Sagittarius is one of the constellations of the zodiac. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its name is Latin for the archer, and its symbol is a stylized arrow. Sagittarius is commonly represented as a centaur pulling back a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east. Sagittarius is the 15th largest constellation in the sky, occupying an area of 867 square degrees. It is located in the fourth quadrant of the southern hemisphere and can be seen at latitudes between +55° and -90°. The neighbouring constellations are Aquila, Capricornus, Corona, Australis, Indus, Microscopium, Australis Ophiuchus, Scutum, Scorpius, Serpens Cauda and Telescopium. Sagittarius belongs to the Zodiac family of constellations, along with Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Capricornus, Aquarius, and Pisces. The Milky Way is at its densest near Sagittarius, as this is where the galactic centre lies. As a result, Sagittarius contains many star clusters and nebulae.
Epsilon Sagittarii is a binary star approximately 143 light years distant. It is a blue class B giant with an apparent magnitude of 1.79 and a luminosity 375 times that of the Sun. Kaus Australis is the brightest star in the constellation Sagittarius and the 36th brightest star in the sky. It has a faint 14th magnitude companion 32 arc seconds away. The star’s traditional name, Kaus Australis, comes from the Arabic word for “bow” (qaws) and the Latin word for “southern” (australis). The star marks the base of the archer’s bow. Together with the stars Delta (Kaus Media) and Lambda Sagittarii (Kaus Borealis), Epsilon Sagittarii represents the archer’s bow. Sigma Sagittarii is the second brightest star in the constellation Sagittarius. It is a hydrogen fusing dwarf star that belongs to the spectral type B2.5 V. It has an apparent magnitude of 2.1. The star has a luminosity 3,300 times that of the Sun and about seven solar masses. It is a very fast rotator, spinning with a speed of more than 200 kilometres per second, which is about 100 times faster than the Sun. The star lies approximately 228 light years from Earth. Sigma Sagittarii is sometimes also known as Nunki. Nunki has a faint (magnitude 9.5) companion about 5.2 arc minutes away. It is located close to the ecliptic and it can sometimes be occulted by the Moon and, very rarely, by planets. Sigma Sagittarii is also notable for being the brightest star that can be occulted by an exterior planet. This, however, only applies to Mars and it happens extremely rarely.
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. 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. It is fairly bright, with an integrated magnitude of 3.0. The Lagoon Nebula was discovered independently by John Flamsteed in 1680, Guillaume Le Gentil in 1747, and Charles Messier in 1764. 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. 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. The Omega Nebula is a fairly bright nebula, sometimes called the Horseshoe Nebula or Swan Nebula. 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. 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 colour, by a central triple star, which formed in the intersection of the two dark bands. The Red Spider Nebula (NGC 6537) is a planetary nebula located at a distance of about 4000 light-years from Earth. NGC 6559 is a star-forming region located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius, showing both emission (red) and reflection (blue) regions. 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. 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. 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 an intermediate concentration at its core.
In 1999 a violent outburst at V4641 Sgr was thought to have revealed the location of the closest known black hole to Earth, but later investigation increased its estimated distance by a factor of 15. The complex radio source Sagittarius A is also in Sagittarius, near its western boundary with Ophiuchus. Astronomers believe that one of its components, known as Sagittarius A*, is associated with a supermassive black hole at the centre of the galaxy, with a mass of 2.6 million solar masses. 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 centre that 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. 2MASS-GC02, also known as Hurt 2, is a globular cluster at a distance of about 16 thousand light-years from Earth. It was discovered in 2000 by Joselino Vasquez, and confirmed by a team of astronomers under the leadership of R. J. Hurt at 2MASS. Credits: Constellation Guide, Wikipedia.