Corona Borealis is a small constellation in the Northern Celestial Hemisphere. Its brightest stars form a semicircular arc. Its Latin name, inspired by its shape, means ‘northern crown.’ Covering 179 square degrees and hence 0.433% of the sky, Corona Borealis ranks 73rd of the 88 modern constellations by area. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 50°S. It is bordered by Boötes to the north and west, Serpens Caput to the south, and Hercules to the east. In the equatorial coordinate system, the right ascension coordinates of the constellation lie between 15h 16.0m and 16h 25.1m, while the declination coordinates are between 39.71° and 25.54°. It has a counterpart- Corona Australis- in the Southern Celestial Hemisphere.
Corona Borealis, a jewelled crown, shown in the Atlas Coelestis of John Flamsteed (1729)
[http://www.ianridpath.com/startales/coronaborealis.htm]
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. 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 been 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. Corona Borealis was one of the 48 constellations mentioned in the Almagest of classical astronomer Ptolemy.
The Arabs called the constellation Alphecca (a name later given to Alpha Coronae Borealis), which means ‘separated’ or ‘broken up,’ a reference to the resemblance of the stars of Corona Borealis to a loose string of jewels. This was also interpreted as a broken dish. Among the Bedouins, the constellation was known as ‘qaṣʿat al-masākīn,’ 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 symbolized the smoke-hole over a fireplace, which conveyed their messages to the gods, as well as how chiefs should come together to consider matters of importance. 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. 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; 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.
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.
[http://www.thespacewriter.com/2003_04_01_thespacewriter_archive.html]
[https://constellations-urs.wikispaces.com/Science+-+Corona+Borealis]
Within the constellation’s borders, there are 37 stars brighter than or equal to apparent magnitude 6.5. 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.
Also called Alphekka or Gemma, Alpha Coronae Borealis appears as a blue-white star of magnitude 2.2. In fact, it is an Algol-type eclipsing binary that varies by 0.1 magnitude with a period of 17.4 days. The primary is a white main-sequence star that is 2.91 times the mass of the Sun and 57 times as luminous, 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 that is a little smaller (0.9 times) the diameter of the Sun. Lying 75 light-years 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 about 112 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 star, pulsating with a period of 16.2 minutes. With a surface temperature of around 7980 K, it has around 2.1 solar masses, 2.6 solar radii, and luminosity 25.3 suns. The smaller star has a surface temperature of around 6750 K, and around 1.4 solar masses, 1.56 solar radii, and luminosity between 4 and 5 times that of the sun.
Near Nusakan is Theta Coronae Borealis, a binary system that shines with a combined magnitude of 4.13 located about 380 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. 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 components are main sequence stars.
Located about 170 light-years away, 4.06-magnitude Delta Coronae Borealis is a yellow giant star that is around 2.4 solar masses and has swollen to 7.4 solar radii. 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, having finished burning core hydrogen, has just begun burning hydrogen in a shell that surrounds the core.
The spectrum of Epsilon Coronae Borealis was analyzed 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 solar masses orange giant of spectral type K2III that has swollen to 21 solar radii and 151 solar luminosities.
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 T Pyxidis 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 white dwarf secondary, the two stars orbiting each other over a period of 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.
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 102,831 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.
Sigma Coronae Borealis is a true multiple star system divisible by small amateur telescopes. 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.
ADS 9731 is an even rarer multiple system in the constellation, composed of six stars, two of which are spectroscopic binaries.
Extrasolar planets have been confirmed in five star systems.
NGC 6085 (above, left) and NGC 6086
[https://en.wikipedia.org/wiki/NGC_6085]
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 away:
Abell 2142: Clash of the Galaxy Clusters
Over the course of billions of years, whole clusters of galaxies merge. Above is an X-ray image of Abell 2142, the result of the collision of two huge clusters of galaxies, and one of the most massive objects known in the universe. This false-color image shows a concentration of gas 50 million degrees hot near the center of the resulting cluster. Oddly, it is the relative coldness of the gas that makes this situation particularly interesting. The center of Abell 2142 is surrounded by gas fully twice as hot, a temperature thought to have been created by energy released during the colossal collision. Still, since we can only see a snapshot in time, much remains unknown about how clusters of galaxies form and coalesce.
[http://apod.nasa.gov/apod/ap000306.html]
Another galaxy cluster in the constellation, RX J1532.9+3021, is approximately 3.9 billion light-years from Earth. At the cluster’s center is a large elliptical galaxy containing one of the most massive and most powerful supermassive black holes yet discovered:
RX J1532.9+3021: Extreme Power of Black Hole Revealed
Astronomers have used NASA’s Chandra X-ray Observatory and a suite of other telescopes to reveal one of the most powerful black holes known. The black hole has created enormous structures in the hot gas surrounding it and prevented trillions of stars from forming.
