Corona Borealis

Template:Short description Template:Redirect Template:Featured article Template:Infobox constellation

Corona Borealis is a small constellation in the Northern Celestial Hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Its brightest stars form a semicircular arc. Its Latin name, inspired by its shape, means "northern crown". In classical mythology Corona Borealis generally represented the crown given by the god Dionysus to the Cretan princess Ariadne and set by her in the heavens. Other cultures likened the pattern to a circle of elders, an eagle's nest, a bear's den or a smokehole. Ptolemy also listed a southern counterpart, Corona Australis, with a similar pattern.

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, and is predicted to do the same in 2025. ADS 9731 and Sigma Coronae Borealis are multiple star systems with six and five components respectively. Five stars in the constellation host Jupiter-sized exoplanets. Abell 2065 is a highly concentrated galaxy cluster one billion light-years from the Solar System containing more than 400 members, and is itself part of the larger Corona Borealis Supercluster.

Characteristics

Covering 179 square degrees and hence 0.433% of the sky, Corona Borealis ranks 73rd of the IAU designated constellations by area.<ref name=tirionconst/> Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 50°S.<ref name=tirionconst>Template:Cite web</ref>Template:Efn 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".<ref name="pa30_469">Template:Cite journal</ref> The official constellation boundaries, as set by Belgian astronomer 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 Template:RA and Template:RA, while the declination coordinates are between 39.71° and 25.54°.<ref name="boundary2">Template:Cite journal</ref> It has a counterpart—Corona Australis—in the Southern Celestial Hemisphere.<ref>Template:Cite book</ref>

Features

Stars

Template:See also The seven stars that make up the constellation's distinctive crown-shaped pattern are all 4th-magnitude stars except for the brightest of them, 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. Zeta Coronae Borealis was noted to be a double star by later astronomers and its components designated Zeta¹ and Zeta². John Flamsteed did likewise with Nu Coronae Borealis; classed by Bayer as a single star, it was noted to be two close stars by Flamsteed. He named them 20 and 21 Coronae Borealis in his catalogue, alongside the designations Nu¹ and Nu² respectively.<ref name=wagman>Template:Cite book</ref> Chinese astronomers deemed nine stars to make up the asterism, adding Pi and Rho Coronae Borealis.<ref name=ridpathCB>Template:Cite web</ref> Within the constellation's borders, there are 37 stars brighter than or equal to apparent magnitude 6.5.Template:Efn<ref name=tirionconst/>

File:CoronaBorealisCC.jpg

The constellation Corona Borealis as it can be seen by the naked eye

Alpha Coronae Borealis (officially named Alphecca by the IAU, but sometimes also known as Gemma) appears as a blue-white star of magnitude 2.2.<ref>Template:Cite journal</ref> In fact, it is an Algol-type eclipsing binary that varies by 0.1 magnitude with a period of 17.4 days.Template:Sfn The primary is a white main-sequence star of spectral type A0V that is 2.91 times the mass of the Sun (Template:Solar mass) and 57 times as luminous (Template:Solar luminosity), and is surrounded by a debris disk out to a radius of around 60 astronomical units (AU).<ref name="Pawellek 2014">Template:Cite journal</ref> 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.<ref name="aaa403">Template:Cite journal</ref> Lying 75±0.5 light-years from Earth,<ref name="vanLeeuwen2007">Template:Cite journal</ref> Alphecca is believed to be a member of the Ursa Major Moving Group of stars that have a common motion through space.<ref>Template:Cite journal</ref>

Located 112±3 light-years away,<ref name="vanLeeuwen2007"/> 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.<ref name=kalerbeta>Template:Cite web</ref> The brighter component is a rapidly oscillating Ap star,<ref name="The radius and effective temperatur">Template:Cite journal</ref> pulsating with a period of 16.2 minutes. Of spectral type A5V with a surface temperature of around 7980 K, it has around Template:Solar mass, 2.6 solar radii (Template:Solar radius), and Template:Solar luminosity. The smaller star is of spectral type F2V with a surface temperature of around 6750 K, and has around Template:Solar mass, Template:Solar radius, and between 4 and Template:Solar luminosity.<ref name="The radius and effective temperatur"/> Near Nusakan is Theta Coronae Borealis, a binary system that shines with a combined magnitude of 4.13 located 380±20 light-years distant.<ref name="vanLeeuwen2007"/> 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.<ref name=kalerthet>Template:Cite web</ref>

