|Pronunciation||/ /, genitive //|
|Symbolism||The Northern Crown|
|Right ascension||15h 16m 03.8205s–16h 25m 07.1526s|
|Area||179 sq. deg. (73rd)|
|Stars with planets||5|
|Stars brighter than 3.00m||4|
|Stars within 10.00 pc (32.62 ly)||0|
|Brightest star||α CrB (Alphecca or Gemma) (2.21m)|
|Nearest star||HD 144579
(46.86 ly, 17.25 pc)
|Visible at latitudes between +90° and −50°.
Best visible at 21:00 (9 p.m.) during the month of July.
Corona Borealis / / is a small constellation in the Northern Celestial Hemisphere. Its Latin name means "northern crown", a name inspired by its shape, as its brightest stars form a semicircular arc. It is the northern counterpart of Corona Australis, the southern crown. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations. In classical mythology Corona Borealis generally represented the crown given by the god Dionysus to the Cretan princess Ariadne and set by him in the heavens. Other cultures likened the pattern to a circle of elders, eagle's nest, bear's den, or even a smokehole.
The brightest star is the magnitude 2.2 Alpha Coronae Borealis. The yellow supergiant R Coronae Borealis is the prototype of a rare class of giant stars—the R Coronae Borealis variables—that are extremely hydrogen deficient, and thought to result from the merger of two white dwarfs. T Coronae Borealis, also known as the Blaze Star, is another unusual type of variable star known as a recurrent nova. Normally of magnitude 10, it last flared up to magnitude 2 in 1946. ADS 9731 and Sigma Coronae Borealis are multiple star systems with six and five components respectively. Five star systems have been found to have Jupiter-sized exoplanets. Abell 2065 is a highly concentrated galaxy cluster located one billion light-years from our Solar System containing over 400 members, and is itself part of the larger Corona Borealis Supercluster.
Covering 179 square degrees and hence 0.433% of the sky, Corona Borealis ranks 73rd of the 88 constellations in area. It is bordered by Boötes to the north and west, Serpens Caput to the south, and Hercules to the east. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is 'CrB'. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of eight segments (illustrated in infobox). In the equatorial coordinate system, the right ascension coordinates of these borders lie between 15h 16.0m and 16h 25.1m, while the declination coordinates are between 39.71° and 25.54°. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 50°S.[a]
The seven stars that make up the constellation's distinctive crown-shaped pattern are all 4th-magnitude stars, except for the constellation's brightest star, Alpha Coronae Borealis. The other six stars are Theta, Beta, Gamma, Delta, Epsilon, and Iota Coronae Borealis. The German cartographer Johann Bayer gave twenty stars in Corona Borealis Bayer designations from Alpha to Upsilon in his 1603 star atlas Uranometria. The components of the double star Zeta Coronae Borealis have since been designated Zeta1 and Zeta2, and John Flamsteed equated his 20 and 21 Coronae Borealis with Nu1 and Nu2. Chinese astronomers considered nine stars to make up the asterism, adding Pi and Rho Coronae Borealis. Within the constellation's borders, there are 37 stars brighter than or equal to apparent magnitude 6.5.[b]
Also called Alphekka or Gemma, Alpha Coronae Borealis appears as a blue-white star of apparent magnitude 2.2, though it is an Algol-type eclipsing binary. It varies by 0.1 magnitude with a period of 17.4 days. The primary is a white main sequence star of spectral type A0V that is 2.91 times the mass of the Sun (M☉) and 57 times as luminous (L☉), and is surrounded by a debris disk out to a radius of around 60 astronomical units (AU). The secondary companion is a yellow main sequence star of spectral type G5V that is a little smaller (0.9) times the diameter of the Sun. Lying 75±0.5 light-years distant from Earth, Alphekka is believed to be a member of the Ursa Major Moving Group of stars that have a common motion through space.
Located 112±3 light-years away, Beta Coronae Borealis or Nusakan is a spectroscopic binary system whose two components are separated by 10 AU and orbit each other every 10.5 years. The brighter component is a rapidly oscillating Ap star, pulsating with a period of 16.2 minutes. Of spectral type A5V with a surface temperature of around 7980 K, it has around 2.09 M☉, 2.63 solar radii (R☉), and 25.3 L☉. The smaller star is of spectral type F2V with a surface temperature of around 6750 K, and has around 1.4 M☉, 1.56 R☉, and between 4 and 5 L☉. Near Nusakan is Theta Coronae Borealis, a binary system that shines with a combined magnitude of 4.13 located 380±20 light-years distant. The brighter component, Theta Coronae Borealis A, is a blue-white star that spins extremely rapidly—at a rate of around 393 km per second. A Be star, it is surrounded by a debris disk.
