Constellations: Gemini 'the Twins' Content from the guide to life, the universe and everything

Constellations: Gemini 'the Twins'

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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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The constellation of Gemini, the Twins.
Short form:Gem
Area:514 sq deg
Co-ordinates1:Right Ascension 07h, Declination +20°

Gemini, the twins, is one of the 12 zodiacal constellations; it is found on the ecliptic2 north-east3 of Orion and is at its most prominent during the late winter months of January and February. Other pointers to locating Gemini are the prominent star Procyon in Canis Minor to the south, and Sirius (alpha Canis Majoris), the brightest star in the northern sky, even further south. Gemini is bounded by the zodiacal constellations of Cancer to the east and Taurus to the west.


The pairing of the two principal stars that form Gemini is mentioned in the culture of most early civilisations; the stars are nearly always associated with pairs or twins. The Anglo-Saxons knew them as 'ge Twisan', the Anglo-Normans as 'Frères', and in Germany they were known as 'Zwillinge'. Our perception today is from classical times with the association of Castor and Pollux from Greek mythology through the listing of the 48 constellations in Ptolemy's Almagest.

Castor and Pollux were the sons of Queen Leda, wife of King Tyndareus of Sparta. Castor was the son of Tyndareus and was mortal, but Pollux was the son of Zeus after a clandestine association with Leda on her wedding night, and as a result was endowed with immortality. Castor became famed as a horseman and Pollux as a pugilist. Together they shared many adventures and sailed with Jason as Argonauts in search of the Golden Fleece. Castor was killed in battle with his cousin Idas, and Pollux pleaded with Zeus to be allowed to share his immortality with him. Zeus, impressed by their brotherly love, granted the wish and placed them in the sky together.

Principal Stars

Gemini's two principal stars Castor and Pollux mark the heads of the twins, with lesser magnitude stars in two lines stretching south-west and their feet firmly anchored in the edge of the Milky Way. In ancient times Castor was perceived as the brighter of the two, but now Pollux definitely outshines it. Consequently, the convention of classifying the brighter star with the Greek letter alpha has been reversed in this instance, with Castor, the lesser magnitude star, retaining the designation alpha. The two are separated by almost five degrees, with Castor being the more northerly of the two.

  • Alpha Geminorum, Castor, is a complex system of stars in its own right, which lies at a distance of 46 light years from Earth. It is primarily a double star which can be separated with a small telescope, and was the first to be recognised as an eclipsing binary, by William Herschel in 1802. The two stars have an orbital period of 470 years while a third star, Castor C - also known as YY Geminorum - also orbits the pair. To further complicate the matter, each of the three stars have been shown to be a spectroscopic binary, making this system consist of no fewer than six individual stars.

  • Beta Geminorum, Pollux, is an individual K-class star with a distinctive orange-red colour and is 35 light years distant. A planet was discovered in orbit around Pollux in 2006. It was named Thestias by the IAU in December 2015 after a public poll.

  • Gamma Geminorum, Alhena, is a brilliant white, class A star, at a distance of 85 light years. Its name is derived from the mark sometimes found on the right side of a horse's or camel's neck. An earlier Arabic derivation is from Al Maisan, 'the proudly marching one'.

  • Delta Geminorum, Wasat, is derived from Al Wasat 'The Middle', reflecting its position at the waist of Pollux. It is a binary star with the primary star white and the secondary blue, at a distance of 55 light years.

  • Epsilon Geminorum, Mebsuta 'The Outstretched', is located at Castor's thigh. It is named from an earlier Arabic constellation of a lion, in which it depicted the outstretched paw of the beast. It is another double star with the primary brilliant white and the secondary blue.

  • Zeta Geminorum, Mekbuda 'The Drawn-in Paw', is also named from the earlier constellation and is a Cepheid variable with a magnitude range between +4.4 and +5.2 over a period of ten days. It lies at a distance of 1,650 light years from Earth.

  • Eta Geminorum, Propus, is the 'Forward Foot' of Castor. It is a red giant and is a spectroscopic binary which dims at intervals of about eight years. An earlier, but now less-used name, is Tejat Prior, an Arabian anatomical term.