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.<ref name=kalergamma>Template:Cite web</ref> The brighter component has been classed as a Delta Scuti variable star,<ref name=SIMBADgamma>Template:Cite web</ref> though this view is not universal.<ref name=kalergamma/> The components are main sequence stars of spectral types B9V and A3V.<ref>Template:Cite journal</ref> Located 170±2 light-years away,<ref name="vanLeeuwen2007"/> 4.06-magnitude Delta Coronae Borealis is a yellow giant star of spectral type G3.5III that is around Template:Solar mass and has swollen to Template:Solar radius. It has a surface temperature of 5180 K.<ref name="Gondoin">Template:Cite journal</ref> 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.<ref name=kalerdelta>Template:Cite web</ref>

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.<ref>Template:Cite book</ref> 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.<ref name=kalernu>Template:Cite web</ref> The primary, Nu¹ Coronae Borealis, is a red giant of spectral type M2III and magnitude 5.2, lying 640±30 light-years distant,<ref name="vanLeeuwen2007"/><ref name=SIMBADnu1>Template:Cite web</ref> and the secondary, Nu² Coronae Borealis, is an orange-hued giant star of spectral type K5III and magnitude 5.4, estimated to be 590±30 light-years away.<ref name="vanLeeuwen2007"/><ref name=SIMBADnu2>Template:Cite web</ref> Sigma Coronae Borealis, on the other hand, is a true multiple star system divisible by small amateur telescopes.Template:Sfn It is actually a complex 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.<ref name="Raghavan 2009">Template:Cite journal</ref> ADS 9731 is an even rarer multiple system in the constellation, composed of six stars, two of which are spectroscopic binaries.<ref name=Tokovinin>Template:Cite journal</ref>Template:Efn

Corona Borealis is home to two remarkable variable stars.<ref name="Levy 1998">Template:Cite book</ref> T Coronae Borealis is a cataclysmic variable star also known as the Blaze Star.Template:Sfn 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 T Pyxidis and U Scorpii. An outburst of T Coronae Borealis was first recorded in 1866; its second recorded outburst was in February 1946.<ref name=Schaefer2009>Template:Cite journal</ref><ref name=Sanford1949>Template:Cite journal</ref> T Coronae Borealis started dimming in March 2023 and it is known that before it goes nova it dims for about a year; for this reason it was initially expected to go nova at any time between March and September, 2024.<ref>Template:Cite news</ref> T Coronae Borealis is a binary star with a red-hued giant primary and a white dwarf secondary, the two stars orbiting each other over a period of approximately 8 months.<ref>Template:Cite book</ref> 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.<ref>Template:Cite web</ref> 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.<ref>Template:Cite journal</ref>

There are several other variables of reasonable brightness for amateur astronomer to observe, including three Mira-type long period variables:<ref name="Levy 1998"/> S Coronae Borealis ranges between magnitudes 5.8 and 14.1 over a period of 360 days.<ref name="AAVSOS">Template:Cite web</ref> Located around 1946 light-years distant, it shines with a luminosity 16,643 times that of the Sun and has a surface temperature of 3033 K.<ref name=Mcdonald>Template:Cite journal</ref> One of the reddest stars in the sky,<ref name="Levy 1998"/> V Coronae Borealis is a cool star with a surface temperature of 2877 K that shines with a luminosity 102,831 times that of the Sun and is a remote 8810 light-years distant from Earth.<ref name=Mcdonald/> Varying between magnitudes 6.9 and 12.6 over a period of 357 days,<ref name="AAVSOV">Template:Cite web</ref> it is located near the junction of the border of Corona Borealis with Hercules and Bootes.<ref name="Levy 1998"/> 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.<ref name="AAVSOW">Template:Cite web</ref> 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.<ref name=AAVSORR>Template:Cite web</ref> RS Coronae Borealis is yet another semiregular variable red giant, which ranges between magnitudes 8.7 to 11.6 over 332 days.<ref name=AAVSORS>Template:Cite web</ref> It is unusual in that it is a red star with a high proper motion (greater than 50 milliarcseconds a year).<ref name=" Jiménez-Esteban">Template:Cite journal</ref> Meanwhile, U Coronae Borealis is an Algol-type eclipsing binary star system whose magnitude varies between 7.66 and 8.79 over a period of 3.45 days<ref name="AAVSOU">Template:Cite web</ref>