Flanking Alpha to the east is Gamma Coronae Borealis, yet another binary star system, whose components orbit each other every 92.94 years and are roughly as far apart from each other as the Sun and Neptune. The brighter component has been classed as a Delta Scuti variable star, though this view is not universal. The components are main sequence stars of spectral types B9V and A3V. Located 170±2 light-years away, 4.06-magnitude Delta Coronae Borealis is a yellow giant star of spectral type G3.5III that is around 2.4 M☉ and has swollen to 7.4 R☉. It has a surface temperature of 5180 K. For most of its existence, Delta Coronae Borealis was a blue-white main sequence star of spectral type B before it ran out of hydrogen fuel in its core. Its luminosity and spectrum suggest it has just crossed the Hertzsprung gap, having finished burning core hydrogen and just begun burning hydrogen in a shell that surrounds the core.
Zeta Coronae Borealis is a double star with two blue-white components 6.3 arcseconds apart that can be readily separated at 100x magnification. The primary is of magnitude 5.1 and the secondary is of magnitude 6.0. Nu Coronae Borealis is an optical double, whose components are a similar distance from Earth but have different radial velocities, hence are assumed to be unrelated. The primary, Nu1 Coronae Borealis, is a red giant of spectral type M2III and magnitude 5.2 640±30 light-years distant, and the secondary, Nu2 Coronae Borealis is an orange-hued giant star of spectral type K5III and magnitude 5.4 590±30 light-years away. Sigma Coronae Borealis, on the other hand, is a true multiple star system divisible by small amateur telescopes. It is actually a complex multiple star system composed of two stars around as massive as the Sun that orbit each other every 1.14 days, orbited by a third sun-like star every 726 years. The fourth and fifth components are a binary red dwarf system that is 14,000 AU distant from the other three stars. ADS 9731 is an even rarer multiple system in the constellation, composed of six stars, two of which are spectroscopic binaries.[c]
Corona Borealis is home to two remarkable variable stars. T Coronae Borealis is a cataclysmic variable star also known as the Blaze Star. Normally placid around magnitude 10—it has a minimum of 10.2 and maximum of 9.9—it brightens to magnitude 2 in a period of hours, caused by a nuclear chain reaction and the subsequent explosion. T Coronae Borealis is one of a handful of stars called recurrent novae, which include RS Ophiuchi, T Pyxidis, V1017 Sagitarii, and U Scorpii. An outburst of T Coronae Borealis was first recorded in 1866; its second recorded outburst was in February 1946. T Coronae Borealis is a binary star with a red-hued giant primary and a small blue secondary; its period is approximately 8 months. R Coronae Borealis is a yellow-hued variable supergiant star, over 7000 light-years from Earth, and prototype of a class of stars known as R Coronae Borealis variables. Normally of magnitude 6, its brightness periodically drops as low as magnitude 15 and then slowly increases over the next several months. These declines in magnitude come about as dust that has been ejected from the star obscures it. Direct imaging with the Hubble Space Telescope shows extensive dust clouds out to a radius of around 2000 AU from the star, corresponding with a stream of fine dust (composed of grains 5 nm in diameter) associated with the star's stellar wind and coarser dust (composed of grains with a diameter of around 0.14 µm) ejected periodically.
There are several other variables of reasonable brightness for amateur astronomer to observe, including three Mira-type long period variables: S Coronae Borealis ranges between magnitudes 5.8 and 14.1 over a period of 360 days. Located around 1946 light-years distant, it shines with a luminosity 16643 times that of the Sun and has a surface temperature of 3033 K. One of the reddest stars in the sky, V Coronae Borealis is a cool star with a surface temperature of 2877 K that shines with a luminosity 102831 times that of the Sun and is a remote 8810 light-years distant from Earth. Varying between magnitudes 6.9 and 12.6 over a period of 357 days, it is located near the junction of the border of Corona Borealis with Hercules and Bootes. Located 1.5° northeast of Tau Coronae Borealis, W Coronae Borealis ranges between magnitudes 7.8 and 14.3 over a period of 238 days. Another red giant, RR Coronae Borealis is a M3-type semiregular variable star that varies between magnitudes 7.3 and 8.2 over 60.8 days. Meanwhile U Coronae Borealis varies in magnitude from 7.7 to 8.8 over 5 hours, but is an eclipsing binary with a period of 3 days and 19 hours.