Star Table

StarDesignationNameMagnitudeDistance (light-years)Remarks
α Gemalpha GemCastor (The Horseman)+1.5846Consists of three binary stars
β Gembeta GemPollux (The Pugilist)+1.1536Orange coloured
γ Gemgamma GemAlhena+2.285Brilliant white
δ Gemdelta GemWasat+3.53 and +8.255Binary
ε Gemepsilon GemMebsuta+2.98685G class supergiant
η Gemeta GemPropus+3.1186Double/Variable
μ Gemmu GemTejat Posterior+2.88231Irregular variable with +9.8 mag companion
PSR B0633+17-Geminga+25 var815Neutron star/pulsar

Clusters and Nebulae

Messier 35, also known as NGC 2168, lies at Castor's feet. It is an open cluster of approximately 150 stars and is about 2,200 light years distant. Two other open clusters, NGC 2158 and NGC 2129, lie nearby, nearly overlapping M35. NGC 2158 borders M35 and is almost in the same line of sight, but is much further away at about 13,000 light years.

NGC 2392, the Eskimo Nebula, is a planetary nebula discovered in 1787 by William Herschel. It is three degrees south-west of delta Geminorum at Pollux's waist, and shows as a light blue disc in larger telescopes.

Clusters and Nebulae Table

Catalogue NoNameTypeMagnitudeDistance (light-years)
M35(NGC 2168)Open Cluster+5.52,800
NGC 2129 Open Cluster+6.77,200
NGC 2158 Open Cluster+8.613,000
NGC 2392Eskimo NebulaPlanetary Nebula+9.210,000

Meteor Shower

The Geminids occur in mid-December each year, peaking between 11 and 17 December with a rate up to 110 meteors per hour. Their origin is something of a mystery. The usual source of a meteor shower stems from the Earth passing through the debris trail left by a comet as their paths cross. The first noticeable occurrence of the Geminids shower was in 1862, but no comet was found that could be attributed as the source. For over a hundred years that situation remained until in 1983 NASA's IRAS4 satellite spotted a rocky body in the same orbit as the Geminids, which has now been named 3200 Phaethon.

It is by no means certain just what the several-kilometre-wide 3200 Phaethon is. At first it was thought to be an asteroid as the spectra of the Geminids indicate a rocky origin. Current theory postulates an extinct comet that is unable any longer to produce a 'tail' from solar heating when it passes close to the Sun. The volatile elements that boiled off on previous passes have been exhausted, leaving only the rocky parts of the comet remaining.

Extrasolar Planets in Gemini

Some extrasolar planets have been discovered lying in the direction of Gemini. One, Thestias, which orbits the beta star Pollux, was first suspected as early as 1993 by Hatzes and Cochran, but it wasn't confirmed until 2006.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 50554HD 50554 b4.91,2792002Superjovian
HD 59686HD 59686 b5.253032003Superjovian/
circular orbit
Pollux/HD 62509Thestias2.9589.64(published) 2006Suspected in 1993
HAT-P-20HAT-P-20 b7.252.8752010Superjovian
HAT-P-24HAT-P-24 b0.6853.352010Hot gas giant
HAT-P-33HAT-P-33 b0.763.472011Hot gas giant
HAT-P-39HAT-P-39 b0.63.52012Hot gas giant
HAT-P-54HAT-P-54 b0.763.82014Hot gas giant
HAT-P-50HAT-P-50 b1.363.122015Hot superjovian
HAT-P-56HAT-P-56 b2.22.82015Superjovian
HD 67087HD 67087 b33522015Superjovian/habitable zone
HD 67087HD 67087 c4.852,3742015Superjovian

The Other Faces of Gemini

The constellation of Gemini has, in the past, been a happy hunting ground for the discovery of new planets. On 13 March, 1781, William Herschel announced that he had discovered the planet Uranus near eta Geminorum (Propus), and later in 1930, Clive Tombaugh, discovered the planet Pluto5 near delta Geminorum (Wasat).

The name Gemini was used for the United States' 'Project Gemini' - a series of manned space flights into Earth orbit during the 1960s. In all, ten manned flights were flown between 1965 and 1966, with the name reflecting the two-man crews. Project Gemini was used to test the feasibility of long-term space flight and spacecraft systems as a forerunner to the Apollo lunar landing programme. Many of the Apollo astronauts, including Neil Armstrong and Buzz Aldrin, flew in Gemini spacecraft.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2The apparent path of the Sun through the sky.3Note that points of the compass work differently in the heavens; if north is at the top, east is to the left and west is to the right.4IRAS: Infra-Red Astronomy Satellite.5In 2006 Pluto lost its status as a planet when the International Astronomical Union (IAU) reclassified it as a 'minor planet'.

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