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, that is located 34 light-years distant.
Represents the Heavenly Twins
The constellation of Gemini represents the legendary twins Castor and Pollux, who were the offspring of Queen Leda of Sparta, the wife of King Tyndareus. Also known as the Dioscuri (“Sons of Zeus”), they featured in a number of prominent stories from Greek mythology, including joining Jason and the Argonauts on the quest to find the Golden Fleece, and taking part in the Calydonian Boar Hunt. They also led the Spartan army that helped retireve their sister Helen after she was abducted by Theseus, king of Attica, an event that took place a while before Paris eloped with Helen to Troy.
Shape: Resembles Parallel Lines
In terms of its shape, Gemini actually looks like two “human-like” figures holding hands, with its two brightest stars, Castor and Pollux, marking the positions of each of their heads. However, the outline of two young men might be less obvious from the southern hemisphere, from which vantage point their figures appear to standing on their heads.
Location: A Northern Constellation
Although Gemini is located in the northern hemisphere, this zodiac constellation lies along 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. 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. 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.
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), the constellation’s brightest star, is an orange giant (K0III) situated about 34 light years away with an apparent magnitude of 1.14. It is 10 times bigger than our sun, with twice its mass, and 32 times its luminosity. Pollux is also the 17th brightest star in the entire sky, and is sometimes referred to as “The Head of the Second Twin”, which derives from the Arabic phrase “Al-Ras al-Tau’am al-Mu’akhar”.
– Castor (Alpha Geminorum), the second most luminous star in Gemini, is a multiple system situated around 51 light years distant that shines with a combined apparent magnitude of 1.58. It appears as a single blue-white star to the naked eye, although the system is actually composed of three binary pairs gravitationally bound together, which have an orbital period of 467 years. Castor is the 44th most luminous star in the sky, and was known in Arabic culture as “The Head of the Foremost Twin,” or Al-Ras al-Taum al-Muqadim.
– Alhena (Gamma Geminorum), the third brightest star in Gemini, is a white subgiant (A1 IV) found 109 light years from our solar system that shines with an apparent magnitude of 1.915. It is around 3 times bigger and more massive than the Sun, with about 123 times its brightness. Amongst the Arabs, it was sometimes called Almeisan, meaning “the shining one.”
Other stars of interest in Gemini includes the red giant Tejat Posterior (“Back Foot”) which has a visual magnitude of 2.88; the triple star system Propus (“Forward Foot”) of magnitude 3.28; and the variable supergiant Mekbuda (Zeta Geminorum), whose brightness varies from between 3.68 and 4.16 over a period of 10.148 days. Gemini also contains the blue supergiant Mebsuta (“Outstretched Paw”), which is 840 light years away, has a visual magnitude of 3.06, and is around 150 times bigger than the Sun, 19 times more massive, and at least 8,500 times brighter.
Notable Objects: Open Clusters and Nebulae
-Eskimo Nebula (NGC 2392, Caldwell 39) 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 sized telescopes 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, with the nebula found southwestward of the luminous star Castor.
– Messier 35 (NGC 2168) is a magnitude 5.30 open cluster that is 24 light years across and located about 2,800 light years away. The star cluster covers about the same area of sky as the full Moon, and can be found at the “feet” of Castor.
Other deep-sky objects of interest in Gemini includes the Jellyfish Nebula, the Medusa Nebula, the neutron star Geminga, and the open clusters NGC 2129 and NGC 2355.
Meteor Showers: The Geminids
-The Geminids is the most prolific meteor shower of the year, and runs from December 4th to 17th, with a peak on the 13th/14th when 120–160 meteors per hour can be seen . The Geminids are visible from all over the globe, and are 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 meteor 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.
Planets: 9 Stars with 9 Planets
As of 2017, Gemini has nine stars with one planet each. One of them, the orange giant star Pollux, has a giant gas planet, Pollux b, that is about 2.3 times as massive as Jupiter, and orbits it once every 590 days.
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 King Tyndareus of Sparta and his wife Queen Leda. The constellation Cygnus represents the swan Zeus transformed himself into on his adulterous visit to Leda, and of the twins, Pollux was the son of Zeus, whereas Castor was the son of Leda’s husband, and was therefore mortal. Throughout their lives the twins were inseparable and had many legendary adventures together, including joining Jason and the Argonauts on their quest for the Golden Fleece. 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.