Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and is one of the 88 constellations recognized today. Covering 1121 square degrees, Pegasus is the seventh-largest of the 88 constellations. Pegasus is bordered by Andromeda to the north and east, Lacerta to the north, Cygnus to the northwest, Vulpecula, Delphinus and Equuleus to the west, Aquarius to the south and Pisces to the south and east. In the equatorial coordinate system the right ascension coordinates of the constellation lie between 21h 12.6m and 00h 14.6m, while the declination coordinates are between 2.33° and 36.61°. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 53°S.
Only the front half of the flying horse is depicted in the sky, but enough to include his wings. His body is outlined by four stars that form the Square of Pegasus (although one of these is now assigned to Andromeda). In front of Pegasus is Equuleus, the foal, whose head alone is imagined among the stars. This illustration is from Chart X of the Uranographia of Johann Bode (1801).
[http://www.ianridpath.com/startales/pegasus.htm]
The Babylonian constellation IKU (field) had four stars of which three were later part of the Greek constellation Hippos (Pegasus).
Pegasus, in Greek mythology, was a winged horse with magical powers. One myth regarding his powers says that his hooves dug out a spring, Hippocrene, which blessed those who drank its water with the ability to write poetry. Pegasus was the one who delivered Medusa’s head to Polydectes, after which he travelled to Mount Olympus in order to be the bearer of thunder and lightning for Zeus. Eventually, he became the horse to Bellerophon, who was asked to kill the Chimera and succeeded with the help of Athena and Pegasus. Despite this success, after the death of his children, Bellerophon asked Pegasus to take him to Mount Olympus. Though Pegasus agreed, he plummeted back to Earth after Zeus either threw a thunderbolt at him or made Pegasus buck him off.
In ancient Persia, Pegasus was depicted by al-Sufi as a complete horse facing east, unlike most other uranographers, who had depicted Pegasus as half of a horse, rising out of the ocean. In al-Sufi’s depiction, Pegasus’s head is made up of the stars of Lacerta the lizard. Its right foreleg is represented by β Peg and its left foreleg is represented by η Peg, μ Peg, and λ Peg; its hind legs are marked by 9 Peg. The back is represented by π Peg and μ Cyg, and the belly is represented by ι Peg and κ Peg.
In Chinese astronomy, the modern constellation of Pegasus lies in The Black Tortoise of the north, where the stars were classified in several separate asterisms of stars. Epsilon and Theta Pegasi are joined with Alpha Aquarii to form Wei, ‘rooftop,’ with Theta forming the roof apex.
In Hindu astronomy, the Great Square of Pegasus contained the 26th and 27th lunar mansions. More specifically, it represented a bedstead that was a resting place for the Moon.
For the Warrau and Arawak peoples in Guyana the stars in the Great Square, corresponding to parts of Pegasus and of Andromeda, represented a barbecue, taken up to the sky by the seven hunters of the myth of Siritjo.
[http://www.dibonsmith.com/peg_con.htm]
Pegasus is dominated by a roughly square asterism, although one of the stars, Delta Pegasi or Sirrah, is now officially considered to be Alpha Andromedae, part of Andromeda, and is more usually called ‘Alpheratz.’ Traditionally, the body of the horse consists of a quadrilateral formed by the stars α Peg, β Peg, γ Peg, and α And. The front legs of the winged horse are formed by two crooked lines of stars, one leading from η Peg to κ Peg and the other from μ Peg to 1 Pegasi. Another crooked line of stars from α Peg via θ Peg to ε Peg forms the neck and head; ε is the snout.
[http://utahsadventurefamily.com/find-pegasus-constellation/]
[https://einstein.stanford.edu/highlights/hl_061104.html]
Epsilon Pegasi is the brightest star of Pegasus. It has the traditional name Enif (EE-nif). The name Enif is derived from the Arabic word for nose, due to its position as the muzzle of Pegasus. With an apparent visual magnitude of 2.4, this is a second-magnitude star that is readily visible to the naked eye. The distance to this star is around 690 light-years (210 parsecs).
[https://en.wikipedia.org/wiki/Epsilon_Pegasi]
Beta Pegasi is a red giant star. The apparent visual magnitude of this star averages 2.42, making it the second brightest star in the constellation after Epsilon Pegasi. Its traditional name is Scheat (a name that has also been used for Delta Aquarii) probably comes from the Arabic ‘Al Sa'id’ for ‘the upper arm,’ or from Sa'd. Arabian astronomers named it ‘Mankib al Faras,’ meaning the ‘Horse’s shoulder.’ It forms the upper right corner of the Great Square of Pegasus. It is located about 196 light-years (60 parsecs) from the Earth.
