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Sagittarius (constellation)

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Sagittarius
Constellation
Sagittarius IAU
List of stars in Sagittarius
Abbreviation Sgr
Genitive Sagittarii
Pronunciation /ˌsæɨˈtɛəriəs/, genitive /ˌsæɨˈtɛəri./
Symbolism the Archer
Right ascension 19 h
Declination −25°
Quadrant SQ4
Area 867 sq. deg. (15th)
Main stars 12, 8
Bayer/Flamsteed
stars
68
Stars with planets 25
Stars brighter than 3.00m 7
Stars within 10.00 pc (32.62 ly) 2
Brightest star ε Sgr (Kaus Australis) (1.79m)
Nearest star Ross 154
(9.69 ly, 2.97 pc)
Messier objects 15
Bordering
constellations
Template:Plainlist
Visible at latitudes between +55° and −90°.
Best visible at 21:00 (9 p.m.) during the month of August.

Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is Sagittarius (Unicode U+2650 ♐), a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow. It lies between Ophiuchus to the west and Capricornus to the east.

Visualizations

SagittariusCC

The constellation Sagittarius as it can be seen with the naked eye.

The constellation's brighter stars (from left to right on the map: τ, ζ, σ, φ, λ, ε, δ, and γ2 Sagittarii) form an easily recognizable asterism known as 'the Teapot'. The stars δ Sgr (Kaus Media), ε Sgr (Kaus Australis), ζ Sgr (Ascella), and φ Sgr form the body of the pot; λ Sgr (Kaus Borealis) is the point of the lid; γ2 Sgr (Alnasl) is the tip of the spout; and σ Sgr (Nunki) and τ Sgr the handle.[1][2]

The constellation as a whole is often depicted as having the rough appearance of a stick-figure archer drawing its bow, with the fainter stars providing its horse body.

Notable features

Stars

Sagittarius-teapot-asterism

The Teapot asterism in Sagittarius.

α Sgr (Rukbat) is not the brightest star of the constellation, having a magnitude of only 3.96 (not shown on the main map as it is located below the map's southeastern corner, north is up).

Deep-sky objects

Sagittarius constellation detail long exposure

An image showing Sagittarius.

The Milky Way is at its densest near Sagittarius, as this is where the galactic center lies. Consequently, Sagittarius contains many star clusters and nebulae. One of the brightest of the star clusters is Messier 55, about 7.5° west of δ Sgr.

The constellation contains nebulae such as the Lagoon Nebula (Messier 8), near λ Sagittarii, the Omega Nebula (Messier 17), also known as the Swan or Horseshoe Nebula, near the border with Scutum; and the Trifid Nebula (Messier 20), a large nebula containing some very young, hot stars. The grouping of the Lagoon Nebula, the Trifid Nebula, and NGC 6559 is often called the Sagittarius triplet.

The Omega Nebula (M17, the Loon Nebula, The Swan Nebula, or the Horseshoe Nebula) is a fairly bright nebula in Sagittarius; it has an integrated magnitude of 6.0 and is 4890 light-years from Earth. It was discovered in 1746 by Philippe Loys de Chésaux; observers since him have differed greatly in how they view the nebula, hence its myriad of names. Most often viewed as a checkmark, it was seen as a swan by George Chambers in 1889, a loon by Roy Bishop, and as a curl of smoke by Camille Flammarion.[3]

Messier 54 HST

Messier 54 was the first globular cluster found that is outside the Milky Way.[4]

The Lagoon Nebula (M8) is an emission nebula that is located 5,000 light-years from Earth and measures 140 light-years by 60 light-years (1.5°). Though it appears grey in telescopes to the unaided eye, long-exposure photographs reveal its pink hue, common to emission nebulae.[5] It is fairly bright, with an integrated magnitude of 3.0.[6] The Lagoon Nebula was discovered independently by John Flamsteed in 1680,[7] Guillaume Le Gentil in 1747,[5] and Charles Messier in 1764.[7] The central area of the Lagoon Nebula is also known as the Hourglass Nebula, so named for its distinctive shape. The Hourglass Nebula has its shape because of matter propelled by Herschel 36. The Lagoon Nebula also features three dark nebulae catalogued in Barnard's Catalog.[5] The Lagoon Nebula was instrumental in the discovery of Bok globules, as Bart Bok studied prints of the nebula intensively in 1947. Approximately 17,000 Bok globules were discovered in the nebula nine years later as a part of the Palomar Sky Survey; studies later showed that Bok's hypothesis that the globules held protostars was correct.[8]

The Trifid Nebula (M20, NGC 6514) is an emission nebula in Sagittarius that lies less than two degrees from the Lagoon Nebula. Discovered by French comet-hunter Charles Messier, it is located between 2,000 and 9,000 light-years from Earth and has a diameter of approximately 50 light-years. The outside of the Trifid Nebula is a bluish reflection nebula; the interior is pink with two dark bands that divide it into three areas, sometimes called "lobes". Hydrogen in the nebula is ionized, creating its characteristic color, by a central triple star, which formed in the intersection of the two dark bands.[5] M20 is associated with a cluster that has a magnitude of 6.3.[9]

