Star Constellations: Gemini, the Twins

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Latin: Gemini
Short form:Gem
Area:514 square degrees
Co-ordinates:07 45h, +28.00°
Zodiac:  Yes

Gemini, the twins, is one of the twelve zodiacal constellations and is found on the ecliptic1 north-east 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, and Sirius, the brightest star in the northern sky, to the south. Gemini is bounded by other zodiacal constellations of Cancer to the east, and Taurus to the west.


The pairing of the two principle stars that form Gemini are mentioned in the culture of most early civilisations and are nearly always associated with pairs or twins. The Anglo-Saxon knew them as 'ge Twisan', the Anglo-Normans as 'Frère' and in Germany 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 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. 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. 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, Gemini C, also orbits the pair. To further complicate the matter, each of the three stars has been shown to be a spectroscopic binary, making this system comprise 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.

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 and 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.

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.

StarNameBrightness (m)Distance (light-years)Remarks
α GemCastor (The Horseman)+1.5846Comprised of triple binary stars.
β GemPollux (The Pugilist)+1.1536Orange coloured.
γ GemAlhena (Proud marching one)+2.285Brilliant white
δ GemWasat (The middle)+3.53 and +8.255Binary
ε GemMebsuta (The outstretched)+2.98685G class super giant
η GemPropus+3.1186Double/Variable
μ GemTejat Posterior+2.88231Irregular variable with +9.8 mag companion.

Clusters and Nebulae

Messier 35 lies at Castors feet. It is an open cluster of approximately 150 stars and is about 2,200 light years distant. Two other open clusters, NGC2158 and NGC2129, lie nearby, nearly overlapping M35. NGC2158 borders M35 and is almost in the same line of sight, but is much further away at about 13,000 light years.

NGC2392, 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
Catalogue NoNameTypeBrightness (m)Distance (light-years)
M35(NGC2168)Open cluster+5.52,800
NGC2129 Open Cluster+6.7 
NGC2158 Open Cluster+8.613,000
NGC2392Eskimo NebulaPlanetary Nebula+9.210,000

Meteor Shower

The Geminids occur in mid December each year, peaking between 11-17 December with a rate up to 110 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 it 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 IRAS 2satellite 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.

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 Pluto3 near delta Geminorum (Wasat).

The name Gemini was used for the United States' 'Project Gemini' which was 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.

1The apparent path of the Sun through the sky2IRAS: Infra-Red Astronomy Satellite3In 2006 Pluto lost its status as a planet when the International Astronomical Union (IAU) reclassified it as 'Minor Planet'

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