|Pronunciation||/ /, genitive //|
|Symbolism||The Northern Crown|
|Right ascension||15h 16m 03.8205s–16h 25m 07.1526s|
|Area||179 sq. deg. (73rd)|
|Stars with planets||4|
|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; its main 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.
The brightest star is the 2.2-magnitude Alpha Coronae Borealis. Four star systems have been found to have exoplanets to date; three of these are orange giants, while the fourth—Rho Coronae Borealis—is a solar twin, very like our own Sun. The yellow supergiant R Coronae Borealis is the prototype of a rare class of unusual stars—the R Coronae Borealis variables—that are extremely hydrogen deficient, and thought to result from the merger of two white dwarfs. Abell 2065 is a highly concentrated galaxy cluster located one billion light-years from our Solar System containing over 400 members, the brightest of which are 16th magnitude.
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. 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 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 around 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 star 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 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 around 112 light-years away, Beta Coronae Borealis or Nusakan is a spectroscopic binary system whose two components are separated by 10 astronomical units 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 around 375 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 light-years away, 4.06-magnitude Delta Coronae Borealis is a yellow giant star of spectral type G3.5III that 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 its shell.
Zeta Coronae Borealis is a double star divisible in small telescopes. It has two blue-white components, 470 light-years from Earth. The primary is of magnitude 5.0 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 around 640 light-years distant, and the secondary, Nu2 Coronae Borealis is an orange-hued giant star of spectral type K5III and magnitude 5.4 around 590 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 astronomical units distant from the other three stars. ADS 9731 is a system composed of six stars, two of which are spectroscopic binaries.
Corona Borealis is home to two remarkable variable stars. T Coronae Borealis is an exploding 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 most recent 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. Though small dips in brightness occur sporadically, extreme decreases happened most recently in 1962, 1972, and 1977. Small carbon particles building up in the stellar atmosphere may be responsible.
S Coronae Borealis is a Mira-type long period variable that ranges between magnitudes 5.8 and 14.1 over a period of 360 days. RR Coronae Borealis is a M3-type semiregular variable star that varies between magnitudes 7.3 and 8.2 over 60.8 days. 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.
Extrasolar planets have been confirmed in four star systems 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 astronomical units., 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. The discovery of a Jupiter-sized planetary companion was announced in 1997 via analysis of the radial velocity of Rho Coronae Borealis, is a Solar twin, yellow dwarf around 57 light-years distant. However, more accurate measurement of data from the Hipparchos satellite subsequently showed it instead to be a low-mass star somewhere between 100 and 200 times the mass of Jupiter.
Corona Borealis contains no bright deep-sky objects. 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. Abell 2142 is a huge, X-ray luminous galaxy cluster that is the result of a still ongoing merger between two galaxy clusters. The combined cluster is six million light years across, contains hundreds of galaxies and enough gas to make a thousand more. It has a heliocentric 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. 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.
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 1.6–2.1, corresponding to a distance of approximately 10 billion light-years away from Earth.
History and mythology
In Greek mythology, Corona Borealis was sometimes considered to represent a crown that was given by Dionysus to Ariadne, the daughter of Minos of Crete. When she wore the crown to her wedding, where she married Dionysus, he placed her crown in the heavens to commemorate the wedding. In Welsh mythology, it was called Caer Arianrhod, "the Castle of the Silver Circle", and was the heavenly abode of the Lady Arianrhod.
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", since the stars form an unsymmetrical pattern with an indent in one side.
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.
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.
The Cheyenne nation of Native Americans called the main stars of this constellation the "Camp Circle", as they arranged their camps in a semicircle. Native Americans also used stars to make designs in the ground at the Medicine Wheel in Bighorn National Forest, Wyoming, USA.
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- Distance calculated from redshift.
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