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Capella (astronomy)

Capella is the sixth brightest star in the night sky and the third brightest in the Northern Hemisphere, located in the constellation Auriga. To the naked eye, it appears as a single star, but it is actually a binary star system composed of two giant yellow stars, which are about ten times the diameter of the Sun, and a pair of red dwarfs. The system is situated approximately 42.8 light-years from Earth. The name "Capella" derives from the Latin term for "little goat," reflecting its historical association with goats in various cultures, including Greek and Mesopotamian traditions.

In terms of brightness, Capella has an apparent magnitude of 0.08. The bright binary component, consisting of two type-G giant stars, orbits a common center every 104 days, while the companion binary, which includes the red dwarfs, is estimated to take around 388 years to complete its orbit. The Capella system is also a member of the Hyades stream, suggesting a shared origin or similar movement patterns with other stars. Notably, Capella emits X-rays, which are believed to originate from the corona of its largest star, making it a point of interest in astronomical studies.

Authored By: Kte’pi, Bill, MA 1 of 4
Published In: 2020 2 of 4
Related Topics:
Binary stars;Constellation;Earth;Milky Way;Ptolemy (astronomer);Red Dwarf Stars;Red giant stars;Sun
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Full Article

The sixth brightest star in the night sky and third brightest in the Northern Hemisphere, Capella is a star in the 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 43 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 Latin word for “little she-goat.” Capella had been called the “goat star” even before Ptolemy incorporated it into the constellation of Auriga, perhaps reflecting the importance of goats to Greek culture and the star’s brightness in the sky. Mesopotamian cultures represented Capella and several of the other stars that make 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 designated Capella A and Capella HL. 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; Capella Aa is a helium-burning giant, while Capella Ab is evolving into a red giant. The companion binary, known as Capella HL, 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 an orbital period of several centuries.

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 documented from 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 hot coronas of the giant stars.

Bibliography

“Capella Is One of the Sky’s Brightest Stars.” EarthSky, 8 Jan. 2026, earthsky.org/brightest-stars/capella-is-the-stellar-beacon-of-auriga-the-charioteer/. Accessed 5 June 2026.

Dinwiddie, Robert, et al. Nature Guide: Stars and Planets. DK, 2012.

Hislop, Susanna, and Hannah Waldron. Stories in the Stars: An Atlas of Constellations. Penguin, 2015. Print.

Lang, Kenneth R. The Life and Death of Stars. Cambridge UP, 2013.

Marini, E., et al. “An Insight into Capella (α Aurigae): From the Extent of Core Overshoot to Its Evolutionary History.” Astronomy & Astrophysics, vol. 676, Aug. 2023, article A19, doi:10.1051/0004-6361/202346758. Accessed 5 June 2026.

Miche, Dinah L. Astronomy: A Self-Teaching Guide. Wiley, 2014.

Pasachoff, Jay M. A Field Guide to Stars and Planets. Houghton Mifflin, 1999.

Rozelot, Jean-Pierre, and Coralie Neiner, editors. Cartography of the Sun and the Stars. Springer, 2016.

Seeds, Michael A., et al. Montgomery. Horizons: Exploring the Universe. Brooks/Cole, 2016.

Sparrow, Giles. Constellations: A Field Guide to the Night Sky. Quercus, 2013.

Torres, Guillermo, et al. “Capella (α Aurigae) Revisited: New Binary Orbit, Physical Properties, and Evolutionary State.” The Astrophysical Journal, vol. 807, no. 1, 2015, article 26, doi:10.1088/0004-637X/807/1/26. Accessed 5 June 2026.