Gemini is the most northern of all the zodiac constellations, and actually resembles “the twins” it depicts, with its two brightest stars marking their heads, and its fainter stars delineating their bodies. It is the 30th largest constellation in the entire night sky, with its brightest star, Pollux, an orange giant star of magnitude 1.2, and located 34 light-years distant.
Location: A Northern Constellation
Although Gemini is located in the northern hemisphere, it is bisected by the ecliptic, which makes it visible to observers in much of the southern hemisphere as well. Consequently, Gemini is visible to anyone located between +90° and -60° of latitude, and is easily found to the northeast of neighboring Orion, between Taurus to the west and Cancer to the east, with Canis Minor lying to its south.
Best Seen: Winter/Spring
Being the most northern of all the zodiac constellations, Gemini is easily seen overhead during the winter months for Northern Hemisphere observers, or in the summer if you live in the Southern Hemisphere. Note that Gemini is lost in the Sun’s glare during late spring/early summer in the northern hemisphere and during late autumn/early winter in the southern hemisphere. The constellation reaches its highest point above the horizon at about 10 PM Local Time on February 9th, regardless of the observer’s position on Earth.
Represents the Heavenly Twins
The constellation represents the “Dioscuri”, also known as Castor and Pollux. These legendary twins are featured in a number of prominent stories from Greek mythology, including the quest to find the Golden Fleece by Jason and the Argonauts, as well as The Siege of Troy.
Shape: Resembles Parallel Lines
In terms of its shape, Gemini actually looks like two “human-like” figures holding “hands”. The two brightest stars in the constellation, Castor and Pollux mark the position of each figures’ head. However, the figures of two young men might be less obvious from the southern hemisphere, from which vantage point the constellation stands on its “head(s)”.
Notable Stars: Pollux (1st Magnitude)
What Gemini lacks in deep sky objects, it makes up for with bright, massive stars, some of which are described below.
– Pollux (Beta Geminorum) is a class K0III, magnitude 1.14 orange giant located about 34 light years away. It is the most luminous star in Gemini, and the 17th most luminous in the entire sky. It is also twice as massive as the Sun, and nine times as big. It is sometimes referred to as “The Head of the Second Twin”, which derives from the Arabic “Al-Ras al-Tau’am al-Mu’akhar”.
– Castor (Alpha Geminorum) is the second most luminous star in Gemini, and the 44th most luminous in the sky. It is blue-white in appearance, 51 light years away and actually consists of six stars gravitationally bound together, which together have a combined apparent magnitude of 1.58. The system has an orbital period of around 467 years.
– Mebsuta (Epsilon Geminorum) is a class G8 IB supergiant with an apparent visual magnitude of 3.06, which at its distance of about 840 light years, makes it at least 8,500 times as bright as the Sun. It is also 19 times as massive as the Sun, and between 105 and 175 times as big. Lying close to the ecliptic, Mebsuta, which means “the outstretched paw” in Arabic, can be occulted by the Moon, and sometimes by the planets, albeit very rarely.
– Mekbuda (Zeta Geminorum) is a supergiant, Population I Cepheid, with an intermediate luminosity classification. Stars of this class can be anything between 4 and 20 times as massive as the Sun, and up to 100,000 times as bright. Mekbuda falls neatly into this class, being 2,900 times as bright as the Sun. Its luminosity, which is a function of its mass, varies between 3.68 and 4.16 over a period of 10.148 days.
Notable Objects: Open Clusters and Nebulae
– Messier 35 (NGC 2168) is a magnitude 5.30 open cluster located about 2,800 light years away. The cluster covers about the same area of sky as the full Moon, and can be found at the “feet” of Castor.
-Eskimo Nebula (NGC 2392, Caldwell 39) (NGC 2392) is a spectacular planetary nebula, and the first object photographed by the Hubble Space Telescope after a successful servicing mission in 1999. It is a bi-polar, double-shell nebula with the remains of the progenitor star clearly visible near the center of the structure. The name “Eskimo Nebula” derives from the fact that the nebula resembles a face enclosed by a fur-collared parka. The Eskimo Nebula lies about 2,870 light years away, and with an apparent visual magnitude of 10.1, is an easy target even for modest amateur equipment in dark skies.
– NGC 2371-2 is a dual-lobed planetary nebula, which appears as if it is two separate objects, which is why it has two designations in the New General catalogue; NGC 2371 and NGC 2372. Located about 4,400 light years away, its apparent magnitude of 13 makes it an easy target in medium-sized amateur equipment. Look for the nebula towards the southwestward of the luminous star Castor.
Planets: 9 Stars with 9 Planets
All told, Gemini has nine stars with one planet each. One star, Pollux, has a giant gas planet about 2.3 times as massive as Jupiter. The planet, named Pollux b, orbits Pollux once every 590 days.
Meteor Showers: The Geminids
-The Geminids is the most prolific meteor shower of the year, and runs from December 4th to December 17th, and with a peak of about 120–160 meteors per hour expected during its most active period on December 13th/14th. The Geminids are visible from all over the globe, and are also a slow meteor shower, which have the ability to penetrate deep and burn up low in the Earth’s atmosphere. As a result, beautiful long arcs are created across the night sky, many lasting a second or two. Unlike most other meteor showers that originate from cometary debris, the Geminids are associated with the asteroid 3200 Phaeton.
– The Rho Geminids is a singularly unspectacular shower that runs from December 28th to January 28th, with a maximum of fewer than 8 meteors per hour on the peak that falls on January 8th.
In astrology the Sun is said to be in the sign of Gemini from May 21st to June 21st. Due to precession, however, in astronomy the Sun currently passes in front of the constellation Gemini from June 21st to July 20th, which is almost exactly a month later. Other astrological associations are:
Date of Birth: May 21 to June 21
Sign Ruler: Mercury
Birth Stone: Agate, sapphire
Characteristics: Perspicacious, smart, cheerful, quick-witted, charming
Compatibility: Aquarius, Libra
Mythology of the Inseparable Twins
In Greek mythology, Pollux and Castor were the sons of Zeus and the mortal Queen Leda. The constellation Cygnus represents the swan Zeus transformed himself into on his adulterous visit to Leda, the king of Sparta’s wife. Of the twins, Pollux was the son of Zeus and immortal whereas Castor was the son of Leda’s husband, and was mortal.
Throughout their lives the twins were inseparable and had many legendary adventures together including The Siege of Troy, and joining Jason and the Argonauts. Eventually Castor died and Pollux unable to be parted from his brother asked Zeus to make him mortal too. Zeus refused but when Pollux followed his brother into the underworld Zeus was so impressed by his devotion that he granted him the request on the condition that each could only spend half of the year on Olympus; the other half was to be spent in the Underworld. Zeus also moved them both into the heavens as the constellation Gemini.