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.<ref>Template:Cite journal</ref> 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.<ref name="Mason 2012">Template:Cite journal</ref>

Extrasolar planetary systems

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 (Template:Jupiter mass) orbiting every 418 days at an average distance of around 1.3 AU.<ref name="lee 2012">Template:Cite journal</ref> Epsilon itself is a Template:Solar mass orange giant of spectral type K2III that has swollen to Template:Solar radius and Template:Solar luminosity.<ref name=aj135_1_209>Template:Cite journal</ref> Kappa Coronae Borealis is a spectral type K1IV orange subgiant nearly twice as massive as the Sun; around it lies a dust debris disk,<ref name="Pawellek 2014"/> and one planet with a period of 3.4 years.<ref name="PASJ">Template:Cite journal</ref> This planet's mass is estimated at Template:Jupiter mass. The dimensions of the debris disk indicate it is likely there is a second substellar companion.<ref name=Bonsor13>Template:Cite journal</ref> Omicron Coronae Borealis is a K-type clump giant with one confirmed planet with a mass of Template:Jupiter mass that orbits every 187 days—one of the two least massive planets known around clump giants.<ref name="PASJ"/> HD 145457 is an orange giant of spectral type K0III found to have one planet of Template:Jupiter mass. Discovered by the Doppler method in 2010, it takes 176 days to complete an orbit.<ref name="sato 2010">Template:Cite journal</ref> XO-1 is a magnitude 11 yellow main-sequence star located approximately Template:Convert light-years away,<ref name="Torres2008">Template:Cite journal</ref> of spectral type G1V 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.<ref name="McCullough2006">Template:Cite journal</ref>

The discovery of a Jupiter-sized planetary companion was announced in 1997 via analysis of the radial velocity of Rho Coronae Borealis, a yellow main sequence star and Solar analog of spectral type G0V,<ref name="Belle">Template:Cite journal</ref> around 57 light-years distant from Earth.<ref>Template:Cite journal</ref> 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.<ref name="ReffertQuirrenbach">Template:Cite journal</ref> Possible stable planetary orbits in the habitable zone were calculated for the binary star Eta Coronae Borealis,<ref>Template:Cite journal</ref> 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.<ref name=kalereta>Template:Cite web</ref> No planet has been found, but a brown dwarf companion about 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.<ref name="brown">Template:Cite journal</ref>

Deep-sky objects

File:Abell2142 chandra xray.jpg

X-ray image of galaxy cluster Abell 2142

Corona Borealis contains few galaxies observable with amateur telescopes.<ref>Template:Cite book</ref> 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.<ref name="Rumistrzewicz">Template:Cite book</ref> 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 away.Template:Efn<ref name="NED">Template:Cite web</ref> Another galaxy cluster in the constellation, RX J1532.9+3021, is approximately 3.9 billion light-years from Earth.<ref name="chandra">Template:Cite web</ref> At the cluster's center is a large elliptical galaxy containing one of the most massive and most powerful supermassive black holes yet discovered.<ref name="chandra"/> Abell 2065 is a highly concentrated galaxy cluster containing more than 400 members, the brightest of which are 16th magnitude; the cluster is more than one billion light-years from Earth.Template:Sfn 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.<ref name=postman88>Template:Cite journal</ref> Another galaxy cluster, Abell 2162, is a member of the Hercules Superclusters.<ref name=kopylova>Template:Cite journal</ref>Template:Clear