The black hole is in a galaxy cluster named RX J1532.9+3021 (RX J1532 for short), located about 3.9 billion light years from Earth. The image here is a composite of X-ray data from Chandra revealing hot gas in the cluster in purple and optical data from the Hubble Space Telescope showing galaxies in yellow. The cluster is very bright in X-rays implying that it is extremely massive, with a mass about a quadrillion- a thousand trillion- times that of the sun. At the center of the cluster is a large elliptical galaxy containing the supermassive black hole.
The large amount of hot gas near the center of the cluster presents a puzzle. Hot gas glowing with X-rays should cool, and the dense gas in the center of the cluster should cool the fastest. The pressure in this cool central gas is then expected to drop, causing gas further out to sink in towards the galaxy, forming trillions of stars along the way. However, astronomers have found no such evidence for this burst of stars forming at the center of this cluster.
This problem has been noted in many galaxy clusters but RX J1532 is an extreme case, where the cooling of gas should be especially dramatic because of the high density of gas near the center. Out of the thousands of clusters known to date, less than a dozen are as extreme as RX J1532. The Phoenix Cluster is the most extreme, where, conversely, large numbers of stars have been observed to be forming.
What is stopping large numbers of stars from forming in RX J1532? Images from the Chandra X-ray Observatory and the NSF’s Karl G. Jansky Very Large Array (VLA) have provided an answer to this question. The X-ray image shows two large cavities in the hot gas on either side of the central galaxy. The Chandra image has been specially processed to emphasize the cavities. Both cavities are aligned with jets seen in radio images from the VLA. The location of the supermassive black hole between the cavities is strong evidence that the supersonic jets generated by the black hole have drilled into the hot gas and pushed it aside, forming the cavities.
Shock fronts- akin to sonic booms- caused by the expanding cavities and the release of energy by sound waves reverberating through the hot gas provide a source of heat that prevents most of the gas from cooling and forming new stars.
The cavities are each about 100,000 light years across, roughly equal to the width of the Milky Way galaxy. The power needed to generate them is among the largest known in galaxy clusters. For example, the power is almost 10 times greater than required to create the well-known cavities in Perseus.
Although the energy to power the jets must have been generated by matter falling toward the black hole, no X-ray emission has been detected from in-falling material. This result can be explained if the black hole is ‘ultra-massive’ rather than supermassive, with a mass more than 10 billion times that of the sun. Such a black hole should be able to produce powerful jets without consuming large amounts of mass, resulting in very little radiation from material falling inwards.
Another possible explanation is that the black hole has a mass only about a billion times that of the sun but is spinning extremely rapidly. Such a black hole can produce more powerful jets than a slowly spinning black hole when consuming the same amount of matter. In both explanations the black hole is extremely massive.
A more distant cavity is also seen at a different angle with respect to the jets, along a north-south direction. This cavity is likely to have been produced by a jet from a much older outburst from the black hole. This raises the question of why this cavity is no longer aligned with the jets. There are two possible explanations. Either large-scale motion of the gas in the cluster has pushed it to the side or the black hole is precessing, that is, wobbling like a spinning top.
[http://chandra.harvard.edu/photo/2014/rxj1532/index.html]
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. 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:
A2065 - The Corona Borealis Cluster
Although it is one billion light years from us, the Corona Borealis supercluster is famous because it is a fairly obvious concentration of clusters of galaxies. Above is a picture of the center of the A2065 cluster. This cluster is often called the Corona Borealis cluster. It is the richest cluster of galaxies in the Corona Borealis supercluster.
The Corona Borealis cluster (A2065) was discovered by Edwin Hubble in the 1930’s. In 1936, Milton Humason measured the redshift of one of the galaxies (PGC 54876) in the cluster as part of a project to demonstrate that velocity is proportional to distance for a large number of distant clusters. This was powerful evidence that the universe is expanding.
The possibility that there might be a supercluster in Corona Borealis was first suggested in the 1950’s. George Abell examined his own catalogue of Rich Clusters of Galaxies (published in 1958) and in a paper published in 1961, he included the Corona Borealis supercluster as supercluster number 13 in a list of 17 possible superclusters.
The first proper study of the Corona Borealis supercluster was published by M Postman, M Geller and J Huchra in 1988. They studied the motion of seven of the clusters in the supercluster, and they estimated the mass of the supercluster.
More recently, in 1997 and 1998, T Small, C Ma, W Sargent and D Hamilton, published three papers about this supercluster (1, 2, 3). They noticed the presence of another supercluster behind the Corona Borealis supercluster (associated with A2034, A2049, A2062, A2069 and A2083) at a distance of 1.5 billion light years (redshift 0.113). They also believe that the clusters at the center of the Corona Borealis supercluster are collapsing together and will eventually form one big cluster.
[http://www.atlasoftheuniverse.com/superc/cbo.html]
GRB 140903A: Chandra Finds Evidence for Violent Stellar Merger
Astronomers have the strongest evidence to date that violent stellar mergers produce pencil-thin jets. This means that a majority of these events will not be detected because they will not be pointed where telescopes can detect them. This result has implications for estimating the number of such mergers that may be detected with gravitational wave observatories. Chandra was used to study X-ray emission from the gamma-ray burst, allowing the width of the jet to be estimated.
