Sextans (“the Sextant”) is an extremely faint southern hemisphere constellation that was created by the Polish astronomer Johannes Hevelius in 1687. It is found close to the celestial equator, with its brightest star, Alpha Sextantis, a white giant situated 287 light years from Earth with a visual magnitude of just +4.48.
Sextans is the 47th largest constellation, taking up an area of 314 square degrees of the southern sky. It can be seen by observers located between +80° and -90° of latitude, although best seen from January to May. Constellations bordering Sextans includes Leo to its north, crater to its east, and Hydra to its west and south.
Hercules Constellation Family
Sextans is a member of the Hercules family of constellations, together with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Serpens, Triangulum Australe and Vulpecula.
In the late 17th century, Johannes Hevelius devised seven constellations which are still recognized today, including Sextans. The constellation honors the astronomical sextant, an instrument primarily used by astronomers for measuring star positions, and upon which Hevelius relied, despite having access to telescopes at that time. Hevelius was inspired to name the constellation Sextans in honor of the instrument he lost following his observatory catching fire in September of 1679.
– Alpha Sextantis, the constellation’s brightest star, is a white giant (A0III) found 287 light years from our solar system that shines with an apparent magnitude of 4.48. This 300 million year old star is around 4.5 times bigger than the Sun, with 3 times its mass, and 122 times its luminosity. It is located just south of Regulus, the brightest star in Leo, and roughly a quarter of a degree to the south of the celestial equator, earning it the informal title of the “equator star”.
– Beta Sextantis, the second brightest star in Sextans, is a blue-white dwarf (B6V) situated 345 light years from Earth. Being a variable star, however, its apparent magnitude varies between 5 and 5.1 over a 15.4 day period. Beta Sextantis is 1.95 times larger than the Sun, and 184 times brighter.
– Gamma Sextantis, the constellation’s third brightest star, is a triple star system found 262 light years distant of magnitude 5.07. It consists of two stellar type A1 bluish-white stars separated by just 0.38 arcseconds, and with an orbital period of 77.6 years. Their visual magnitudes are 5.8 and 6.2, while 36 arcseconds away is a 12th magnitude companion.
Other stars of interest in Sextans includes the blue-white dwarf Delta Sextantis; the yellow-white giant Epsilon Sextantis; the yellow subgiant 24 Sextantis; the yellow-white dwarf star HD 86081; the orange dwarf stars BD-08°2823 and WASP-43; and the red dwarf star LHS 292.
Notable Deep-Sky Objects
There are no Messier objects in Sextans, but it does contains a number of notable deep-sky objects.
– Spindle Galaxy (NGC 3115) is a +9.9 magnitude lenticular galaxy which is unconnected to another Spindle Galaxy (Messier 102) of the same name found in the constellation of Draco. NGC 3115 is located 31.6 million light years away, and has a two billion-solar-mass supermassive black hole at its core, making it the nearest such galaxy to Earth. The Spindle Galaxy predominantly contains very old stars, and has already consumed most of the gas and dust that would have enabled it to form new stars.
– Cosmos Redshift 7 (CR7) is situated 12.9 billion from Earth, making it one of the most distant galaxies ever discovered, and as such is a high-redshift galaxy that is a Lyman-alpha emitter. It is also one of the oldest galaxies, and contains first generation (Population III) stars that were formed just 800 million years after the Big Bang, and may have been responsible for heavier than hydrogen chemical elements being produced, such as oxygen, nitrogen, carbon, calcium and iron, which are necessary elements in the later formation of planets and life in the Universe.
– CL J1001+0220 is a cluster of 17 galaxies found 11.1 billion light-years away, making it the most distant galaxy cluster yet discovered. Another remarkable feature of CL J1001+0220 is that 9 out of the 11 galaxies within 250,000 light years of its central region are showing high levels of star formation at around 3,400 suns each year, in contrast to other such distant galaxies which show little evidence of growth.
– NGC 3169 is a spiral galaxy 70 million light years distant of magnitude 10.3. It is located south of the bright star Regulus in Leo, and has distorted, sweeping spiral arms as a result of the gravitational interaction taking place with the nearby galaxy NGC 3166.
– CID-42 is a quasar, or a supermassive black hole orbited by gas that resides at the centre of a galaxy. It is situated 3.9 billion light years away, and is the result of two smaller galaxies colliding and their central black holes merging to form one supermassive black hole. One of the black holes has since recoiled from the resulting gravitational waves created by the merger, and is subsequently being ejected from the galaxy at a rate of around 2000 km/s. Once expelled, it will become a displaced quasar for somewhere between 10 million to 10 billion years until its fuel is exhausted.
Other objects of interest in Sextans includes the dwarf irregular galaxies Sextans A and Sextans B; the Sextans Dwarf Spheroidal galaxy; and the emission line galaxy UGC 5797.