TY Coronae Borealis is a pulsating white dwarf (of ZZ Ceti) type, which is around 70% as massive as the Sun, yet has only 1.1% of its diameter. Discovered in 1990, UW Coronae Borealis is a low-mass X-ray binary system composed of a star less massive than the sun and a neutron star surrounded by an accretion disk that draws material from the companion star. It varies in brightness in an unusually complex manner: the two stars orbit each other every 111 minutes, yet there is another cycle of 112.6 minutes, which corresponds to the orbit of the disk around the degenerate star. The beat period of 5.5 days indicates the time the accretion disk—which is asymmetrical—takes to precess around the star.
Extrasolar planets have been confirmed in five star systems, four of which were found by the radial velocity method. The spectrum of Epsilon Coronae Borealis was analysed for seven years from 2005 to 2012, revealing a planet around 6.7 times as massive as Jupiter (MJ) orbiting every 418 days at an average distance of around 1.3 AU., Epsilon itself is a 1.7 M☉ orange giant of spectral type K2III that has swollen to 21 R☉ and 151 L☉. Kappa Coronae Borealis is an orange subgiant nearly twice as massive as the Sun with a spectral type K1IV with a dust debris disk, and one planet with a period of around 3.4 years. This planet is around 2.5 MJ. The dimensions of the debris disk indicate it is likely there is a second substellar companion. Omicron Coronae Borealis is another K-type clump giant with one confirmed planet with a mass of 0.83 MJ that orbits every 187 days—one of the two least massive planets known around clump giants. HD 145457 is an orange giant of spectral type K0III found to have one planet around 2.9 MJ. Discovered by the Doppler method in 2010, it takes 176 days to complete an orbit. XO-1 is a magnitude 11 yellow dwarf star located approximately 560 light-years away, with a mass and radius similar to the Sun. In 2006 the hot Jupiter exoplanet XO-1b was discovered orbiting XO-1 by the transit method using the XO Telescope. Roughly the size of Jupiter, it completes an orbit around its star every three days.
The discovery of a Jupiter-sized planetary companion was announced in 1997 via analysis of the radial velocity of Rho Coronae Borealis, a Solar twin, yellow dwarf around 57 light-years distant. However, more accurate measurement of data from the Hipparcos satellite subsequently showed it instead to be a low-mass star somewhere between 100 and 200 times the mass of Jupiter. Possible stable planetary orbits in the habitable zone were calculated for the binary star Eta Coronae Borealis, which is composed of two stars—yellow main sequence stars of spectral type G1V and G3V respectively—similar in mass and spectrum to the Sun. No planet has been found but a brown dwarf companion around 63 times as massive as Jupiter with a spectral type of L8 was discovered at a distance of 3640 AU from the pair in 2001.
Corona Borealis contains few galaxies observable with amateur telescopes. NGC 6085 and 6086 are a faint spiral and elliptical galaxy respectively close enough to each other to be seen in the same visual field through a telescope. Abell 2142 is a huge (six million light-year diameter), X-ray luminous galaxy cluster that is the result of an ongoing merger between two galaxy clusters. It has a redshift of 0.0909 (meaning it is moving away from us at 27,250 km/s) and a visual magnitude of 16.0. It is about 1.2 billion light years (380 Mpc) away.[d] Another galaxy cluster in the constellation, RX J1532.9+3021, is located approximately 3.9 billion light years from Earth. At the cluster's center is a large elliptical galaxy containing the supermassive black hole. Abell 2065 is a highly concentrated galaxy cluster containing over 400 members, the brightest of which are 16th magnitude. The cluster is more than one billion light-years from Earth. On a larger scale still, Abell 2065, along with Abell 2061, Abell 2067, Abell 2079, Abell 2089, and Abell 2092, make up the Corona Borealis Supercluster.
