1. Introduction

Corona Borealis, Latin for "Northern Crown," is a small but visually striking constellation in the northern celestial hemisphere. It is distinguished by its distinctive semicircular arrangement of stars, which resembles a crown or wreath. Positioned between the constellations of Hercules and Boötes, Corona Borealis occupies an area of approximately 179 square degrees in the sky. The celestial coordinates of Corona Borealis lie between approximately right ascension 15h 16m to 16h 25m and declination +25° to +40°. This places the constellation in close proximity to the celestial north pole, making it visible to observers in the northern hemisphere year-round. Its location along the ecliptic path also means that the Sun passes through Corona Borealis during the summer solstice, further enhancing its prominence in the night sky (Figure 1).

Corona Borealis is characterized by its distinct semicircular shape, formed by a loop of bright stars that represent the points of the crown. The constellation's brightest star, known as Alphecca or Alpha Coronae Borealis, marks the jewel at the crown's apex. Despite its modest size and lack of bright stars, Corona Borealis contains several interesting celestial objects, including variable stars, binary star systems, and galaxy clusters, making it a captivating area of exploration for astronomers and stargazers alike.

Figure 1. IAU chart of Corona Borealis. Source: https://www.iau.org/static/archives/images/screen/crb.jpg. Credit: IAU and Sky & Telescope. Reproduced under CC BY 4.0 license.

2. Historical Background and Mythology

In Greek mythology, Corona Borealis is often associated with the story of Ariadne, the daughter of King Minos of Crete. According to the myth, Ariadne fell in love with Theseus, the Athenian hero who slew the Minotaur. To help him navigate the labyrinth and defeat the Minotaur, Ariadne provided Theseus with a ball of thread, which he used to find his way out. In return for her assistance, Theseus promised to marry Ariadne and take her to Athens. However, Theseus abandoned Ariadne on the island of Naxos on their journey home. Heartbroken and despondent, Ariadne was discovered by the god Dionysus, who fell in love with her and made her his wife. As a wedding gift, Dionysus placed Ariadne's crown, the Corona Borealis, in the heavens as a constellation to honor her memory.

Corona Borealis is also associated with other cultures and mythologies. In Roman mythology, the constellation is sometimes linked to the story of Bacchus (the Roman equivalent of Dionysus) and his bride, who is often identified as Ariadne. Bacchus is said to have placed Ariadne's crown in the heavens as a reminder of their eternal bond.

In Chinese astronomy, Corona Borealis is part of the Azure Dragon constellation, one of the four symbols of the Chinese constellations. In Chinese mythology, the Azure Dragon represents the east and is associated with the spring season and the element of wood. While Corona Borealis itself is not explicitly mentioned in Chinese mythology, its position within the Azure Dragon constellation reflects its cultural significance within Chinese cosmology.

3. Notable Stars

Alphecca (Alpha Coronae Borealis): Alphecca, also known as Alpha Coronae Borealis, is the brightest star in the Corona Borealis constellation. It is a binary star system located approximately 75 light-years away from Earth. The primary star is a white main-sequence star, while the secondary star is a fainter companion. Alphecca serves as the jewel at the apex of the celestial crown, marking the prominent point in the constellation's semicircular arrangement of stars. Despite its relatively close distance, Alphecca is not exceptionally bright, with an apparent magnitude of around 2.2.

Nusakan (Beta Coronae Borealis): Nusakan, also known as Beta Coronae Borealis, is another notable star in the constellation. It is a spectroscopic binary star system located approximately 114 light-years away from Earth. The primary star is a yellow-white dwarf star, while the secondary star is a fainter companion. Nusakan is a relatively bright star, with an apparent magnitude of around 3.7, making it visible to the naked eye under favorable viewing conditions. Its name, Nusakan, derives from the Arabic phrase meaning "the two series" or "the two orders," referring to its binary nature.

Gamma Coronae Borealis (γ Coronae Borealis) is a binary star system located in the constellation Corona Borealis. It is one of the notable stars in this constellation, although it is not particularly bright to the naked eye. The primary star, Gamma Coronae Borealis A, is a yellow-white dwarf star of spectral type F2V. It has a visual magnitude of approximately 3.83, making it visible under good viewing conditions. The secondary component, Gamma Coronae Borealis B, is a fainter companion star that orbits around the primary star. The two stars are separated by a distance of about 4 arcseconds, which is relatively close in astronomical terms. Gamma Coronae Borealis is classified as a binary star system because of the orbital motion of its components around a common center of mass. This binary system is located at a distance of approximately 93 light-years from Earth.

Zeta Coronae Borealis (ζ Coronae Borealis) is a binary star system located in the constellation Corona Borealis. It is one of the prominent stars in this constellation, visible to the naked eye with a combined apparent magnitude of approximately 3.9. Zeta Coronae Borealis is situated at a distance of about 114 light-years from Earth. The primary component, Zeta Coronae Borealis A, is a yellow-white dwarf star of spectral type F9V. It has a visual magnitude of approximately 5.1, making it fainter than its companion star. The secondary component, Zeta Coronae Borealis B, is a fainter star that orbits around the primary star. The two stars are separated by a distance of about 1.7 arcseconds, which is relatively close in astronomical terms. In addition to its binary nature, Zeta Coronae Borealis has significance in traditional Chinese astronomy. It is part of the Chinese asterism known as Nán Hé (南河), which translates to "the Southern River." This asterism is associated with the mythological figure of the river god in Chinese folklore.

R Coronae Borealis: R Coronae Borealis is a variable star located in the Corona Borealis constellation. It belongs to a rare class of stars known as R Coronae Borealis variables, which exhibit irregular declines in brightness due to the formation of clouds of carbon dust in their atmospheres. These dust clouds obscure the starlight, causing periodic dimming events that can last for weeks or months. R Coronae Borealis is one of the brightest and most well-known stars of this type, with a maximum apparent magnitude of around 6.0.

T Coronae Borealis: T Coronae Borealis is another variable star in the constellation, belonging to the class of stars known as T Coronae Borealis variables. Like R Coronae Borealis stars, T Coronae Borealis variables undergo irregular declines in brightness due to the formation of carbon dust clouds in their atmospheres. However, T Coronae Borealis stars typically exhibit more frequent and rapid variations in brightness compared to their R Coronae Borealis counterparts.