Gemini headerGemini star map

Gemini, one of the constellations of the Zodiac lying between Taurus and Cancer on the ecliptic, can currently be found in our Northern sky, just north east of the prominent Orion.

In Greek mythology, Castor and Pollux the two brightest stars of the constellation, were the offspring of Leda and Jupiter (or Zeus) who had seduced Leda in the guise of a swan.  (In other myths, Pollux was the son of Leda’s husband King Tyndareus and Castor the son of Jupiter).

Castor was famous for taming and managing horses while Pollux was know for his skill as a boxer.  The twins were inseparable.

The twins were part of the Argonautic expedition. During a violent sea storm Orpheus played his harp and prayed to the sea god Poseidon to save them.  The storm dispersed and stars appeared on the heads of Castor and Pollux.

They then became known as the protectors of voyagers and seamen.

Castor was later slain while at war.  Pollux,  who was immortal and in grief for his twin went to Jupiter begging to share his immortality with Castor.  Jupiter rewarded the attachment of the brothers by placing them amongst the stars as Gemini the twins.

Objects to observe:

Pollux: Beta (β) Geminorum is the brightest star in the constellation at 1.2 mag. An orange giant – bright golden/orange in colour 34ly away, 60 times as luminous as the sun.

Castor:  Alpha (α) Gem is a multiple star. Using the naked eye it should appear as a 1.6 mag blue-white star.  A small telescope of 60mm should resolve 2 to 3 components at high power.  The double with a separation of about 3 arc seconds of 1.9 / 3.0 mag are both binaries, too close to split any further with a small scope. I could just split the components at 79x with my 8mm Radian, increasing the magnification to 158x using a 2x Barlow they were easily split A third wide red dwarf companion should be seen with a small scope, this eclipsing binary varies between mag 9.3 and 9.8.

The twins are a good guide for measuring angular distances, they are exactly 4.5˚ apart.

Wasat:  Delta (δ), is also a double star 3.5/ 8.2 mag, a telescope larger than 75mm will be needed to split these.

Propus & Mekbuda: Eta (η) and zeta ( ζ ) are variable double stars.

M35 is a bright open cluster of mag. 5 northwest of Eta Gem. On a clear night it should be visible with the naked eye and is easily found with binoculars. It is most impressive at low magnification, my best observing was at 37 x with a 17mm Plossl. The 200 – 300 stars appear to form lines within the cluster. This open cluster is relatively nearby at 2800 light years distant and only 150 million years old it spreads out over a a volume of 30 light years.

On the southwestern edge of M35 lies NGC 2158. It is of similar size to M35, however is much fainter lying 4x further away. It can be seen as a fuzzy haze unless using >100mm aperature. This slightly denser open cluster carries approximately the same number of stars and is 10x older.

M35-NGC2158M35 on the upper left with NGC2158 on the lower right

The Geminid Meter Showers radiate from a point near Castor with maximum on December 13 – 14.ngc2392

NGC 2392: The most challenging and most rewarding object to search for in Gemini.  The Eskimo or Clown Face planetary nebula is 8.9 mag., 3000 ly away. Only large telescopes show the face and frill it gets the name from. Certainly a challenge with the 78mm.  It was found by using Delta and Lambda as markers and a small triangle of stars near delta (see map above). It was visible with 26mm and 17mm as a faint fuzzy green glow, averted vision is always best with these elusive characters.  No central star was visible.  Seeing didn’t allow higher magnification.  I hopped back onto Saturn which was brilliant 2 hours earlier and the views were shocking due to upper atmospheric turbulence. I hope to get better views of The Eskimo during our new moon this meeting night.


Sources: Myths of Greece and Rome, Thomas Pulfinch 1979; Collins Stars & Planets, Ridpath & Tirion 2000; Hartungs Astronomical Objects for Southern Telescopes, Malin & Frew1995.

Astronomy Picture of the day: 2013 July 30 2013 January 3