Sagittarius constellation lies in the southern sky. It is one of the constellations of the zodiac. It represents the archer. Sagittarius is the 15th largest constellation in the sky. It occupies an area of 867 square degrees. It is located in the fourth quadrant of the southern hemisphere (SQ4) and can be seen at latitudes between +55° and -90°. The neighboring constellations are Aquila, Capricornus, Corona Australis, Indus, Microscopium, Ophiuchus, Scutum, Scorpius, Serpens Cauda and Telescopium.
[http://www.constellation-guide.com/constellation-list/sagittarius-constellation/]
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.
Sagittarius, the centaur-like archer, shown drawing his bow on Chart XV of the Uranographia of Johann Bode (1801).
[http://www.ianridpath.com/startales/sagittarius.htm]
The Babylonians identified Sagittarius as the god Nergal, a strange centaur-like creature firing an arrow from a bow. 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.’ The figure is reminiscent of modern depictions of Sagittarius.
In Greek mythology, Sagittarius is usually identified as a centaur: half human, half horse. However, perhaps due to the Greek’s adoption of the Sumerian constellation, some confusion surrounds the identity of the archer. Some identify Sagittarius as the centaur Chiron, the son of Philyra and Saturn and tutor to Jason, who was said to have changed himself into a horse to escape his jealous wife, Rhea. However, Chiron is in fact represented by the constellation Centaurus, the other heavenly centaur. An alternative tradition is that Chiron merely invented the constellation Sagittarius to help in guiding the Argonauts in their quest for the Golden Fleece.
A competing mythological tradition, as espoused by Eratosthenes, identified the Archer not as a centaur but as the satyr Crotus, son of Pan, who Greeks credited with the invention of archery. According to myth, Crotus often went hunting on horseback and lived among the Muses, who requested that Zeus place him in the sky, where he is seen demonstrating archery.
The arrow of this constellation points towards the star Antares, the ‘heart of the scorpion,’ and Sagittarius stands poised to attack should Scorpius ever attack the nearby Hercules, or to avenge Scorpius’s slaying of Orion.
The ‘Teapot’ asterism is in Sagittarius. The Milky Way is the ‘steam’ coming from the spout.
As seen from the northern hemisphere, the constellation’s brighter stars 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. These same stars originally formed the bow and arrow of Sagittarius.
To complete the teapot metaphor, under good conditions, a particularly dense area of the Milky Way can be seen rising in a north-westerly arc above the spout, like a puff of steam rising from a boiling kettle.
Sagittarius the Archer, the most Southerly of the zodiac constellations, photographed close to meridian transit from a latitude of 51º.5 North. This is about as high in the sky as Sagittarius gets when seen from Northern Europe, Southern Canada and Southern Russia; observers situated South of latitude 51º.5 North will see more of the constellation, whilst those situated to the North of this latitude will see less.
[http://www.aenigmatis.com/astronomy/find/sagittarius.htm]
Riding low in the summer sky is the constellation Sagittarius, looking like a teapot and containing some of the finest deep sky objects.
[https://www.space.com/21653-sagittarius-constellation.html]
Kaus Australis (Epsilon Sagittarii)
[http://www.daviddarling.info/encyclopedia/K/Kaus_Australis.html]
Epsilon Sagittarii (Kaus Australis) is a binary star system. The apparent visual magnitude of +1.85 makes it the brightest star in the constellation. The distance to this star is around 143 light-years (44 parsecs). The traditional name of this star ‘Kaus Australis,’ stems from Arabic ‘qaws,’ ‘bow’ and Latin ‘austrālis,’ ‘southern.’
[https://en.wikipedia.org/wiki/Epsilon_Sagittarii]
[https://www.universeguide.com/star/nunki]
Sigma Sagittarii (Nunki) is the second brightest star in the constellation Sagittarius. Nunki has an apparent magnitude of +2.05, making it readily visible to the naked eye. The distance to this star is about 230 light-years (70 parsecs) from Earth. The total luminosity of σ Sgr is 3300 times that of the Sun while it has a surface temperature of 18,890 K. Its modern name Nunki is an Assyrian or Babylonian name recovered by archaeologists and made public by R. H. Allen. Sigma Sagittarii has a 10th magnitude optical companion located 5.2 arcminutes away.
Because it is close to the ecliptic, Nunki can be occulted by the Moon and very rarely by planets. The last occultation of Nunki by a planet took place on November 17, 1981, when it was occulted by Venus. Furthermore, Nunki is the brightest star that can be principally occulted by an exterior planet between 5000 BCE and 5000 AD. However, only Mars can do this, and only rarely; the last time was on September 3, 423.
