Corona Borealis

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: Corona Borealis (also called the Northern Crown) is a constellation, or a group of stars, in a semicircular shape that resembles a crown. This constellation is found in the Northern Hemisphere and is most visible around July. Corona Borealis has a rich history. Cultures around the world identified it and linked it to a wide variety of myths, tales, and beliefs. Astronomers are interested in the constellation because it contains an assortment of various types of stars including binary stars, variable stars, and recurrent novas.

The Crown Constellation

Corona Borealis is a constellation, an officially designated section of the night sky. Since ancient times, people have imagined patterns of stars, or asterisms, to form various images. Different cultures around the world have identified their own sets of patterns in the stars. Since 1930, the International Astronomical Union has recognized eighty-eight official constellations.

The Latin name Corona Borealis, meaning Northern Crown, comes from the semicircular shape of the circumpolar constellation. The ancient Greeks saw it as a crown. It was one of the original forty-eight constellations noted by the early astronomer Ptolemy (ca. 100–170 CE). People in many different cultures have identified Corona Borealis in different ways. Some have seen a wreath, dish, boomerang, castle, or even a group of dancers.

Attributes of Corona Borealis

Corona Borealis can be found in the Northern Hemisphere between the latitudes of 90 degrees north and 50 degrees south. It is far north of the celestial equator, with a declination of 30 degrees, and has a right ascension of sixteen hours. It is most visible during its highest point in July. The form of Corona Borealis is made of many stars, which are designated with Greek letters alpha, beta, gamma, and so on. Some of the more notable stars have separate names, such as Alphecca and Nusakan. The stars of Corona Borealis vary in brightness, but the major ones are visible to the naked eye.

Stargazers can find Corona Borealis between several other constellations. To its north and west is the Boötes constellation, which has been interpreted as a farmer or shepherd. To its south is Serpens Caput, or Serpent’s Head. To its east is the Hercules constellation, named after the mythical hero. Astronomers consider Corona Borealis to be a counterpart to a similar constellation in the Southern Hemisphere known as Corona Australis, or the Southern Crown.

Overall, among the many constellations, Corona Borealis is not particularly large. However, it is still highly significant to astronomers. Perhaps the most notable object is the central star of the semicircle. This star is known as Alphecca, Gemma, Gnosia, Asteroth, or alpha Coronae Borealis. The name Gemma means "gem" or "jewel," a name the star received due to its resemblance to a jewel on a crown. This star, the brightest star in Corona Borealis, is actually not one star but two. It is an eclipsing binary star, a system in which two nearby stars revolve around a common center of mass. About every 17.4 days the two stars eclipse each other. This creates a small drop in brightness.

Another important star in the Corona Borealis is known as R Coronae Borealis (or R CrB). This star is known to some astronomers as the Fadeout Star or the Reverse Nova due to its unusual behavior. Although normally somewhat bright, it suddenly and unpredictably dims before returning to normal. Astronomers believe this odd change occurs when the star emits clouds of carbon that fill its atmosphere and block light from escaping.

A third notable star from the constellation is T Coronae Borealis, known to many astronomers as the Blaze Star. This star appears to have opposite behavior from R Coronae Borealis. T Coronae Borealis is a recurrent nova, a star that occasionally and briefly increases its brightness greatly due to explosions within the star. Normally, the Blaze Star is not very bright, but it can become as bright as Alphecca during its eruptions.

Myths and Tales of the Ancient World

Since ancient times, people around the world have been fascinated by the night sky. These people looked for constellations and created stories about the patterns (asterisms) they saw. Their stories often reflected religious beliefs or myths and folktales that helped explain the mysteries of nature. Stars and constellations were also important for practical reasons. People used these heavenly bodies as a sort of calendar because the apparent movement of the stars followed similar paths each year. Ancient farmers, for instance, often studied the positions of constellations to decide when it was time to plant or harvest crops.

Corona Borealis is thought to be one of the earliest constellations identified, and different groups around the world made their own interpretations of the semicircle shape. The most famous story, and the one from which the official name derives, originated in ancient Greece. Ancient Greek astronomers felt the semicircle looked like a crown. This observation linked with myths of the princess Ariadne of Crete, who helped the hero Theseus defeat a monster called the Minotaur. Afterward, according to some versions of the myth, Ariadne married Dionysus, the Greek god of wine and fertility. The god presented his wife with a splendid bejeweled crown, which he later placed in the heavens as an eternal memorial to Ariadne.

In Western Asia, astronomers believed the semicircular shape represented a cracked dish or a broken pot. In Australia, some native people felt the star shape resembled a boomerang, a curved wooden stick used for hunting. The Cheyennes and Shawnees of North America believed the constellation represented a circle of eternally dancing maidens in the sky. The circle became a semicircle when one maiden left to marry. In ancient Celtic lands, some people envisioned the stars as a celestial castle called Caer Arianrhod. Although the Greek interpretation is the most famous and influential due to the work of Ptolemy, astronomers and historians preserve the other tales for their cultural importance.

Astronomical Importance

Constellations like Corona Borealis are still a favorite of stargazers as well as a helpful tool for astronomers. Astronomers can use constellations as maps to quickly locate stars and determine whether stars have changed location, appearance, or patterns of movement.

Corona Borealis is most important to modern astronomers and stargazers only for the wide variety of space objects that can be located within the constellation. There are several binary stars in the grouping, including Alphecca, beta (or Nusakan), and gamma. These stars are interesting to astronomers, who monitor the revolutions and eclipses of the star pairs.

The activity of R Coronae Borealis makes it an astronomical curiosity that has led space researchers to study the different factors that can affect a star’s brightness. Astronomers began to use this star as a prototype star, or an example. Officially, all stars that exhibit similar changes in luminosity are classified as R Coronae Borealis (RCB) Type Variables. Similarly, astronomers have taken great interest in T Coronae Borealis for its qualities as a recurrent nova, another unusual and interesting type of star.

Another interesting feature of the constellation is the Corona Borealis galaxy cluster, also called Abell 2065. This extremely distant cluster of about four hundred galaxies is more than 1,200 million light-years from Earth. Due to its extreme distance, this formation can only be seen with very powerful telescopes.

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 pattern of stars that appear in the night sky near a celestial pole and can be seen all night long, year-round.
• declination: the north-south position of a celestial body relative to the celestial equator expressed in degrees of arc.
• constellation: a section of the night sky identified by patterns of stars seen from Earth.
• International Astronomical Union: a worldwide association of professional astronomers that sets the rules for naming celestial bodies and features on them and defines scientific constants of importance to astronomy.
• right ascension: the east-west position of a celestial body defined in relation to the celestial equator and expressed in hours and minutes, not degrees of arc.

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