Constellations: Sextans 'the Sextant' Content from the guide to life, the universe and everything

Constellations: Sextans 'the Sextant'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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A sextant.
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)

Short form:Sex
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.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years)
Spectral classification
and/or comments
α Sexalpha SextantisHD 87887+4.5 var280White giant
β Sexbeta SextantisHD 90994+5.1 var300Blue-white dwarf
γ Sexgamma SextantisHD 85558+5.1 var240Triple system
δ Sexdelta SextantisHD 90882+5.2 var300Blue-white dwarf
ε Sexepsilon SextantisHD 89254+5.25175Yellow-white sub-giant
LHS 292LHS 292LP 731-58+15 var15Red dwarf flare star
SW SexSW SextantisSW SextantisVariable946Cataclysmic variable
HD 92788GC 14729SPOCS 461+7.3 var107Has a planet
HD 86081HIP 48711SAO 137236+8.73297Has 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.

Other Galaxies

Sextans A and Sextans B are both dwarf irregular galaxies.

Galaxy Table

CatalogueGalaxy TypeBrightness (m)Distance
(light years)
NGC 3115Lenticular+9.132mSpindle Galaxy; Caldwell 53; Bennett 42
NGC 3165Irregular+14.564mBeing absorbed by NGC 3166
NGC 3166Lenticular+10.664mDistorted by NGC 3169
NGC 3169Seyfert spiral+10.564mPaired with NGC 3166
NGC 2967Spiral+11.680mGrand design/face on
NGC 2974Spiral+10.893mDiscovered in 1785 by William Herschel
NGC 3423Spiral+11.272mGrand design/face on

Meteor Showers

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
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 92788HD 92788 b3.863252000Superjovian; habitable zone
HD 92788HD 92788 c0.91622013Gas giant; habitable zone
HD 86081HD 86081 b1.52.142006Hot Jupiter
BD-082823BD-082823 b0.0455.62009Hot terrestrial
BD-082823BD-082823 c0.33237.62009Gas giant/
habitable zone
24 Sex24 Sex b24502010Gas giant
24 Sex24 Sex c0.868832010Gas giant
WASP-43WASP-43 b1.780.812011Hot Jupiter
WASP-127WASP-127 b0.184.182016Hot 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.

1A lament for their passing.2That is, it is below the ecliptic if you are in the Northern Hemisphere. In the Southern Hemisphere, it is above the ecliptic.3The apparent path of the Sun across the sky.4In this instance 'close' means five million light years.5Lunar topography.6Known as 'the mother of moon charts', her life was commemorated in the 2006 book Die Sternjägerin (The Star Huntress) by Eric Walz.7A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.8A 'cosmic trainwreck' of two (or more) colliding galaxies can take around a billion years.

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