[https://en.wikipedia.org/wiki/Sigma_Sagittarii]
Zeta Sagittarii
[https://it.wikipedia.org/wiki/Zeta_Sagittarii]
Zeta Sagittarii (Ascella) is the third brightest star system in the constellation Sagittarius, with an apparent visual magnitude of +2.59. Its traditional name Ascella comes from a Latin word meaning armpit. The distance to Ascella is about 88 ly (27 pc). Ascella is a binary star system, with the two components orbiting each other over a period of 21 years at an eccentricity of 0.211. The combined mass of the system is about 5 times that of the Sun and their blended stellar classification is A2.5 Va.
[https://en.wikipedia.org/wiki/Zeta_Sagittarii]
Representation of the Milky Way with Kaus Media in the center
[https://es.wikipedia.org/wiki/Kaus_Medius]
Delta Sagittarii (Kaus Media) is a double star in the southern zodiac constellation of Sagittarius. It has the traditional names Kaus Media, Kaus Meridionalis, and Media, which come from the Arabic ‘qaws,’ ‘bow’ and Latin media ‘middle.’ The apparent visual magnitude of this star is +2.70, making it easily visible to the naked eye. It is roughly 348 light-years (107 parsecs) distant from Earth. This is a giant star with a stellar classification K3 III. It has three dim companions
[https://en.wikipedia.org/wiki/Delta_Sagittarii]
Kaus Borealis Star, Lambda Sagittarii (top)
[https://astrologyking.com/kaus-borealis-star/]
Lambda Sagittarii (Kaus Borealis) marks the top of the Archer’s bow, whence its traditional name Kaus Borealis (‘Northern bow’). With an apparent visual magnitude of +2.82, this is the fifth brighter member of the constellation and it is readily visible to the naked eye. It is located at a distance of 78.2 light-years (24.0 parsecs) from Earth. Being near the ecliptic, it is sometimes occulted by the Moon and, more rarely, by a planet. The last planet to pass in front of it was Venus, on 19 November 1984. The previous occasion was on 5 December 1865, when it was occulted by the planet Mercury.
[https://en.wikipedia.org/wiki/Lambda_Sagittarii]
False-color image of the Pistol Star and Pistol Nebula
The Pistol Star is a blue hypergiant star, one of the most luminous known in the Milky Way. It is one of many massive young stars in the Quintuplet cluster in the Galactic Center region. The star owes its name to the shape of the Pistol Nebula, which it illuminates. It is located approximately 25,000 light years from Earth in the direction of Sagittarius. It would be visible to the naked eye as a fourth magnitude star if it were not for the interstellar dust that completely hides it from view in visible light.
The star is thought to have ejected almost 10 solar masses of material in giant outbursts perhaps 4,000 to 6,000 years ago (as observed from Earth). Its stellar wind is over 10 billion times stronger than the Sun’s. Its exact age and future are not known, but it is expected to end in a brilliant supernova or hypernova in 1 to 3 million years. The mass is equally uncertain, thought to have been over 100 times the Sun when initially formed but now considerably less due to extreme mass loss. Modelling the star itself to match its spectrum gives a mass of 27.5 M☉, while matching its current properties to an evolutionary model gives a much higher mass (86-92 M☉).
Early reports suggested that it might be the most luminous star known, being almost 10 million times as luminous as the Sun. Later studies, however, have reduced its estimated luminosity, making it a candidate luminous blue variable about one-third as luminous as the binary star system Eta Carinae. Even so, it radiates about as much energy in 20 seconds as the Sun does in a year.
The Pistol Star was discovered using the Hubble Space Telescope in the early 1990s by Don Figer, an astronomer at UCLA.
[https://en.wikipedia.org/wiki/Pistol_Star]
Location of KW Sagittarii in Sagittarius
[https://www.universeguide.com/star/kwsagittarii]
KW Sagittarii is a red supergiant, located approximately 2,400 parsecs away from our Sun in the direction of the constellation Sagittarius. It has a size over 1,000 R☉ (Sun radii) making it one of the largest known stars. If placed at the center of the Solar System, the star’s surface would engulf Mars.
[https://en.wikipedia.org/wiki/KW_Sagittarii]
The image of the Peony Nebula star was taken by NASA’s Spitzer Space Telescope in its naturally dusty region.
Peony Star, or WR 102ka, is one of the most luminous stars known in our galaxy. Named after the nebula that surrounds it- the Peony Nebula- the Peony Star is a Wolf-Rayet star with an absolute magnitude of -11.5. It lies at a distance of 26,100 light years from Earth. The star’s luminosity is uncertain- it is estimated to be about 3.2 million times that of the Sun- because the star is heavily obscured by dust and not visible in optical bands. It must be observed in infrared wavelengths.