Gamma-ray bursts, or GRBs, are some of the most violent and energetic events in the Universe. Although these events are the most luminous explosions in the universe, a new study using NASA’s Chandra X-ray Observatory, NASA’s Swift satellite and other telescopes suggests that scientists may be missing a majority of these powerful cosmic detonations.
Astronomers think that some GRBs are the product of the collision and merger of two neutron stars or a neutron star and a black hole. The new research gives the best evidence to date that such collisions will generate a very narrow beam, or jet, of gamma rays. If such a narrow jet is not pointed toward Earth, the GRB produced by the collision will not be detected.
Collisions between two neutron stars or a neutron star and black hole are expected to be strong sources of gravitational waves that could be detected whether or not the jet is pointed towards the Earth. Therefore, this result has important implications for the number of events that will be detectable by the Laser Interferometry Gravitational-Wave Observatory (LIGO) and other gravitational wave observatories.
On September 3, 2014, NASA’s Swift observatory picked up a GRB - dubbed GRB 140903A due to the date it was detected. Scientists used optical observations with the Gemini Observatory telescope in Hawaii to determine that GRB 140903A was located in a galaxy about 3.9 billion light years away, relatively nearby for a GRB.
The large panel in the graphic is an illustration showing the aftermath of a neutron star merger, including the generation of a GRB. In the center is a compact object- either a black hole or a massive neutron star- and in red is a disk of material left over from the merger, containing material falling towards the compact object. Energy from this in-falling material drives the GRB jet shown in yellow. In orange is a wind of particles blowing away from the disk and in blue is material ejected from the compact object and expanding at very high speeds of about one tenth the speed of light.
The image on the left of the two smaller panels shows an optical view from the Discovery Channel Telescope (DCT) with GRB 140903A in the middle of the square and a close-up X-ray view from Chandra on the right. The bright star in the optical image is unrelated to the GRB.
The gamma-ray blast lasted less than two seconds. This placed it into the ‘short GRB’ category, which astronomers think are the output from neutron star-neutron star or black hole-neutron star collisions eventually forming either a black hole or a neutron star with a strong magnetic field. (The scientific consensus is that GRBs that last longer than two seconds result from the collapse of a massive star.)
About three weeks after the Swift discovery of GRB 140903A, a team of researchers led by Eleonora Troja of the University of Maryland, College Park (UMD), observed the aftermath of the GRB in X-rays with Chandra. Chandra observations of how the X-ray emission from this GRB decreases over time provide important information about the properties of the jet.
Specifically, the researchers found that the jet is beamed into an angle of only about five degrees based on the X-ray observations, plus optical observations with the Gemini Observatory and the DCT and radio observations with the National Science Foundation’s Karl G. Jansky Very Large Array. This is roughly equivalent to a circle with the diameter of your three middle fingers held at arms length. This means that astronomers are detecting only about 0.4% of this type of GRB when it goes off, since in most cases the jet will not be pointed directly at us.
Previous studies by other astronomers had suggested that these mergers could produce narrow jets. However, the evidence in those cases was not as strong because the rapid decline in light was not observed at multiple wavelengths, allowing for explanations not involving jets.
Several pieces of evidence link this event to the merger of two neutron stars, or between a neutron star and black hole. These include the properties of the gamma ray emission, the old age and the low rate of stars forming in the GRB’s host galaxy and the lack of a bright supernova. In some previous cases strong evidence for this connection was not found.
New studies have suggested that such mergers could be the production site of elements heavier than iron, such as gold. Therefore, the rate of these events is also important to estimate the total amount of heavy elements produced by these mergers and compare it with the amounts observed in the Milky Way galaxy.
[http://chandra.harvard.edu/photo/2016/grb/index.html]
Perseid Below
Denizens of planet Earth watched this year’s Perseid meteor shower by looking up into the moonlit night sky. But this remarkable view captured by astronaut Ron Garan looks down on a Perseid meteor. From Garan’s perspective onboard the International Space Station orbiting at an altitude of about 380 kilometers, the Perseid meteors streak below, swept up dust left from comet Swift-Tuttle heated to incandescence. The glowing comet dust grains are traveling at about 60 kilometers per second through the denser atmosphere around 100 kilometers above Earth's surface. In this case, the foreshortened meteor flash is right of frame center, below the curving limb of the Earth and a layer of greenish airglow. Out of the frame, the Sun is on the horizon beyond one of the station’s solar panel arrays at the upper right. Seen above the meteor near the horizon is bright star Arcturus and a star field that includes the constellations Bootes and Corona Borealis. The image was recorded on August 13 while the space station orbited above an area of China approximately 400 kilometers to the northwest of Beijing.
[http://apod.nasa.gov/apod/ap110817.html]
[https://en.wikipedia.org/wiki/Corona_Borealis]
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