Constellations: Sextans 'the Sextant'
Created | Updated Nov 12, 2017
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæleon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
Frosted eyes there were that lifted altars;
And silent answers crept across the stars.
Compass, quadrant and sextant contrive
No farther tides... High in the azure steeps
Monody1 shall not wake the mariner.
This fabulous shadow only the sea keeps.
– At Melville's Tomb by Harold Hart Crane (1899 - 1932)
|Area:||314 sq deg|
|Co-ordinates:||Right Ascension 10h, Declination 0°|
|Origin:||Modern (17th Century)|
Sextans is an unremarkable constellation situated just below2 the ecliptic3, positioned on top of Hydra and also sharing borders with Leo and Crater. There are neither any named nor any bright stars, but there's an interesting nearby red dwarf and a prototype variable star in the list. Within the borders of Sextans are three extrasolar planetary systems and it is the radiant of a daylight meteor shower. Far beyond the stars of Sextans are some galaxies which appear in the New General Catalogue, and also Sextans A, a close4 dwarf galaxy which is so mis-shapen that instead of the usual curvaceousness, it appears square.
Hydra, at over 1,300 square degrees in area, is the biggest of the 88 constellations we acknowledge today. However, in ancient times, the sea serpent was portrayed much larger. It included the stars which now make up three smaller constellations. On the original Hydra the tiny threesome were the fins on its back, or the 'humps' as it undulated through the 'water'. Later on, the smaller constellations Sextans, Corvus 'the crow' and Crater 'the cup' were created to make Hydra a more manageable size.
Johannes Hevelius (1611 - 1687), the founder of selenography5, delineated seven star formations – one of them was Sextans Uranise, the sextant of Urania, to honour his sextant, the brass instrument he used to measure the position of stars. The word sextant comes from the Latin word for one sixth of the arc of a circle, because of the instrument's resemblance to this shape. Sextants were used not only by astronomers but also on sea voyages by ancient mariners, who plotted their navigational course with celestial assistance.
The other six constellations created by Hevelius were: Scutum, Lacerta, Lynx, Leo Minor, Canes Venatici and Vulpecula. They were all introduced in his 1690 atlas Firmamentum Sobiescianum, which was published posthumously by his second wife Elisabeth6 (1647 - 1693), also an astronomer.
By the 19th Century there were over 100 constellations in existence, honouring some unusual objects that were in vogue at the time. In 1922, the International Astronomical Union (IAU) eliminated some but retained Sextans Uranise (shortened to 'Sextans') in the 88 internationally recognised today.
Stars are identified by Greek letters combined with the genitive of the constellation name. This is known as the 'Bayer designation' after the man who devised the system. Some stars have proper names as well, for example, alpha Tauri is Aldebaran, but there are no named stars in Sextans. Other stars, like LHS 292, are known by their catalogue number. Recently discovered variable stars, like SW Sextantis, are given upper case English letters.
Stars of Sextans
Beta Sextantis is a blue-white dwarf classified as an alpha2 Canum Venaticorum variable.
Gamma Sextantis is a circumfluent triple star system comprising a binary pair of white dwarfs which orbit each other, and a much more distant companion which encompasses them both.
LHS 292, a +15 magnitude red dwarf flare star, is a close neighbour of ours (in galactic terms) at just 15 light years7 distance.
SW Sextantis is a cataclysmic variable star given its own sub-class. This means it is a prototype, and other similar, but later discovered stars, can be classed SW Sex-types. Cataclysmic variables (CVs) are binary systems which have one partner (the donor) constantly supplying the other until the feeder has gobbled so much starstuff that it explodes. The orgasm does not destroy the phoenix-like recipient; what occurs is a dwarf nova, and as soon as the fireworks have subsided, the cycle of activity repeats itself. SW Sex-types have a high mass transference rate but the rhythmic supply dies down before the partner swells to overcapacity, so there is no culmination. All we see is a rise in the magnitude as the pair pulsate to their highest peak. The whole shebang begins again almost immediately; there's no quiet afterglow, not even time for a quick shower before the hot heavenly bodies embrace again.
Other SW Sex-types are TT Trianguli, V380 Ophiuchi, AH Mensae, HL Aquarii, BO Ceti and TT Arietis.
|α Sex||alpha Sextantis||HD 87887||+4.5 var||280||White giant|
|β Sex||beta Sextantis||HD 90994||+5.1 var||300||Blue-white dwarf|
|γ Sex||gamma Sextantis||HD 85558||+5.1 var||240||Triple system|
|δ Sex||delta Sextantis||HD 90882||+5.2 var||300||Blue-white dwarf|
|ε Sex||epsilon Sextantis||HD 89254||+5.25||175||Yellow-white sub-giant|
|LHS 292||LHS 292||LP 731-58||+15 var||15||Red dwarf flare star|
|SW Sex||SW Sextantis||SW Sextantis||Variable||946||Cataclysmic variable|
|HD 92788||GC 14729||SPOCS 461||+7.3 var||107||Has a planet|
|HD 86081||HIP 48711||SAO 137236||+8.73||297||Has a planet|
New General Catalogue (NGC)
The NGC was compiled by John Louis Emil Dreyer (the director of the Armagh Observatory from 1882 to 1916). All of the NGC objects in Sextans are galaxies.