[https://en.wikipedia.org/wiki/Beta_Pegasi]
Alpha Pegasi is the third brightest star in the constellation Pegasus and one of the four stars in the Great Square of Pegasus. It has the traditional name Markab (or Marchab), which comes from an Arabic word ‘markab,’ ‘the saddle of the horse.’ Markab has a stellar classification of B9 III, indicating that it is a B-type giant star that has exhausted the hydrogen at its core and has evolved beyond the main sequence. It is located about 133 ly (40.9 pc) distant from Earth.
[https://en.wikipedia.org/wiki/Alpha_Pegasi]
Gamma Pegasi is located at the southwest corner of the asterism known as the Great Square. It also has the traditional name Algenib; confusingly however, this name is also used for Alpha Persei. The average apparent visual magnitude of +2.84 puts this at fourth place among the brightest stars in the constellation. The distance to this star is roughly 390 light-years (120 parsecs) from Earth.
[https://en.wikipedia.org/wiki/Gamma_Pegasi]
51 Pegasi
[https://en.wikipedia.org/wiki/51_Pegasi]
Twelve star systems have been found to have exoplanets. 51 Pegasi was the first Sun-like star discovered to have an exoplanet companion; 51 Pegasi b (unofficially named Bellerophon) is a hot Jupiter close to its sun, completing an orbit every four days. Spectroscopic analysis of HD 209458 b, an extrasolar planet in this constellation, has provided the first evidence of atmospheric water vapor beyond the solar system, while extrasolar planets orbiting the star HR 8799 also in Pegasus are the first to be directly imaged. V391 Pegasi is a hot subdwarf star that has been found to have a planetary companion.
Deep-sky objects in Pegasus include:
M15 photographed by HST. The planetary nebula Pease 1 can be seen as a small blue object to the upper left of the core of the cluster.
Messier 15 or M15 (also designated NGC 7078) is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier’s catalogue of comet-like objects in 1764. At an estimated 12.0 billion years old, it is one of the oldest known globular clusters.
M15 is about 33,600 light-years from Earth, and 175 light years in diameter. It has an absolute magnitude of -9.2, which translates to a total luminosity of 360,000 times that of the Sun. Messier 15 is one of the most densely packed globulars known in the Milky Way galaxy. Its core has undergone a contraction known as ‘core collapse’ and it has a central density cusp with an enormous number of stars surrounding what may be a central black hole.
Home to over 100,000 stars, the cluster is notable for containing a large number of variable stars (112) and pulsars (8), including one double neutron star system, M15 C. M15 also contains Pease 1, the first planetary nebula discovered within a globular cluster in 1928. Just three others have been found in globular clusters since then.
At magnitude 6.2, M15 approaches naked eye visibility under good conditions and can be observed with binoculars or a small telescope, appearing as a fuzzy star.
[https://en.wikipedia.org/wiki/Messier_15]
Galaxies in Pegasus
This wide, sharp telescopic view reveals galaxies scattered beyond the stars of the Milky Way at the northern boundary of the high-flying constellation Pegasus. Prominent at the upper right is NGC 7331. A mere 50 million light-years away, the large spiral is one of the brighter galaxies not included in Charles Messier’s famous 18th century catalog. The disturbed looking group of galaxies at the lower left is well-known as Stephan’s Quintet. About 300 million light-years distant, the quintet dramatically illustrates a multiple galaxy collision, its powerful, ongoing interactions posed for a brief cosmic snapshot. On the sky, the quintet and NGC 7331 are separated by about half a degree.
[https://apod.nasa.gov/apod/ap161203.html]
NGC 7331 and Beyond
Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way. About 50 million light-years distant in the northern constellation Pegasus, NGC 7331 was recognized early on as a spiral nebula and is actually one of the brighter galaxies not included in Charles Messier’s famous 18th century catalog. Since the galaxy’s disk is inclined to our line-of-sight, long telescopic exposures often result in an image that evokes a strong sense of depth. The effect is further enhanced in this sharp image by galaxies that lie beyond the gorgeous island universe. The background galaxies are about one tenth the apparent size of NGC 7331 and so lie roughly ten times farther away. Their close alignment on the sky with NGC 7331 occurs just by chance. Seen here through faint foreground dust clouds lingering above the plane of Milky Way, this visual grouping of galaxies is also known as the Deer Lick Group.