Peony Star has an estimated mass of about 100 solar masses and a radius 92 times solar. It is less that 3 million years old, but it has shed a lot of its initial mass and is expected to end its life in a supernova or hypernova explosion in the next few million years.
[http://www.constellation-guide.com/constellation-list/sagittarius-constellation/]
Ross 154
Ross 154 is a nearby red dwarf. Also known as V1216 Sagittarii, it lies in the eastern part of the constellation Sagittarius, northeast of Kaus Borealis (Lambda Sagittarii) at a distance of only 9.7 light-years (3.0 parsecs). It was discovered in 1925 by Frank Elmore Ross. Its closest neighbor is Barnard’s Star, 5.41 light-years (1.66 parsecs) away.
[http://www.daviddarling.info/encyclopedia/R/Ross_154.html]
The Milky Way is at its densest near Sagittarius, as this is where the galactic center lies:
Galactic Center: Scientists Take Viewers to the Center of the Milky Way
A new visualization allows viewers to control their own exploration of the center of the Milky Way galaxy. This 360-degree movie is based on data from NASA’s Chandra X-ray Observatory and other telescopes. The viewer observes this region from the vantage point of Sagittarius A*, the Galaxy’s supermassive black hole.
The Earth is located about 26,000 light years, or about 150,000 trillion miles, from the center of the Galaxy. While humans cannot physically travel there, scientists have been able to study this region by using data from powerful telescopes that can detect light in a variety of forms, including X-ray and infrared light.
This visualization builds on infrared data with the European Southern Observatory’s Very Large Telescope of 30 massive stellar giants called Wolf-Rayet stars that orbit within about 1.5 light years of the center of our Galaxy. Powerful winds of gas streaming from the surface of these stars are carrying some of their outer layers into interstellar space.
When the outflowing gas collides with previously ejected gas from other stars, the collisions produce shock waves, similar to sonic booms, which permeate the area. These shock waves heat the gas to millions of degrees, which causes it to glow in X-rays. Extensive observations with Chandra of the central regions of the Milky Way have provided critical data about the temperature and distribution of this multimillion-degree gas.
Astronomers are interested in better understanding what role these Wolf-Rayet stars play in the cosmic neighborhood at the Milky Way’s center. In particular, they would like to know how the stars interact with the Galactic center’s most dominant resident: the supermassive black hole known as Sagittarius A* (abbreviated Sgr A*). Pre-eminent yet invisible, Sgr A* has the mass equivalent to some four million Suns.
Scientists have used the visualization to examine the effects Sgr A* has on its stellar neighbors. As the strong gravity of Sgr A* pulls clumps of material inwards, tidal forces stretch the clumps as they get closer to the black hole. Sgr A* also impacts its surroundings through occasional outbursts from its vicinity that result in the expulsion of material away from the giant black hole. These outbursts can have the effect of clearing away some of the gas produced by the Wolf-Rayet winds.
The researchers, led by Christopher Russell of the Pontifical Catholic University of Chile, used the visualization to understand the presence of previously detected X-rays in the shape of a disk that extend about 0.6 light years outward from Sgr A*. Their work shows that the amount of X-rays generated by these colliding winds depends on the strength of outbursts powered by Sgr A*, and also the amount of time that has elapsed since an eruption occurred. Stronger and more recent outbursts result in weaker X-ray emission.
The information provided by the theoretical modeling and a comparison with the strength of X-ray emission observed with Chandra led Russell and his colleagues to determine that Sgr A* most likely had a relatively powerful outburst that started within the last few centuries. Moreover, their findings suggest the outburst from the supermassive black hole is still affecting the region around Sgr A* even though it ended about one hundred years ago.
[http://chandra.harvard.edu/photo/2018/gcenter360/index.html]
Sagittarius contains several well-known nebulae, including the Lagoon Nebula (Messier 8), near λ Sagittarii, and the Trifid Nebula (Messier 20), while NGC 6559 is a star-forming region located at a distance of about 5,000 light-years from Earth, showing both emission (red) and reflection (bluish) regions:
A Sagittarius Triplet
These three bright nebulae are often featured on telescopic tours of the constellation Sagittarius and the crowded starfields of the central Milky Way. In fact, 18th century cosmic tourist Charles Messier cataloged two of them; M8, the large nebula above and left of center, and colorful M20 near the bottom of the frame. The third emission region includes NGC 6559, right of M8 and separated from the larger nebula by a dark dust lane. All three are stellar nurseries about five thousand light-years or so distant. Over a hundred light-years across the expansive M8 is also known as the Lagoon Nebula. M20’s popular moniker is the Trifid. Glowing hydrogen gas creates the dominant red color of the emission nebulae. In striking contrast, blue hues in the Trifid are due to dust reflected starlight. The colorful composite skyscape was recorded with two different telescopes to capture a widefield image of the area and individual close-ups at higher resolution.
