Constellations: Pyxis 'the Mariner's Compass' Content from the guide to life, the universe and everything

Constellations: Pyxis 'the Mariner's Compass'

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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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Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
Name:Pyxis
Genitive:Pyxidis
Meaning:the Mariner's Compass
Short form:Pyx
Area:221 sq deg
Co-ordinates1:Right Ascension 09h, Declination −30°
Origin:Modern

The constellation of Pyxis the Mariner's Compass lies completely within the Southern Hemisphere of the celestial globe. It represents the magnetic compass used for directional navigation, not to be confused with the drawing instrument used by designers to measure circles, for which the constellation Circinus, the Geometer's Compass exists. Pyxis was originally named Pyxis Nautica, although the second part of the name has generally dropped out of use. It is surrounded by the constellations Hydra, Puppis, Antlia and Vela. It is difficult to see from northern latitudes as it only just clears the horizon when seen from around +50 degrees North in December.

Mythology and History

Pyxis is part of a one-time giant constellation, Argo Navis, the ship Argo. Argo Navis was one of the 48 constellations listed in Ptolemy's star catalogue the Almagest. It is also the only one of Ptolemy's constellations not to be accepted as a constellation in its own right by the IAU2 when in 1930 they formalised specific boundaries to 88 constellations covering the whole celestial globe. The complete Argo was so large that to make it more manageable convention was to refer to individual stars as being in 'the sail', 'the keel' or 'the stern'. When the IAU delineated the boundaries of the constellations they created four new constellations out of the old. These are now recognised as Carina the Keel3, Vela the Sail, Puppis the Stern4 and Pyxis.

This four-way split created the anomalous situation with most of the other constellations whereby the brightest star in the constellation is given the Greek letter alpha, the next brightest beta, followed by gamma and so on through the complete Greek alphabet. The split did not take into account the positions of the graded stars, so we now have, for example, alpha and beta in Carina, gamma and delta in Vela and the others spread around those two and Puppis.

The exception to this is Pyxis, which was created by French astronomer Abbé Nicolas Louis de la Caille5 by utilising some of the stars at the top of Argo's mast and giving them the normal classification of alpha, beta, gamma etc. It is a strange place for a compass to be situated, but as ancient Greek mariners never had the benefit of the magnetic compass it is also a strange artefact to be on this particular ship anyway.

In Greek mythology Argo Navis is the ship that was built by Argo for the celebrated Jason and his 50 Argonauts. The tale tells the adventures of the Argonauts and how they undertook a quest to retrieve the golden fleece of Aries the ram from Colchis. Other stories link Argo with the boat that carries Isis and Osiris in Egyptian myth, while Christian bibliology links it with Noah and his ark, and the great deluge.

Principal Stars

Northern observers may be lucky to observe anything of this constellation as it just about clears the horizon in December at latitudes of +50° North. Its most northerly star gamma Pyxidis is therefore the easiest to see from those latitudes. With alpha and beta it forms a straight line pointing directly south.

Pyxis contains a noteworthy star with a tendency to violent outbursts of activity as a re-occurring nova. T Pyxidis is a dense, white dwarf with a companion star from which hydrogen is transferred to the white dwarf. As material builds up on T Pyxidis, it reaches a critical temperature and density when spontaneous nuclear fusion takes place. This results in a nova explosion and a significant brightening of the star to naked eye proportions, only to die away when the new material has been used up. This cycle repeats itself approximately every 20-25 years, the last three were in 1920, 1944 and 1966. The next occurrence was in 2011, some 20 years overdue. It is now thought to be close to the Chandrasekhar limit when it might explode as a type 1a supernova.

Star Table

StarDesignationBrightness (m)Distance
(light years6)
Remarks
α Pyxalpha Pyxidis+3.701,000Hot blue-white
β Pyxbeta Pyxidis+3.97320Spectroscopic binary
γ Pyxgamma Pyxidis+4.01215Orange giant
T PyxT Pyxidis+15.03,000Recurrent Nova

Star Clusters, Nebulae and Galaxies

Despite its proximity to the Milky Way and its background of stars, there are few deep sky objects within the borders of Pyxis that are easily seen without recourse to expensive equipment. NGC 2613 is a beautifully symmetrical spiral galaxy with a magnitude of +10.4, discernible with larger aperture telescopes and long time-exposure photography. Another deep space object, NGC 2883, is a barely discernible galaxy of +14.4 magnitude. It is unusual insofar as it is of an irregular shape, possibly caused by a 'close' encounter with another nearby galaxy whose gravitational effect has distorted it away from the usual spiral shape.

NGC 2627 is a loose cluster of stars with a magnitude of +8.4, located near zeta Pyxidis. NGC 2818 is another loose cluster with a planetary nebula in its centre. The nebula is probably associated with the cluster. Other clusters, albeit difficult to see, include NGC 2658 and NGC 2635, which are +9.0 and +11.2 magnitudes respectively.

Meteor Shower

Pyxis is a relatively small constellation and although several meteor showers have their radiant in neighbouring constellations, there are no significant showers originating in Pyxis.

Extrasolar Planets

Pyxis can lay claim to at least three stars with extrasolar planets. HD 73256, a variable star also catalogued CS Pyxidis, has a confirmed planet HD 73256 b, 1.87 times the mass of our own Jupiter. It is a so-called 'Hot Jupiter' orbiting its parent star every 2.5 days.

Another, Gliese 317 b (now known as GJ 317 b), is just 20% greater mass than Jupiter and complets an orbit around nearby7 red dwarf Gliese 317 every 693 days. Gliese 317 has another, unconfirmed candidate planet Gliese 317 c, a suspected gas giant in a highly eccentric orbit, whose year is 2,700 Earth days long.

HD 73267 is a yellow dwarf star not unlike our own Sun. In 2008 a superjovian world was detected in orbit around it. None of these extrasolar planets are very good prospects in the search for extra-terrestrial life.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2The International Astronomical Union.3Otherwise known as 'the Hull'.4Also known as 'the Poopdeck'.5Another way his name is given is Nicolas Louis de Lacaille.6A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.7This is in galactic terms, Gliese 317 is just under 30 light years away from our Solar System.

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