[http://apod.nasa.gov/apod/ap140301.html]
Stephan’s Quintet Plus One
The first identified compact galaxy group, Stephan’s Quintet is featured in this remarkable image constructed with data drawn from Hubble Legacy Archive and the Subaru Telescope on the summit of Mauna Kea. The galaxies of the quintet are gathered near the center of the field, but really only four of the five are locked in a cosmic dance of repeated close encounters taking place some 300 million light-years away. The odd man out is easy to spot, though. The interacting galaxies, NGC 7319, 7318A, 7318B, and 7317 have a more dominant yellowish cast. They also tend to have distorted loops and tails, grown under the influence of disruptive gravitational tides. The mostly bluish galaxy, NGC 7320, is in the foreground about 40 million light-years distant, and isn’t part of the interacting group. Still, captured in this field above and to the left of Stephan’s Quintet is another galaxy, NGC 7320C, that is also 300 million light-years distant. Of course, including it would bring the four interacting galaxies back up to quintet status. Stephan’s Quintet lies within the boundaries of the high flying constellation Pegasus. At the estimated distance of the quintet’s interacting galaxies, this field of view spans over 500,000 light-years.
[http://apod.nasa.gov/apod/ap140327.html]
Einstein’s Cross is a quasar that has been lensed by a foreground galaxy. The elliptical galaxy is 400 million light-years away with a redshift of 0.0394, but the quasar is 8 billion light-years away. The lensed quasar resembles a cross because the gravitational force of the foreground galaxy on its light creates four images of the quasar:
Einstein’s Cross
An important example of the gravitational lens effect is the quadruple image shown above. This system is called the Einstein Cross, because it is such an excellent example of the phenomenon of gravitational lensing. This phenomenon was postulated by Einstein as soon as he realized that gravity would be able to bend light and thus could have lens-like effects. This system is also known as Huchra’s Lens, after its founder. The four separate images have the same redshift of Z= 1.695. This suggests that they are quasar images, and in fact multiple images of the same quasar. The image is interpreted as gravitational lensing by an almost perfectly aligned galaxy for which the red shift was measured to be Z= 0.0394. Using a Hubble constant of 71 km/s /Megaparsec as indicated by the WMAP project, the Z measurements imply a distance of 500 million light years (ly) for the lensing galaxy and 10.4 billion ly for the quasar. The quasar is over 20 times further away than the galaxy that lenses it to give the four images. Modeling suggests that the alignment of the lensing galaxy and the quasar is within 0.05 arcseconds.
[http://hyperphysics.phy-astr.gsu.edu/hbase/astro/eincros.html]
VLA J2130+12 is a black hole close to the globular cluster Messier 15:
VLA J2130+12: Clandestine black hole may represent new population
The true identity of an unusual source in the Milky Way galaxy has been revealed. This object contains a very quiet black hole, a few times the Sun’s mass, about 7,200 light years from Earth. This discovery implies that there could be a much larger number of black holes in the Galaxy that have previously been unaccounted for.
The result was made by combining data from many different telescopes that detect various forms of light, each providing key pieces of information. These telescopes included NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, NSF’s Karl G. Jansky Very Large Array (VLA), Green Bank Telescope, Arecibo Observatory, and the European Very Long Baseline Interferometry Network.
The collaborative nature of this study is depicted in this multi-panel graphic. The large panel shows a composite Chandra and optical image of the globular cluster M15 located in our galaxy, where the X-ray data are purple and the optical data are red, green and blue. The source being studied here is bright in radio waves, as shown in the close-up VLA image, but the Chandra data reveal it can only be giving off a very small amount of X-rays.
This new study indicates this source, called VLA J213002.08+120904 (VLA J2130+12 for short), contains a black hole a few times the mass of our Sun that is very slowly pulling in material from a companion star. At this paltry feeding rate, VLA J2130+12 was not previously flagged as a black hole since it lacks some of the telltale signs that black holes in binary systems typically display.
Previously, most astronomers thought that VLA J2130+12 was probably a distant galaxy. Precise measurements from the radio telescopes showed that this source was actually well within our Galaxy and about five times closer to us than M15. Hubble data identified the companion star in VLA J2130+12 having only about one-tenth to one-fifth the mass of the Sun.
The observed radio brightness and the limit on the X-ray brightness from Chandra allowed the researchers to rule out other possible interpretations, such as an ultra-cool dwarf star, a neutron star, or a white dwarf pulling material away from a companion star.
Because this study only covered a very small patch of sky, the implication is that there should be many of these quiet black holes around the Milky Way. The estimates are that tens of thousands to millions of these black holes could exist within our Galaxy, about three to thousands of times as many as previous studies have suggested.
[http://chandra.harvard.edu/photo/2016/vla/index.html]
The Eta Pegasids radiate from the area near Eta Pegasi every year on May 30.
[https://en.wikipedia.org/wiki/Pegasus_%28constellation%29]
No comments:
Post a Comment