Mythology

File:Sidney Hall - Urania's Mirror - Hercules and Corona Borealis.jpg

Hercules and Corona Borealis, as depicted in Urania's Mirror (Template:Circa)

In Greek mythology, Corona Borealis was linked to the legend of Theseus and the minotaur. It was generally considered to represent a crown 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 at her marriage to Dionysus, he placed it in the heavens to commemorate their wedding.Template:Sfn An alternative 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. Template:Lang, attributed to 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 been previously hidden as yet another child of Jupiter's trysts with a mortal, in this case Semele. Its proximity to the constellations Hercules (which Template:Lang reports was once attributed to Theseus, among others) and Lyra (Theseus' lyre in one account) could indicate that the three constellations were invented as a group.<ref>Template:Cite book</ref> Corona Borealis was one of the 48 constellations mentioned in the Almagest of classical astronomer Ptolemy.<ref name=wagman/>

In Mesopotamia, Corona Borealis was associated with the goddess Nanaya.<ref>M. Stol, Nanaja [in:] Reallexikon der Assyriologie und vorderasiatischen Archäologie vol 9, 1998, p. 147</ref>

In Welsh mythology, it was called Caer Arianrhod, "the Castle of the Silver Circle", and was the heavenly abode of the Lady Arianrhod.<ref>Template:Cite book</ref> To the ancient Balts, Corona Borealis was known as Darželis, the "flower garden".<ref>Template:Cite journal</ref>

The Arabs called the constellation Alphecca (a name later given to Alpha Coronae Borealis), which means "separated" or "broken up" (Template:Lang Template:Transliteration), a reference to the resemblance of the stars of Corona Borealis to a loose string of jewels.<ref>Template:Cite book</ref> This was also interpreted as a broken dish.<ref name="Raymo">Template:Cite book</ref> Among the Bedouins, the constellation was known as Template:Transliteration (Template:Lang), or "the dish/bowl of the poor people".<ref>Template:Cite book</ref>

The Skidi people of Native Americans saw the stars of Corona Borealis representing a council of stars whose chief was Polaris.<ref name="hall">Template:Cite journal</ref> 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.<ref>Template:Cite book</ref> The Shawnee people saw the stars as the Heavenly Sisters, who descended from the sky every night to dance on earth. Alphecca signifies the youngest and most comely sister, who was seized by a hunter who transformed into a field mouse to get close to her. They married though she later returned to the sky, with her heartbroken husband and son following later.<ref name="Raymo"/> The Mi'kmaq of eastern Canada saw Corona Borealis as Mskegwǒm, the den of the celestial bear (Alpha, Beta, Gamma and Delta Ursae Majoris).<ref>Template:Cite journal</ref>

Polynesian peoples often recognized Corona Borealis; the people of the Tuamotus named it Na Kaua-ki-tokerau and probably Te Hetu. The constellation was likely called Kaua-mea in Hawaii, Rangawhenua in New Zealand, and Te Wale-o-Awitu in the Cook Islands atoll of Pukapuka. Its name in Tonga was uncertain; it was either called Ao-o-Uvea or Kau-kupenga.<ref>Template:Cite book</ref>

In Australian Aboriginal astronomy, the constellation is called womera ("the boomerang") due to the shape of the stars.<ref>Template:Cite book</ref> 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.<ref name="ridley1875">Template:Cite book</ref> 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.<ref>Template:Cite book</ref>

Later references

Corona Borealis was renamed Corona Firmiana in honour of the Archbishop of Salzburg in the 1730 Atlas Mercurii Philosophicii Firmamentum Firminianum Descriptionem by Corbinianus Thomas, but this was not taken up by subsequent cartographers.<ref>Template:Cite book</ref> The constellation 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 short story.<ref>Template:Cite book</ref> Finnish band Cadacross released an album titled Corona Borealis in 2002.<ref>Template:Cite web</ref>