M24, also called the Small Sagittarius Star Cloud, is a star cluster with an approximate magnitude of 3. About 9400 light-years away, it has a diameter of approximately 330 light-years. Embedded in M24 is NGC 6603, a smaller star cluster that is very dense. NGC 6567, a dim planetary nebula, and Barnard 92, a Bok globule, are also nearby.[10]

NGC 6445 is a planetary nebula with an approximate magnitude of 11. A large nebula at over one arcminute in diameter, it appears very close to the globular cluster NGC 6440.[11]

NGC 6638 is a dimmer globular at magnitude 9.2, though it is more distant than M71 at a distance of 26,000 light-years. It is a Shapley class VI cluster; the classification means that it has intermediate concentration at its core. It is approximately a degree away from the brighter globulars M22 and M28; NGC 6638 is southeast and southwest of the clusters respectively.[12]

In 1999 a violent outburst at V4641 Sgr was thought to have revealed the location of the closest known black hole to Earth,[13] but later investigation increased its estimated distance by a factor of 15.[14] The complex radio source Sagittarius A is also here. Astronomers believe that one of its components, known as Sagittarius A*, is associated with a supermassive black hole at the center of the galaxy, with a mass of 2.6 million solar masses.[15] The Sagittarius Dwarf Elliptical Galaxy is located just outside the Milky Way.

Baade's Window is an area with very little obscuring dust that shows objects closer to the Milky Way's center than would normally be visible. NGC 6522, magnitude 8.6, and NGC 6528, magnitude 9.5, are both globular clusters visible through Baade's Window. 20,000 and 24,000 light-years from Earth, with Shapley classes of VI and V respectively, both are moderately concentrated at their cores. NGC 6528 is closer to the galactic core at an approximate distance of 2,000 light-years.[16]

Mythology

Template:Refimprove section

Sagittariusurania

Sagittarius as depicted in Urania's Mirror, a set of constellation cards published in London c.1825.

The Babylonians identified Sagittarius as the god Nerigal or Nergal, a strange centaur-like creature firing an arrow from a bow.[17] It is generally depicted with wings, with two heads, one panther head and one human head, as well as a scorpion's stinger raised above its more conventional horse's tail. The Sumerian name Pabilsag is composed of two elements – Pabil, meaning 'elder paternal kinsman' and Sag, meaning 'chief, head'. The name may thus be translated as the 'Forefather' or 'Chief Ancestor'.[18] The figure is reminiscent of modern depictions of Sagittarius.

In Greek mythology, Sagittarius is identified as a centaur: half human, half horse. In some legends, the Centaur Chiron was the son of Philyra and Saturn, who was said to have changed himself into a horse to escape his jealous wife, Rhea. Chiron was eventually immortalised in the constellation of Centaurus or in some version, Sagittarius.

The arrow of this constellation points towards the star Antares, the "heart of the scorpion."

Astrology

As of 2002, the Sun appears in the constellation Sagittarius from 18 December to 18 January. In tropical astrology, the Sun is considered to be in the sign Sagittarius from 22 November to 21 December, and in sidereal astrology, from 16 December to 14 January.

Notes

  1. "Sagittarius". deepsky.astroinfo.org. http://deepsky.astroinfo.org/Sgr/. Retrieved 30 August 2008. 
  2. skywatchers[dead link]
  3. Levy 2005, p. 103.
  4. "First Globular Cluster Outside the Milky Way". ESA/Hubble Photo of the Week. http://spacetelescope.org/images/potw1145a/. Retrieved 9 November 2011. 
  5. 5.0 5.1 5.2 5.3 Wilkins, Jamie; Dunn, Robert (2006). 300 Astronomical Objects: A Visual Reference to the Universe (1st ed.). Buffalo, New York: Firefly Books. ISBN 978-1-55407-175-3. 
  6. Levy 2005, p. 108.
  7. 7.0 7.1 Levy 2005, p. 109.
  8. Levy 2005, pp. 111–112.
  9. Levy 2005, p. 114.
  10. Levy 2005, pp. 143–144.
  11. Levy 2005, p. 133.
  12. Levy 2005, pp. 167–168.
  13. "Dramatic Outburst Reveals Nearest Black Hole". National Radio Astronomy Observatory. http://www.nrao.edu/pr/2000/v4641/. Retrieved 30 August 2008. 
  14. A Black Hole in the Superluminal Source SAX J1819.3-2525 (V4641 SGR), 2001: "Finally, we find a distance in the range 7.40 ≤ d ≤ 12.31 kpc (90% confidence), which is at least a factor of ≈ 15 larger than the initially assumed distance of ≈ 500 pc."
  15. Levy 2005, p. 143.
  16. Levy 2005, pp. 174–175.
  17. Page 15 of Origins of the ancient constellations: I. The Mesopotamian traditions, by J. H. Rogers
  18. White, Gavin (2008). Babylonian Star-lore. Solaria Pubs. p. 155. 

References

External links

Coordinates: Celestia 19h 00m 00s, −25° 00′ 00″



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