Capella (astronomy)

The sixth brightest star in the night sky and third brightest in the Northern Hemisphere, Capella is a star in the constellation Auriga. Capella appears to be a single star to the naked eye, but it is actually made up of two binary stars: a binary of giant yellow stars that are about ten times the diameter of the sun and a binary of red dwarfs in a multiple star system approximately 42.8 light-years from Earth.

Overview

Capella’s constellation, Auriga, is one of the eighty-eight modern constellations that divide the sky into regions and was one of the forty-eight constellations listed in the catalog of the second-century Greco-Egyptian astronomy Ptolemy. The constellation Auriga is named for the Latin word for "charioteer," and the word Capella is the diminutive Latin word for female goat charioteer. Capella had been called the "goat star" even before Ptolemy incorporated it into the constellation of Auriga, in fact, perhaps reflecting the importance of goats to Greek culture and the star’s brightness in the sky. The earlier Mesopotamian culture had represented Capella and several of the other stars making up Auriga as a goatherd.

Capella has an apparent magnitude (brightness as seen from Earth) of 0.08 and an absolute magnitude (brightness from a standardized distance of 10 parsecs) of 0.4. The Capella system’s two binaries are typically called the bright binary and the companion binary. The bright binary is a noneclipsing binary (meaning that from Earth neither star appears to pass in front of the other) consisting of two type-G giant stars, Capella Aa and Capella Ab, that orbit around a common center every 104 days. They are approximately 62 million miles (100 million kilometers) apart, just two-thirds the distance between the sun and Earth and are in the process of expanding into red giants. The companion binary includes two red dwarfs, Capella H and Capella L, that are located 10,000 astronomical units from the bright binary. (The stars named Capella C through G occupy the same visual field but reside in unrelated systems.) The companion binary is less well observed, but estimates of its orbit around the bright binary suggest a period of 388 years.

William Wallace Campbell and Hugh Newall were the first astronomers to deduce that Capella was a binary system in 1899. Astronomer Olin J. Eggen discovered that the Capella system is a member of the Hyades stream, a disparate collection of stars that follow a trajectory through space similar to that of the Hyades cluster, and published his findings in 1960. The Hyades stream has been observed since at least 1869, with the most common explanations being that the stars share a common origin in an earlier massive cluster or that their motion demonstrates the existence of tidal effects from the center of the Milky Way galaxy. Capella is also notable as a source of X-rays, which are thought to emanate from the corona of the system’s largest star.

Bibliography

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