[https://apod.nasa.gov/apod/ap170727.html]
Among the bright nebulae and star clusters of the Sagittarius spiral arm, are the Omega Nebula (also known as the Swan Nebula, or M17), the open cluster M25, and M24, also called the Small Sagittarius Star Cloud:
A Sagittarius Starscape
This rich starscape spans nearly 7 degrees on the sky, toward the Sagittarius spiral arm and the center of our Milky Way galaxy. A telescopic mosaic, it features well-known bright nebulae and star clusters cataloged by 18th century cosmic tourist Charles Messier. Still popular stops for skygazers M16, the Eagle (far right), and M17, the Swan (near center) nebulae are the brightest star-forming emission regions. With wingspans of 100 light-years or so, they shine with the telltale reddish glow of hydrogen atoms from over 5,000 light-years away. Colorful open star cluster M25 near the upper left edge of the scene is closer, a mere 2,000 light-years distant and about 20 light-years across. M24, also known as the Sagittarius Star Cloud, crowds in just left of center along the bottom of the frame, fainter and more distant Milky Way stars seen through a narrow window in obscuring fields of interstellar dust.
[https://apod.nasa.gov/apod/ap140905.html]
Another nebula is the Red Spider Nebula (NGC 6537), a planetary nebula located at a distance of about 4,000 light-years from Earth:
Hubble spins a web into a giant Red Spider Nebula
Huge waves are sculpted in this two-lobed nebula called the Red Spider Nebula, located some 3,000 light-years away in the constellation of Sagittarius. This warm planetary nebula harbors one of the hottest stars known and its powerful stellar winds generate waves 100 billion kilometers (62.4 billion miles) high. The waves are caused by supersonic shocks, formed when the local gas is compressed and heated in front of the rapidly expanding lobes. The atoms caught in the shock emit the spectacular radiation seen in this image.
[https://www.nasa.gov/image-feature/goddard/2016/hubble-spins-a-web-into-a-giant-red-spider-nebula]
Sagittarius A* is a supermassive black hole at the center of the Milky Way galaxy:
Supermassive Black Hole Sagittarius A*
The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*), located in the middle, is revealed in these images. As described in our press release, astronomers have used NASA’s Chandra X-ray Observatory to take a major step in understanding why material around Sgr A* is extraordinarily faint in X-rays.
The large image contains X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards. This hot gas originates from winds produced by a disk-shaped distribution of young massive stars observed in infrared observations.
These new findings are the result of one of the biggest observing campaigns ever performed by Chandra. During 2012, Chandra collected about five weeks worth of observations to capture unprecedented X-ray images and energy signatures of multi-million degree gas swirling around Sgr A*, a black hole with about 4 million times the mass of the Sun. At just 26,000 light years from Earth, Sgr A* is one of very few black holes in the universe where we can actually witness the flow of matter nearby.
The authors infer that less than 1% of the material initially within the black hole’s gravitational influence reaches the event horizon, or point of no return, because much of it is ejected. Consequently, the X-ray emission from material near Sgr A* is remarkably faint, like that of most of the giant black holes in galaxies in the nearby Universe.
The captured material needs to lose heat and angular momentum before being able to plunge into the black hole. The ejection of matter allows this loss to occur.
This work should impact efforts using radio telescopes to observe and understand the ‘shadow’ cast by the event horizon of Sgr A* against the background of surrounding, glowing matter. It will also be useful for understanding the impact that orbiting stars and gas clouds might make with the matter flowing towards and away from the black hole.