NGC 3115 has the common name of the Spindle Galaxy; from our vantage point it is viewed edge on. At its centre is a supermassive black hole. The Spindle Galaxy features in Sir Patrick Moore's Caldwell catalogue for backyard astronomers at number 53, and the Bennett list positions it at number 42.
NGC 3166 and NGC 3169 are close enough at 50,000 light years to affect each other but this is not yet a cosmic trainwreck8. The pair look quite neighbourly in this 'Best Images of the Advanced Observing Programme' feature. NGC 3165 is a much smaller irregular galaxy which is being consumed by its close neighbour NGC 3166.
Sextans A and Sextans B are both dwarf irregular galaxies.
|Catalogue||Galaxy Type||Brightness (m)||Distance
|NGC 3115||Lenticular||+9.1||32m||Spindle Galaxy; Caldwell 53; Bennett 42|
|NGC 3165||Irregular||+14.5||64m||Being absorbed by NGC 3166|
|NGC 3166||Lenticular||+10.6||64m||Distorted by NGC 3169|
|NGC 3169||Seyfert spiral||+10.5||64m||Paired with NGC 3166|
|NGC 2967||Spiral||+11.6||80m||Grand design/face on|
|NGC 2974||Spiral||+10.8||93m||Discovered in 1785 by William Herschel|
|NGC 3423||Spiral||+11.2||72m||Grand design/face on|
The meteor shower connected with this constellation is called the Sextantids. However, this is a daytime shower so it's not well-known. They were discovered by AA Weiss in September 1957, who recorded 30 meteors per hour. No more Sextantids were reported until 1961, and the next shower noted was in 1969, so it is possibly periodic at four years.
Extrasolar Planets in Sextans
Several extrasolar planetary systems have been found in this constellation up to 2011:
HD 92788 b is almost four times the mass of Jupiter, but it does orbit in the habitable zone.
HD 86081 b is a gas giant which orbits the star HD 86081 so closely that its year is just over two Earth days long. These types of planets are known as 'Hot Jupiters' and they have the fastest atmospheric winds known to science. Because their orbit is so tight, they always show the same face to the star, as the Moon does to the Earth. So, with this 'missionary position', you'd expect one side of the planet to be hellishly hot, and the other constantly cold. However, the atmosphere is superheated to such an extent that it is energised and the excited wind rushes around the planet at supersonic speed, heating up the cooler gas on the far side. The intense activity creates a worldwide mean temperature, too hostile for comfort and with 7,000mph burning winds there's no welcome sign here.
BD-082823 has two attendant planets according to the bumper announcement of 19 October, 2009.
WASP-43 b is a gas giant orbiting its parent star in less than one Earth day.
Extrasolar Planets Table
|Star name or
|Year of discovery||Comments|
|HD 92788||HD 92788 b||3.86||325||2000||Superjovian; habitable zone|
|HD 92788||HD 92788 c||0.9||162||2013||Gas giant; habitable zone|
|HD 86081||HD 86081 b||1.5||2.14||2006||Hot Jupiter|
|BD-082823||BD-082823 b||0.045||5.6||2009||Hot terrestrial|
|BD-082823||BD-082823 c||0.33||237.6||2009||Gas giant/|
|24 Sex||24 Sex b||2||450||2010||Gas giant|
|24 Sex||24 Sex c||0.86||883||2010||Gas giant|
|WASP-43||WASP-43 b||1.78||0.81||2011||Hot Jupiter|
|WASP-127||WASP-127 b||0.18||4.18||2016||Hot Neptune|
There are two types of sextant: a navigator's sextant, which needs to be small and portable, and an astronomical instrument for measuring heavenly bodies like stars. The astronomer's sextant could be built to any scale on a fixed frame, then manipulated to the desired position. Famous astronomers who made use of sextants were Tycho Brahe and the first Astronomer Royal, John Flamsteed (1646 - 1719). Married astronomers Johannes and Elisabeth Hevelius constructed their own device which was twice their height. It was unique in that it had an alidade (a sight) to do the initial lining up, and needed two people to work the equipment. Unfortunately, in 1679 a fire in their observatory destroyed the astronomical instruments, including the sextant. The couple's achievements throughout their 24-year marriage, commemorated in the 1690 Prodromus Astronomiae which contained the positions of over 1,500 stars, is a testament to their remarkable teamwork.