In November 2013 astronomers discovered the largest structure in the universe ever found—the Hercules–Corona Borealis Great Wall, which lies partly within this constellation's borders. The structure is a galaxy filament, or a huge group of galaxies assembled by gravity. It is about 10 billion light-years (3 Gpc) at its longest dimension, which is approximately 1/9 (10.7%) of the diameter of the observable universe, 7.2 billion light-years (2.2 Gpc; 150,000 km/s in redshift space) wide, but only 900 million light-years (300 Mpc) thick, and is the largest known structure in the universe. It is at redshift 2, corresponding to a distance of approximately 10 billion light-years away from Earth.
In Greek mythology, Corona Borealis was linked to the legend of Theseus and the minotaur. It was generally considered to represent a crown that was given by Dionysus to Ariadne, the daughter of Minos of Crete, after she had been abandoned by the Athenian prince Theseus. When she wore the crown to her wedding, where she married Dionysus, he placed her crown in the heavens to commemorate the wedding. An alternate version has the besotted Dionysus give the crown to Ariadne, who in turn gives it to Theseus after he arrives in Crete to kill the minotaur that the Cretans have demanded tribute from Athens to feed. The hero uses the crown's light to escape the labyrinth after disposing of the creature, and Dionysus later sets it in the heavens. The Latin author Hyginus linked it to a crown or wreath worn by Bacchus (Dionysus) to disguise his appearance when first approaching Mount Olympus and revealing himself to the gods, having being previously hidden as yet another child of Jupiter's trysts with a mortal, in this case Semele. In Welsh mythology, it was called Caer Arianrhod, "the Castle of the Silver Circle", and was the heavenly abode of the Lady Arianrhod. It was one of the 48 constellation mentioned in the Almagest of classical astronomer Ptolemy.
The Arabs called the constellation Alphecca (a name later given to Alpha Corona Borealis), which means "separated" or "broken up" (الفكة al-Fakkah), a reference to the resemblance of the stars of Corona Borealis to a loose string of jewels. Among the Bedouins, the constellation was known as qaṣʿat al-masākīn (قصعة المساكين), or "the dish/bowl of the poor people".
The Skidi people of Native Americans saw the stars of Corona Borealis representing a council of stars whose chief was Polaris. The constellation also symbolised the smokehole over a fireplace, which conveyed their messages to the gods, as well as how chiefs should come together to consider matters of importance. The Mi'kmaq of eastern Canada saw Corona Borealis as Mskegwǒm, the den of the celestial bear (Alpha, Beta, Gamma and Delta Ursae Majoris).
Polynesian peoples often recognized Corona Borealis; it was likely called Te Hetu in the Tuamotus, whose people called the constellation Na Kaua-ki-tokerau. In Hawaii, the constellation was likely called Kaua-mea; it was called Rangawhenua in New Zealand. The figure of Corona Borealis was called Te Wale-o-Awitu in Pukapuka. Its name in Tonga was unsure; it was either called Ao-o-Uvea or Kau-kupenga.
In Australian Aboriginal astronomy, the constellation is called womera ("the boomerang") due to the shape of the stars. The Wailwun people of northwestern New South Wales saw Corona Borealis as mullion wollai "eagle's nest", with Altair and Vega—each called mullion—the pair of eagles accompanying it. The Wardaman people of northern Australia held the constellation to be a gathering point for Men's Law, Women's Law and Law of both sexes come together and consider matters of existence.
The constellation of Corona Borealis was featured as a main plot ingredient in the short story "Hypnos" by H. P. Lovecraft, published in 1923; it is the object of fear of one of the protagonists in the novella. Finnish band Cadacross released an album titled Corona Borealis in 2002.
- While parts of the constellation technically rise above the horizon to observers between the 50°S and 64°S, stars within a few degrees of the horizon are to all intents and purposes unobservable.
- Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.
- The components are organised thus: Aa and Ab are yellow-white main sequence stars of spectral types F4V and F5V and 1.35 and 1.32 solar masses respectively, which orbit each other every 3.27 days. This pair is in a 450-year orbit with star B, a star of spectral type G4V that has around the same mass as the Sun. Star C is a yellow white star of spectral type F3V around 1.41 times as massive as the sun, which has just started brightening and moving off the main sequence. It is in a 1000-year orbit with a pair of stars, Da and Db, a yellow-white main sequence star of spectral type F7V and a red dwarf of spectral type M3V. Da and Db take 14.28 days to orbit each other. Finally the system of stars C and Dab, and the system of stars Aab and B, take over 20,000 years to orbit each other.
- Distance calculated from redshift.
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