The Sextans Constellation

Sextans is a minor equatorial constellation which was introduced in 1687 by Johannes Hevelius. Its name is Latin for the astronomical sextant, an instrument that Hevelius made frequent use of in his observations. Sextans is the 47th constellation in size, occupying an area of 314 square degrees. It is located in the second quadrant of the southern hemisphere and can be seen at latitudes between +80° and -90°. Sextans as a constellation covers a rather dim, sparse region of the sky located between the constellations Hydra, Crater, and Leo. The constellation was created by the Polish astronomer Johannes Hevelius in the 17th century. Sextans belongs to the Hercules family of constellations, along with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Serpens, Triangulum Australe and Vulpecula.

Sextans does not have any stars brighter than magnitude 3.00 and contains five stars. The brightest star in the constellation is Alpha Sextantis. It is a white giant star with the stellar classification A0III. It has an apparent magnitude of 4.48 and is approximately 287 light years distant from Earth. It is the brightest star in Sextans. It is 122 times more luminous than the Sun and has a mass three times solar. The star is believed to be about 300 million years old. Alpha Sextantis is informally considered to be an “equator star,” currently located less than a quarter of a degree south of the celestial equator. The star lies almost exactly south of the bright star Regulus in Leo constellation, and only 0.4 arc minutes to the west.

Sextans does not contain any Messier objects NGC 3115, the Spindle Galaxy, is a lenticular galaxy in Sextans. It has an apparent magnitude of 9.9 and is 31.6 million light years from Earth. It is several times bigger than the Milky Way and has a supermassive black hole at its centre. NGC 3115 is the nearest galaxy with a billion-solar-mass black hole to Earth. The galaxy was discovered by William Herschel on February 22, 1787. Most of the stars in it are pretty old, and the galaxy does not have much dust and gas left for new star formation to take place. NGC 3115 should not be confused with Messier 102 (NGC 5866) in Draco constellation, which is also called the Spindle Galaxy. NGC 3169 is a spiral galaxy in Sextans. It has an apparent magnitude of 10.3 and is 70 million light years distant. It can be found just under the bright star Regulus in Leo constellation. The galaxy has a distorted shape as a result of gravitational interaction with nearby galaxy NGC 3166. Both galaxies were discovered by William Herschel in 1783. A supernova, SN 2003cg, was discovered in NGC 3169 in 2003. NGC 3166 is also a spiral galaxy. It lies about 50,000 light years from NGC 3169. The two galaxies will eventually merge into one larger galaxy. Sextans A is a small dwarf irregular galaxy about 5,000 light years across. It has a visual magnitude of 11.9 and is approximately 4.31 million light years from Earth. The galaxy is located within the Local Group of galaxies. Sextans B is an irregular galaxy with an apparent magnitude of 11.9. It is approximately 4.44 million light years distant. Five planetary nebulae have been discovered in Sextans B. It is one of the smallest galaxies in which planetary nebulae have been found. Sextans B forms a pair with its neighbour Sextans A. It might also be gravitationally associated with the galaxies NGC 3109 in Hydra constellation and the Antlia Dwarf in Antlia. The Sextans Dwarf Spheroidal galaxy, also known as Sextans I, lies at a distance of only 290,000 light years from the Sun and is one of the satellites of our own galaxy, the Milky Way. The galaxy is receding from us at 224 km/s.

Sextans I has an apparent magnitude of 10.4. It was discovered in 1990 by Michael Irwin, director of the Cambridge Astronomical Survey Unit and one of the discoverers of the Sagittarius Dwarf Elliptical Galaxy and the Cetus Dwarf. UGC 5797 is an emission line galaxy, one currently undergoing active star formation. It lies at a distance of 34 million light years and has an apparent magnitude of 14.4. CL J1001+0220 is the most distant known galaxy cluster as of 2016. It is located at a distance of 11.1 billion light years from Earth. CL J1001+0220 is also the first cluster observed in the stage of evolving from a proto-cluster to a mature cluster. It contains 17 galaxies. Nine of the eleven massive galaxies in the cluster’s central region are showing evidence of new stars forming at a very high rate. Cosmos Redshift 7 (CR7) is one of the oldest, most distant galaxies known. It is a high-redshift Lyman-alpha emitter galaxy located about 12.9 billion light years from the solar system. The galaxy contains Population III (first generation) stars, formed during the reionization epoch, when the Universe was only 800 million years old, not long after the Big Bang. Lyman-alpha emitters are typically young, extremely distant, low-mass galaxies that have the highest specific star formation rate of any galaxies known. They provide clues into the history of the Universe and are believed to be the progenitors of modern Milky Way-type galaxies. CR7 also contains old, metal-poor Population II stars and is three times brighter than other extremely distant galaxies. The galaxy’s nickname was inspired by the Portuguese footballer Cristiano Ronaldo, who is known as CR7. CID-42 (also catalogued as CXOC J100043.1+020637), is a galaxy quasar located at an approximate distance of 3.9 billion light years. The object is believed to contain a supermassive black hole at its core and be the result of a collision between two smaller galaxies. It has an extended trail of stars. When the two smaller galaxies collided, their black holes also collided to form a single supermassive black hole, which then recoiled from the gravitational waves produced by the collision and is now being ejected from the galaxy. Once it is ejected, the black hole will likely shine as a displaced quasar until it exhausts its fuel, which could take anywhere from 10 million to 10 billion years. Sextans is the location of the field studied by the Cosmic Evolution Survey (COSMOS) project, undertaken by the Hubble Space Telescope to survey a 2-square degree equatorial field in the constellation with the Advanced Camera for Surveys (ACS). More than 2 million galaxies were detected during the survey, spanning 75% of the age of the Universe.

There is one daytime meteor shower associated with the constellation, the Sextantids, occurring in late October and early November. Credits: Constellation Guide, Wikipedia.