Sagittarius is one of the constellations of the zodiac and is located in the Southern celestial hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its old astronomical symbol is (♐︎). Its name is Latin for "archer". Sagittarius is commonly represented as a centaur drawing a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east. The center of the Milky Way lies in the westernmost part of Sagittarius (see Sagittarius A).
Visualizations
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. Marking the bottom of the teapot's "handle" (or the shoulder area of the archer), is the bright star (2.59 magnitude) Zeta Sagittarii (ζ Sgr), named Ascella, and the fainter Tau Sagittarii (τ Sgr). To complete the teapot metaphor, under dark skies a particularly dense area of the Milky Way (the Large Sagittarius Star Cloud) can be seen rising in a north-westerly arc above the spout, like a puff of steam rising from a boiling kettle. 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 the outline of the horse's body. Sagittarius famously points its arrow at the heart of Scorpius, represented by the reddish star Antares, as the two constellations race around the sky. Following the direct line formed by Delta Sagittarii (δ Sgr) and Gamma2 Sagittarii (γ2 Sgr) leads nearly directly to Antares. Fittingly, Gamma2 Sagittarii is Alnasl, the Arabic word for "arrowhead", and Delta Sagittarii is called Kaus Media, the "center of the bow," from which the arrow protrudes. Kaus Media bisects Lambda Sagittarii (λ Sgr) and Epsilon Sagittarii (ε Sgr), whose names Kaus Borealis and Kaus Australis refer to the northern and southern portions of the bow, respectively. Due to its astronomical interest and its status as a Zodiac constellation, Sagittarius is one of the best-known constellations and is considered a prominent feature of the summer skies in the northern hemisphere. However, at locations north of 43°N the constellation either drags along the southern horizon, or it does not rise at all. By contrast, in most of the southern hemisphere Sagittarius can appear overhead or nearly so. It is hidden behind the Sun's glare from mid-November to mid-January and is the location of the Sun at the December solstice. By March, Sagittarius is rising at midnight. In June, it achieves opposition and can be seen all night. The June full moon appears in Sagittarius. In classical antiquity, Capricorn was the location of the Sun at the December solstice, but due to the precession of the equinoxes, this had shifted to Sagittarius by the time of the Roman Empire. By approximately 2700 AD, the Sun will be in Scorpius at the December solstice.
Features
Stars
α Sgr (Rukbat, meaning "the archer's knee") despite having the "alpha" designation, is not the brightest star of the constellation, having a magnitude of only 3.96. It is towards the bottom center of the map as shown. Instead, the brightest star is Epsilon Sagittarii (ε Sgr) (Kaus Australis, or "southern part of the bow"), at magnitude 1.85, or about seven times as bright as α Sgr. Sigma Sagittarii (σ Sgr) (Nunki) is the constellation's second-brightest star at magnitude 2.08. Nunki is a B2V star approximately 260 light-years away. "Nunki" is a Babylonian name of uncertain origin, but thought to represent the sacred Babylonian city of Eridu on the Euphrates, which would make Nunki the oldest star name currently in use. Zeta Sagittarii (ζ Sgr) (Ascella), with apparent magnitude 2.61 of A2 spectra, is actually a double star whose two components have magnitudes 3.3 and 3.5. Delta Sagittarii (δ Sgr) (Kaus Media), is a K2 spectra star with magnitude 2.71 about 350 light years from Earth. Eta Sagittarii (η Sgr) is a double star with component magnitudes of 3.18 and 10, while Pi Sagittarii (π Sgr) (Albaldah) is actually a triple system whose components have magnitudes 3.7, 3.8, and 6.0. The Bayer designation Beta Sagittarii (Beta Sgr, β Sagittarii, β Sgr) is shared by two star systems, β¹ Sagittarii, with apparent magnitude 3.96, and β² Sagittarii, magnitude 7.4. The two stars are separated by 0.36° in the sky and are 378 light-years from earth. Beta Sagittarii, located at a position associated with the forelegs of the centaur, has the traditional name Arkab, meaning "Achilles tendon". Nova Sagittarii 2015 No. 2 was discovered on 15 March 2015, by John Seach of Chatsworth Island, NSW, Australia. It lies near the center of the constellation. It reached a peak magnitude of 4.3 before steadily fading.
Named stars
Deep-sky objects
The Milky Way is at its densest near Sagittarius, as this is where the Galactic Center lies. As a result, Sagittarius contains many star clusters and nebulae.
Star clouds Sagittarius contains two well-known star clouds, both considered fine binocular objects.
The Large Sagittarius Star Cloud is the brightest visible region of the Milky Way. It is a portion of the central bulge of the galaxy seen around the thick dust of the Great Rift, and is the innermost galactic structure that can be observed in visible wavelengths. It has several embedded clusters and superimposed dark nebulae. The Small Sagittarius Star Cloud, also known as Messier 24, has an apparent magnitude of 2.5. The cloud fills a space of significant volume to a depth of 10,000 to 16,000 light-years. Embedded in M24 is NGC 6603, a small star cluster that is very dense. NGC 6567, a dim planetary nebula, and Barnard 92, a Bok globule, are also nearby.
Nebulae Sagittarius contains several well-known nebulae, including the Lagoon Nebula (Messier 8), near λ Sagittarii; the Omega Nebula (Messier 17), near the border with Scutum; and the Trifid Nebula (Messier 20), a large nebula containing some very young, hot stars.
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