Capella

Capella is a prominent star system located in the northern constellation of Auriga. It serves as the brightest star in its constellation and ranks as the sixth-brightest star in the entire night sky. In the northern celestial hemisphere, it is the third-brightest star, following only Arcturus and Vega. To a person looking up with the naked eye, Capella appears as a single, yellowish-white point of light. However, it is actually a complex quadruple star system. This means it consists of four stars that are physically bound together.

The system is organized into two distinct binary pairs. The primary pair, known as Capella Aa and Capella Ab, consists of two bright-yellow giant stars. These two giants are very close to one another. They maintain a tight circular orbit with a separation of only 0.74 astronomical units. They complete one full orbit around each other every 104 days. The second pair, named Capella H and Capella L, is much different. These are faint, small, and relatively cool red dwarfs. They are located much further away from the primary pair, separated by about 10,000 astronomical units.

Each star in the primary pair is in a different stage of life. Both stars have a mass of approximately 2.5 times that of our Sun. They have exhausted the hydrogen in their cores. Because of this, they have cooled and expanded, moving off the main sequence. Capella Aa is the cooler and more luminous of the two. It has a spectral class of G8III, meaning it is an aging red clump star. It is 78.7 times as luminous as the Sun and 11.98 times its radius. It is currently fusing helium into carbon and oxygen in its core. Capella Ab is slightly smaller and hotter, with a spectral class of G0III. It is 72.7 times as luminous as the Sun and 8.83 times its radius. This star is in the Hertzsprung gap. This is a brief subgiant evolutionary phase where it expands and cools to become a red giant.

Humans have observed Capella for many centuries. Its name comes from the Latin word for "little goat." This name relates to the goat Amalthea from classical mythology. In ancient Mesopotamia, it was known as "GAM" or "Gamlum." Some cultures even called it the "Shepherd's Star." In Mexico, the people at Monte Albán built Building J around 275 BC. The steps of this building were aligned to face the rising of Capella.

Scientific understanding of Capella changed greatly in the late 19th and early 20th centuries. In 1899, Professor William Wallace Campbell announced that Capella was a binary star. He used spectroscopic observations to see the stars moving. He noticed a Doppler shift, which showed the stars moving toward and away from Earth. At the same time, Hugh Newall also concluded it was a binary system. In 1919, John Anderson and Francis Pease used an interferometer to resolve the stars. This was the first time an object outside our Solar System was measured using interferometry.

Capella is also a very powerful source of X-rays. In 1974, scientists detected X-ray emissions that were more than 10,000 times the X-ray luminosity of the Sun. These X-rays are thought to come primarily from the corona of Capella Aa. The corona is the outer atmosphere of a star. The high temperature of this corona suggests it might be held in place by magnetic confinement. This makes Capella a very important subject for studying stellar X-ray astronomy.

Finding Capella in the night sky is relatively simple for observers in the north. It is circumpolar for anyone north of 44°N latitude. This means the star never sets below the horizon for those people. It is located halfway between the stars in Orion's Belt and Polaris. Capella is approximately 42.9 light-years away from Earth. Interestingly, the system was likely closer to us in the past. About 237,000 years ago, it passed within 29 light-years of our Solar System. This makes Capella a vital link for understanding how star systems evolve and move through our galaxy.