Corona Australis

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: Corona Australis is a constellation that resembles a crown or wreath. It is located in the Southern Hemisphere and is most visible in August. This constellation has been interpreted in many ways over thousands of years and has a rich history of myths and symbolism. Many stargazers and astronomers study Corona Australis to see examples of binary and variable stars as well as nearby nebulae.

Crown in the Southern Sky

For tens of thousands of years, people have studied the stars and searched for constellations. Corona Australis was one of the earliest constellations to be documented by ancient astronomers. It was given the name Corona Australis, Latin for "southern crown," because its stars form a semicircular shape that resembles a crown or wreath. A similar crown-shaped constellation in the Northern Hemisphere—Corona Borealis, the "northern crown"—is considered by astronomers to be a counterpart to Australis. Corona Borealis is by far the larger and brighter of the two. Both circumpolar constellations are among the eighty-eight constellations that the International Astronomical Union recognizes.

Corona Australis is visible throughout the Southern Hemisphere and in some parts of the Northern Hemisphere, particularly below the latitude of 40 degrees north. Some more northern latitudes can see it during its highest point in August. However, it cannot be seen at any time above 53 degrees north. Corona Australis can be found using its declination (about −41.5 degrees from the celestial equator) and right ascension (18.64 hours).

Attributes of Corona Australis

Corona Australis is a relatively small and compact constellation. It consists of six primary stars and fourteen other space objects. Many other stars, planets, and other bodies are located near the constellation in the night sky, some of which are of significant interest to astronomers.

The main star in Corona Australis formation is alpha Coronae Australis, also known as Alfecca Meridiana. It is the only star in the constellation with a common name. Alfecca Meridiana is about 125 to 140 light-years from Earth. While not particularly bright, it is noteworthy for the high speed at which it rotates—approximately 180 kilometers per second at its equator—and for its excess infrared radiation. Such infrared excess is a sign that a star may be surrounded by a disk of dust. This, in turn, suggests that it may host its own planetary system.

Another notable star in the constellation is epsilon Coronae Australis. It is an eclipsing binary variable star, consisting of two stars that regularly eclipse one another as they orbit a common center of mass. In addition, this star is a contact binary, meaning that its two stars are close enough to share an atmosphere. Gamma Coronae Australis is also a binary star, while kappa Coronae Australis is an optical double star. This means that while its two stars appear close together when viewed from Earth, they are in fact very far away from one another. R Coronae Australis is an irregular variable star that undergoes great changes in brightness.

Despite this assortment of stars, Corona Australis is best known for the other astronomical objects it contains. One of the most famous of these is the Corona Australis molecular cloud complex, an enormous group of nebulae. A nebula is a vast cloud of dust and gas. Nebulae are often the site of new star and planet formation; over millions of years, drifting matter clumps together to form increasingly large bodies. Astronomers have identified many newly created stars in the Corona Australis complex, and more are constantly forming. At a distance of about 420 light-years, the complex is one of the closest star-forming regions to Earth’s solar system. Corona Australis also contains a number of globular clusters, or huge, spherical groupings of orbiting stars. One such cluster is New General Catalog 6541 (NGC 6541). It is estimated to be about fourteen billion years old—as old as the universe itself.

Monsters, Gods, Heroes, and Poets

Although Corona Australis appears mainly in the Southern Hemisphere, the ancient Greeks noticed and took great interest in it. The Greek Egyptian astronomer Ptolemy (ca. 100–170 CE) included it among the forty-eight constellations he charted during the second century CE. Corona Australis achieved its lasting fame thanks largely to the mythical stories of the ancient Greeks, although records of its underlying mythology are uncertain and varied.

The earliest Greek astronomers may not have considered the constellation a crown at all. According to some sources, it was originally rendered as a quiver of arrows in the hand of the nearby constellation Sagittarius, a centaur. In Greek myth, centaurs were generally portrayed as warlike half-human, half-horse creatures that favored bows and arrows during their attacks. Other Greek astronomers interpreted Corona Australis as a crown belonging to Sagittarius, while still others thought it represented a laurel wreath, an ancient Greek symbol of great honor. According to some myths, the god Dionysus honored his dead mother with such a wreath, then hung the wreath in the sky as an eternal monument. Some historians suggest that ancient people may have considered the wreath to be in honor of the great Greek poet Corinna, who won a contest against her rival Pindar.

