21 lines
5.0 KiB
Markdown
21 lines
5.0 KiB
Markdown
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name: Sagittarius
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created_by: skybber
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created_date: 2021-07-22 18:37:57.308352
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updated_by: skybber
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updated_date: 2022-02-11 18:39:22.876582
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---
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A large and very rich in bright objects zodiac constellation in the southern sky between Scorpio and Capricornus. Especially in North America, the striking pattern of bright stars, none of which exceeds 1st magnitude, has been named "Teapot" after its shape. It consists of a group of eight stars, γ, δ, ε, ζ, τ, ρ, π and λ Sagittario. The Sun passes through Sagittarius from December 18 to January 19, at the time of the winter solstice. The best time to observe the objects of this constellation is therefore in summer. However, it lies far enough south that its southernmost parts do not come into view for us, and the very centre of the Galaxy is only a few degrees above the horizon, so we are deprived of some fantastic views.
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The most important fact about the constellation Sagittarius is that in that direction, at a distance of 27,000 light-years, lies the core of our Galaxy, and in front of it, one of its spiral arms, the Arm of Sagittarius. The exact position of the centre is identified with the position of the Sagittarius A radio source and is given by a rectangular value of 17h 42.5min and a declination of -28˚ 59'. In the sky, this point is about 4 degrees WNW of the star γ Sagitttario. However, we will never see it within the distance of the galactic centre because it is obscured by the huge clouds of interstellar dust lying along the spiral plane of our Galaxy. Astronomers have estimated that stars in the region near the galactic centre are dimmed by up to 27 magnitudes! The brightest visually visible star clouds are therefore located a little further north and east, and represent a kind of cross-section of dark dust clouds.
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This is exemplified by the Milky Way's most extensive and brightest cloud ever, the Large Cloud in Sagittarius, visible in part on clear, clear nights perhaps even from our latitudes. It is part of the central bulge and is practically the most distant structure in our Galaxy that can be seen without a telescope. The faintest stars of the cloud lie about 20,000 light years away. In the sky, the cloud has an elongated shape with an average width of about three and a half degrees and a length like the conjunction of the stars μ Sagittario and λ Scorpii. While it is always a bit of an exaggeration to talk about distinct colours, note that it has a yellowish tint. To the west of the Great Cloud there is a dark band with intricate edges about one and a half degrees wide, extending from the boundary with the constellation of the Serpent to the tail of Scorpius. In some places its boundary is sharply defined; in others it passes smoothly into its surroundings, and the exact limits are difficult to determine.
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In the immediate vicinity of the Great Cloud two conspicuous oval nebulae are visible to the naked eye - the Laguna and Trifid nebulae. One of the most interesting places in the entire Milky Way is hidden behind a very distinctive brightening in the form of the Small Cloud in Sagittarius M24. It lies in a milky region about five degrees in width, running north from the Large Cloud towards the Shield, which merges smoothly into its surroundings on the west side and contains several small dark bays on the east. M24 represents only a very small part of the structure of the spiral arm of the Ruler, which, together with the Sagittarius arm, forms the two main spiral arms of our Galaxy. It starts near the galactic center about 10,000 light-years from the Sun, rotates around, disappears behind it, and only appears after the Perseus arm.
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M24 is more than easy to find in the sky about two degrees north of the star μ Sagittario because it is a very conspicuous object. In the binocular it is seen as a hazy white region of uniform brightness and 1x2 degrees in size, from which a few faint stars stand out. It is most conspicuous and contrasted in its eastern part, and best delineated along the southeast, northeast, and northwest sides. In addition, a pair of textbook dark nebulae, Barnard 92 and 93, are located in close proximity. South of M24 lies a small but conspicuous dark smudge and a cluster of stars 15, 16 and 17 Sgr that cannot be individually resolved. The environment is finally completed by another blurred circular spot half a degree in diameter, the open cluster M25 (4.6mag).
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The arm of Sagittarius continues southward through the constellations of Sagittarius and Scorpius to the constellation of the Ship's Keel. Most of the famous and beautiful nebulae or star clusters that can be seen in this part of the sky are therefore located in this arm. It includes a large molecular cloud, on the edges of which we observe the well-known emission nebulae M8, M16, M17 or M20, the open clusters M18, M21 or the association around NGC 6231. It also includes, for example, the star cluster NGC 6193 from the Altar, the nebula Eta Carinae from the constellation of the Ship's Keel or the Jewel Box in the Southern Cross.
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