[http://www.nasa.gov/mission_pages/chandra/multimedia/black-hole-SagittariusA.html]
The Sagittarius Dwarf Irregular Galaxy (SagDIG) is a dwarf galaxy in the constellation of Sagittarius, about 3.4 million light-years away:
[https://en.wikipedia.org/wiki/Sagittarius_Dwarf_Irregular_Galaxy]
Sagittarius Dwarf Irregular Galaxy
How old is this galaxy? The nearby Local Group galaxy dubbed the Sagittarius Dwarf Irregular Galaxy (SagDIG) is not only very small but also has relatively few elements more massive than helium. Now the lack of heavy elements might mean that SagDIG is very young, so that component stars had little time to create and disperse massive elements. Conversely, SagDIG’s diminutive size could indicate that it formed in the early universe, being a surviving building block of modern large galaxies. The above detailed image from the Hubble Space Telescope has now resolved enough stars to solve this mystery: SagDIG is ancient. Although SagDIG does have some groups of young stars, many stars are very old, and the galaxy as a whole helps astronomers to understand how the universe evolved, and show that at least one metal-poor galaxy is almost as old as the universe. Pictured above, SagDIG spans about 1,500 light years and lies about 3.5 million light years away toward the constellation of Sagittarius.
[https://apod.nasa.gov/apod/ap041116.html]
The Sagittarius Dwarf Elliptical Galaxy (SagDEG) is an elliptical galaxy located just outside the Milky Way:
The Sagittarius Dwarf Elliptical, satellite galaxy of our Milky Way
The Sagittarius Dwarf Elliptical Galaxy (Sag DEG, Sgr dE or the Sagittarius Dwarf Spheroidal Galaxy) is a small elliptical loop-shaped satellite galaxy of our Milky Way that lies about 70,000 light-years away from Earth in the constellation of Sagittarius, while it is currently receding from us at approximately 140 kilometers per second. It is roughly 10,000 light-years across and is home to four known globular clusters, including Messier 54.
Sag DEG should not be confused with the Sagittarius Dwarf Irregular Galaxy (Sag DIG), a small galaxy at 3.4 million light-years distant.
SagDEG is one of the most recently discovered members of the Local Group, and is currently in a very close encounter to our Milky Way galaxy. It is apparently in process of being disrupted by tidal gravitational forces of its big massive neighbor in this encounter. It is surprising that the dwarf has not been disrupted for so far. This fact is an indication for the unusually high concentration of dark matter within this small galaxy, which ties the stars stronger to the galaxy by its gravity.
It is a rather old galaxy, with little interstellar dust and composed largely of older and metal-poor stars, even though it has multiple stellar populations, ranging in age from the oldest globular clusters (almost as old as the Universe itself) to smaller populations of stars as young as several hundred million years which are metal-rich.
Sag DEG can be credited with shaping the Milky Way’s spiral arms.
It has orbited the Milky Way, with a period of 550 to 750 million years, about ten times during its billions of years of existence, at a distance of about 50,000 light-years from the galactic core. During these orbits Sag DEG struck our galaxy some 1.9 billion years ago. It then looped over the galactic ‘north pole’ and struck again about 900 million years ago. It is heading back right now, on course for a third clash with the southern face of the Milky Way disk in 10 million years or so.
Telescopic data and detailed simulations show how these galactic collisions have sent streams of stars out in loops in both galaxies. These long streamers continue to swell with stars and are gradually tugged outward by the Milky Way’s rotation into a familiar spiral arm. It’s the dark matter within Sag DEG that provided the initial push.
Sag DEG pays a high price though– sucked inward repeatedly by the Milky Way’s mightier gravity, it’s being ripped apart by the blows, sending huge amounts of its stars and dark matter into the new arms. Its starting mass was about 100 billion times the mass of our Sun, but has already decreased by a factor of two or three.
When all that dark matter first smacked into the Milky Way, 80 percent to 90 percent of it was stripped off. That first impact triggered instabilities that were amplified, and quickly formed the spiral arms of our galaxy.
Impacts between galaxies and their companions are thought to be widespread in the cosmos, and many of the spiral galaxies we can see were probably formed in this way.
[http://annesastronomynews.com/photo-gallery-ii/galaxies-clusters/the-sagittarius-dwarf-elliptical/]
The Wow! signal was a strong narrowband radio signal that appeared to have come from the direction of Sagittarius, on August 15, 1977. The signal was detected by astronomer Jerry R. Ehman while working on a SETI project. The signal appeared to come from the constellation Sagittarius and bore the expected hallmarks of non-terrestrial and non-Solar System origin. Impressed by the result, Ehman circled the signal on the computer printout and wrote the comment Wow! on its side, which became the name of the signal itself. The entire signal sequence lasted for the full 72-second window that Big Ear was able to observe it, but has not been detected since.
The space probe New Horizons is moving on a trajectory out of the Solar System as of 2016 that places the probe in front of Sagittarius as seen from the Earth. New Horizons will exhaust its radioisotope thermoelectric generator long before it reaches any other stars.
[https://en.wikipedia.org/wiki/Sagittarius_%28constellation%29]
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