Although the exact mythological meaning of Corona Australis is unclear, its importance is demonstrated by its inclusion in a "family" of constellations related to the mythical hero Hercules. In addition to the Hercules constellation, other members of this constellation family include Aquila, the eagle; Sagitta, Hercules’s arrow; and Scutum, the shield. Within the same family are several of Hercules’s foes, including Hydra, the sea serpent; Serpens, the serpent; Centaurus, the centaur; and Lupus, the wolf.

Modern Astronomical Importance

Corona Australis is most visible during its highest point in August. Even at its brightest, however, it is a relatively faint constellation, and most of its components are best viewed with binoculars or a telescope. Observers can study alpha, beta, and epsilon Coronae Australis with binoculars, but telescopes of varying power are necessary to see deep-sky objects such as NGC 6541 and many of the more distant clusters and nebulae. The faintness of the constellation is largely due to the amount of cosmic dust in the region. In many cases, though, this dust reflects light and creates stunning patterns of bright colors.

Despite the faintness of the constellation, astronomers often look to Corona Australis to study its binary stars, variable stars, and other types of stellar objects. In mid-March, stargazers frequently monitor this constellation to see the Corona Australids, a meteor shower that can yield about five to seven falls per hour.

Corona Australis is an important region of the southern night sky, easily found and studied in conjunction with other nearby constellations. Its nearest neighbors in the night sky include Sagittarius, Scorpius, Ara, and Telescopium. Observers can view the layout of these stars and constellations as well as study their rich histories of myth and symbolism.

PRINCIPAL TERMS

• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere; it is equally distant from the celestial poles.
• circumpolar constellation: a constellation that is always visible in the night sky near a celestial pole; the Northern and Southern Hemispheres have different sets of circumpolar stars.
• constellation: a region of space defined by a pattern of stars that can be seen in the night sky from Earth.
• declination: a space object’s angular distance north or south of the celestial equator, expressed in degrees of arc.
• International Astronomical Union: an association of professional astronomers from all over the world who define astronomical constants while promoting research, education, and discussion on important astronomical topics.
• right ascension: a space object’s longitudinal arc along the celestial equator, measured eastward from the vernal equinox and expressed in hours.

Bibliography

"Corona Australis." Peoria Astronomical Society. Peoria Astronomical Soc., n.d. Web. 10 Apr. 2015.

Esplin, T. L., and K.L. Luhman. "A Census of Stars and Disks in Corona Australis." The Astronomical Journal, vol. 163, no. 2, 14 Jan. 2022, iopscience.iop.org/article/10.3847/1538-3881/ac3e64/meta. Accessed 14 June 2022.

Kaler, James B. "Alfecca Meridiana." Stars. U of Illinois, n.d. Web. 10 Apr. 2015.

Kaler, James B. "Corona Australis." Stars. U of Illinois, n.d. Web. 10 Apr. 2015.

Kronberg, Christine. "Constellation Families." Munich Astro Archive. U Observatory Munich/Leibniz Supercomputing Centre, 5 Aug. 1996. Web. 10 Apr. 2015.

Kronberg, Christine. "Corona Australis." Munich Astro Archive. U Observatory Munich/Leibniz Supercomputing Centre, 29 Sept. 1997. Web. 10 Apr. 2015.

Malin, David. "The Tail of the Corona Australis Reflection Nebula." Australian Astronomical Observatory. Commonwealth of Australia, 25 July 2010. Web. 10 Apr. 2015.

Nemiroff, Robert, Jerry Bonnell, and Ignacio Diaz Bobillo. "Stars and Dust across Corona Australis." Astronomy Picture of the Day. NASA, 12 Sept. 2013. Web. 10 Apr. 2015.

O’Meara, Stephen James. Southern Gems. New York: Cambridge UP, 2013. Print.