, most of the light reaching our eyes comes from two yellow giant stars, known as Capella Aa and Capella Ab. They are separated by about 0.74 AU, less than the average distance between Earth and the Sun, and complete an orbit around each other every 104 days. From 43 light-years away, their light blends into the single golden point we see as Capella.
Both stars are considerably larger than the Sun. Capella Aa is around 12 times the Sun’s radius, while Capella Ab is about nine times the Sun’s radius. Aa is also the cooler of the two, at around 4,970 K compared with roughly 5,730 K for Ab. Despite being cooler, Aa is slightly more luminous overall because its larger size gives it a greater surface area from which to radiate energy.
The Two Giants Are Already Well Beyond the Sun in Their Life Cycles
Capella Aa and Ab are thought to be only around 600 million years old, making them far younger than our 4.6-billion-year-old Sun. Yet both have already exhausted the hydrogen in their cores and left the main sequence. This is because they contain more than twice the Sun’s mass and have therefore consumed their nuclear fuel much more quickly.
The two stars have also reached different points in their evolution. Capella Aa is already burning helium in its core, while Capella Ab is still making the transition from the main sequence into its giant phase. Although the two stars are roughly the same age, their different masses have caused them to evolve at different rates.
Two Faint Red Dwarfs Complete the Capella System
The two giants are only half of the Capella system. Around 10,000 AU away lies another pair, Capella H and Capella L, two small and faint red dwarf stars. H and L form their own binary pair, just as the two giants Aa and Ab form a much tighter binary.
The two pairs are also gravitationally bound to one another. In other words, Capella is best pictured as two binary systems forming one larger four-star system. Each pair has its own center of mass, while the two pairs themselves move around the center of mass of the entire system. Because the giant pair contains most of the mass, that overall center lies much closer to Aa and Ab.
No Planets Are Known Around Capella
No planets have been discovered around Capella, and planetary orbits in such a system would be more complicated than those around a single star. A planet orbiting close to either giant would also feel the strong gravitational pull of the other, making many possible orbits unstable. Farther out, however, a planet could potentially orbit both giants together.
Capella’s two giants together are around 150 times more luminous than the Sun. For a planet to receive roughly the same amount of energy that Earth receives from the Sun, it would have to orbit beyond the equivalent of Saturn’s orbit — roughly 12 times Earth’s distance from the Sun. From there, it would see the two bright giants moving around each other every 104 days.
The other two stars would matter much less. Capella H and L are so distant from the giants that their heat and light would have little effect on a planet orbiting the central pair.