Gemini, the twins, is one of the original 48 figures passed down to us via Ptolemy’s 2nd century Almagest. It lies along the elliptical, thus is one of the signs of the Zodiac (the Sun passes through this constellation during the height of the summer).




Gemini has been represented by celestial twins in most traditions. The version we currently use originates from the Greek pair of Castor and Pollux, sons of Leda. Castor was an immortal, son of King Tyndareus of Sparta, whilst immortal Pollux was fathered by Zeus (in another one of his guises – a swan, Cygnus).



Gemini is south of Auriga and the faint constellation of Lynx borders to the north east - just above Castor and Pollux. To the East is Cancer, to the west Taurus. Gemini also borders Monoceros and Orion which are both in the southwest. Gemini covers 514 square degress of sky with the fainter winter Milky Way running through its western side.




The two brightest stars of Gemini are Castor (α Gem) and Pollux (β Gem). These form a pair which is five degrees apart.


Castor, ‘the horseman’, is the more northerly and fainter of the two, it has an apparent magnitude of +1.59 and is a class-A star. In small telescopes Castor is easily identifiable as a close double star. It has components of +1.9 and +2.9 mag which are separated by three arc seconds. This pair have an orbital period of just 4.70 days and was at periastron (closest approach to each other) in 1965. The double nature of this star system was most likely observed by the 17th Century observer G.D. Cassini in Paris. Since then a third star has been identified to be part of the system, this is Cassini C which is a 9th magnitude star (+9.1 mag) and lies approximately 72 arc seconds away. All three components have also been shown to be spectroscopic binaries with Castor A and B being made of bright A-class stars and Castro C being made of a couple of red dwarfs.

Pollux, ‘the pugilist’, is a K-class star with a magnitude of +1.15. Since it is a K-class star it appears to be orange in colouration and this means Pollux has a relatively cool photosphere (surface of star) with a temperature of only 4500K. It is only 35 light years away from our star system.


Castor and Pollux mark the twins’ heads, with chains of stars extending southwards making up their bodies.


The feet of Pollux are marked by γ and ξ Geminorum, which are near the border with Monoceros. γ Gem, Alhena (Arabic for a mark on the neck of horse or a camel), is a class-A star with a magnitude of +1.9 and is approximately 82 light years away.


Midway between Alhena and Pollux is δ Gem, Wasat (Arabic for middle). It is a +3.53 F-class star which is 33 light years away. This is a binary star with a +8.2 companion at a distance of 5.8 arc seconds. Slight to the southwest of delta Gem is ζ Gem which is a luminous Cepheid variable which has a magnitude range of +4.4 to +5.2 over a 10.15 day period. This object is around 1650 light years away.


The body of Castor is marked by a line though ε Gem and his feet start at μ Gem.


Eta Gem is a strange star system. It is a spectroscopic binary with a red giant surrounded by a cloud of dust and gas. This star shows irregular variations of about 0.9 mag. It has an average magnitude of +3.28. Every eight years or so (this July coming) and then in August 2012 , the system drops by about half a magnitude due to eclipses by what is thought to be denser regions of the cloud.


Six degrees north of Pollux is U Gem, a cataclysmic variable. Its usually magnitude is +14.9 but it outbursts to as bright as +8.2 mag. This occurs over intervals of about 105 days. This system consists of a white dwarf and a G-class dwarf, with mass exchange between the two. As the material builds up the smaller partner increases in mass and so has more fuel and this leads to a ‘nuclear runaway’ reaction, thus increasing the luminosity produced by the system.


Objects in Gemini


Gemini is close to the plane of the Milky Way and so is home to a number of good open star clusters, most notably M35 (right).


M35 is well seen through binoculars and lies 2200 light years away. It is entangled at the southern edge with NGC 2158 (bottom right of image), which is much further away (13 000 light years) and is seen as a small concentration five arc minutes across is a rich star field, so to see this you require to have a decent telescope. Through a large telescope (200mm aperture) NGC 2158 can be resolved into individual stars. Its appearance is similar to a loose globular cluster and it is suggest that it is a globular cluster that has been gravitationally captured by our galaxy.


Another interesting object is the Eskimo Nebula (left), NGC 2392.


This was discovered by William Herschel in 1787, it is fairly bright with an overall magnitude of +9.2. It is 3 degrees west southwest of δ Gem.


In large telescopes it shows a bluish disc and shows some evidence of the outer ring structure. The central star is a hot O-class star of magnitude +10.5, 3000 light years away.






Object Table:




RA (J2000.0)



Open cluster

06h 01.0m

+23º 18’


Open cluster

06h 07.5m

+24º 06’

NGC2168 (M35)

Open cluster

06h 08.9m

+24º 20’


Variable star

06h 14.9m

+22º 30’


Variable star

06h 23.0m

+22º 30’


Variable star

07h 04.1m

+20º 34’


Double star

07h 20.1m

+21º 59’

Eskimo Nebula

Planetary nebula

07h 29.2m

+20º 55’

Castor (alpha)

Double star

07h 34.6m

+31º 53’


Variable star

07h 55.1m

+22º 00’




Gemini Map -

Gemini Photo -

Gemini Figures:


Eskimo Nebula: