Gemini is the northernmost of all the zodiac constellations, and it actually resembles the “twins” it represents, with its two brightest stars marking their heads and its fainter stars outlining their bodies. It is the 30th largest constellation in the night sky, with its brightest star, Pollux, a giant orange star of magnitude 1.2, located 34 light-years away. Learn all about the constellation Gemini in this post.
When and how can I see the constellation Gemini?
You have many months during the year to observe Gemini, which is one of the brightest constellations in the zodiac. The constellation Gemini lights up the evening sky from January to May, although it will be located in the west two hours earlier with each passing month. As seen from mid-northern latitudes , for example, Gemini rises above the western horizon around 5 a.m. in early February, 3 a.m. in early March, 1 a.m. (2 a.m. daylight saving time) in early April, and 11 p.m. (midnight) daylight saving time in early May.
January and February are a great time to observe the constellation Gemini because it is well in the east at dusk and remains out for most of the night. Gemini rises higher in the sky around 10 p.m. in early February and 9 p.m. in late February. That is local time, the time on your clock, no matter where you live in the world.
Gemini remains visible in the evening sky until May. By late May and June, Gemini can be found in the west-northwest corner of the sky at dusk, and the two brightest stars in Gemini, Castor and Pollux, disappear at sunset around the summer solstice on June 21. Gemini is best identified by its two brightest stars, Castor and Pollux. These two are notable for being bright and close together in the dome of the sky. Like brothers!
- Castor is six stars in one
- Pollux is the brightest of two twin stars.
Incidentally, Gemini and the brightest star in the night sky, Sirius, reach the peak of their nighttime journeys at about the same time. In the mid-latitudes of the northern hemisphere, we see the brightest stars of Gemini, Castor and Pollux, almost overhead while Sirius shines quite low in our southern sky. South of the equator, it’s the opposite: Sirius shines high overhead while Gemini sits low in the northern sky.
The sun passes in front of the constellation Gemini, the Twins, every year from approximately June 21 to July 20. To remember these dates, keep in mind that the sun enters Gemini just after the June solstice and remains within Gemini for the following month. Gemini cannot be seen in late spring or early summer in the northern hemisphere, or in late fall or early winter in the southern hemisphere, because that is when the celestial Twins are lost in the glare of the sun.
Main stars of the Gemini constellation
Castor – α Geminorum (Alpha Geminorum)
Castor is the second brightest star in Gemini and the 44th brightest star in the sky. It is a visual binary with a combined apparent magnitude of 1.58. The two components are separated by 6″ and have a period of revolution of approximately 467 years.
Each component of the Alpha Geminorum system is itself a spectroscopic binary star, making Castor a four-star system. Castor also has a faint companion about 72″ away, an eclipsing binary star system with a period of just under a day.
Both components of the system are red dwarfs (class M). This makes Castor a sextuple star system, as its six components are gravitationally bound. The third component is classified as a variable star and has the designation YY Geminorum.
The primary component belongs to spectral class A1 V, and its companion is believed to be of spectral type M5 V. The stars of the secondary component have stellar classifications A2 Vm and M2 V. Castor is approximately 51 light-years away from the sun. In Arabic culture, it was known as “The Chief of the More Important Twin” or Al-Ras al-Taum al-Muqadim.
Pollux – β Geminorum (Beta Geminorum)
Pollux is the brightest star in Gemini and the 17th brightest star in the night sky. It is an evolved orange giant with a stellar classification of K0 III. It has an apparent magnitude of 1.14 and is 33.78 light-years away from the solar system.
Beta Geminorum has twice the mass of the Sun and approximately nine times the solar radius. It is sometimes referred to as “The Head of the Second Twin,” from the Arabic Al-Ras al-Tau’am al-Mu’akhar. An extrasolar planet was confirmed to be orbiting the star in June 2006. The planet, Pollux b, has a mass at least 2.3 times that of Jupiter and an orbital period of 590 days.
Alhena (Almeisan) γ Geminorum (Gamma Geminorum)
Gamma Geminorum is another bright star in Gemini. With an apparent magnitude of 1.915, it is easily visible to the naked eye. The star is approximately 109 light-years from Earth. It is a white subgiant star with a spectral classification of A1 IV, has 2.8 solar masses, and is 3.3 times the radius of the Sun. It is approximately 123 times more luminous than the Sun.
The traditional name for the star, Alhenam, comes from the Arabic Al Han’ah, meaning “the mark,” in reference to the mark on a camel’s neck. It is also sometimes known as Almeisan, which is derived from the Arabic Al Maisan, meaning “the shining one.”
Mebsuta – ε Geminorum (Epsilon Geminorum)
Epsilon Geminorum marks the extended right leg of Castor. It is a supergiant belonging to the spectral class G8 Ib. It has an apparent magnitude of 3.06 and is approximately 840 light-years away.
The star is approximately 8,500 times brighter than the Sun and has 19 times the solar mass. The star’s radius is about 105-175 times that of the Sun. The traditional name for Epsilon Geminorum, Mebsuta, comes from the Arabic Mabsūṭah, meaning “the extended paw.” In Arabic culture, Epsilon and Zeta Geminorum represented the paws of a lion. Epsilon Geminorum is also sometimes called Melboula or Melucta.
The star is located near the ecliptic and can be obscured by the Moon and planets, eta geminorum, mu geminorum, medusa nebula
Tejat Posterior – μ Geminorum (Mu Geminorum)
Mu Geminorum is the fourth brightest star in the constellation Gemini. It has a visual magnitude of 2.857 and is about 230 light-years away. The star belongs to spectral class M3 III, which means it is a red giant. It is also classified as a slow and irregular variable of type LB. It exhibits variations in brightness between magnitudes 2.75 and 3.02 over a period of 72 days, and has a long-term variation period of 2,000 days.
The traditional name of the star, Tejat Posterior, means “the hind foot,” and refers to the foot of Castor. It is also sometimes known by another Latin name, Calx, meaning “the heel.”
Tejat Prior – η Geminorum (Eta Geminorum)
Eta Geminorum is a multiple star approximately 350 light-years from Earth. It consists of three components, a spectroscopic binary star and a G0-class dwarf star orbiting in pair with a period of over 700 years.
The primary component of the binary system is a semi-regular variable star that exhibits variations in brightness over a period of 234 days. It is a red giant with the spectral classification M3IIIlab. Its brightness ranges between magnitudes 3.15 and 3.9. The secondary star is of spectral type B and orbits the red giant with a period of 8.2 years.
Eta Geminorum is located near the ecliptic and can occasionally be occulted by the Moon and very rarely by a planet.
Alzirr – ξ Geminorum (Xi Geminorum)
Xi Geminorum is a yellow-white subgiant star belonging to spectral class F5 IV, approximately 11 times brighter than the Sun. It has an apparent magnitude of 3.35 and is 58.7 light-years away from Earth. The star is a fast rotator, with a projected rotation speed of 66 km/s. It is suspected to be a spectroscopic binary.
The star has a proper name, Alzirr, which means “the button” in Arabic. It marks one of the four feet of the Gemini twins. It is bright enough to be seen without binoculars.
Wasat – δ Geminorum (Delta Geminorum)
Delta Geminorum is also bright enough to be visible to the naked eye. It has a visual magnitude of 3.53 and is about 60.5 light-years away. It has the stellar classification F0 IV, meaning it is a yellow-white subgiant star. The traditional name for the star, Wasat, means “middle” in Arabic. The star is also sometimes known by its Chinese name, Ta Tsun.
Wasat is located two tenths of a degree south of the ecliptic and is sometimes occulted by the Moon and much less frequently by planets. In 1930, American astronomer Clyde Tombaugh discovered Pluto just 0.5° east of the star.
Wasat is a very fast rotator, with a projected rotation speed of 129.7 km/s. It is believed to be around 1.6 billion years old. In 1.1 billion years, the star will reach 6.7 light-years from the Sun.
Delta Geminorum is actually a three-star system. The inner stars in the system form a spectroscopic binary with components orbiting each other with a period of 6.1 years. The binary system has a K-class companion orbiting the primary component with a period of 1,200 years and can be seen in a small telescope.
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κ Geminorum (Kappa Geminorum)
Kappa Geminorum is another multiple star in Gemini. It has a stellar classification of G8 IIIa and is approximately 143 light-years away from the solar system. The star is 78 times brighter than the Sun. It has an apparent magnitude of 3.57.
λ Geminorum (Lambda Geminorum)
Lambda Geminorum belongs to the spectral class A3 V and is classified as a variable star. It has an apparent magnitude of 3.58 and is 94.3 light-years away. It is 28 times brighter than the Sun, has 2.8 times the solar radius, and 2.1 times the mass of the Sun. The estimated age of the star is around 800 million years.
Propus – ι Geminorum (Iota Geminorum)
Iota Geminorum is a fourth-magnitude star belonging to the spectral class G9III. It has an apparent magnitude of 3.78 and is approximately 326 light-years away. The star’s proper name, Propus, means “the forefoot” in Latin. Propus is classified as a variable star.
Mekbuda – ζ Geminorum (Zeta Geminorum)
Mekbuda, Zeta Geminorum, is an intermediate-luminosity supergiant and a variable star classified as a Classical Cepheid, or Population I Cepheid, named after Delta Cephei, the star that serves as the prototype for this class of variables. Delta Cephei variables are generally 4-20 times more massive than the Sun and can be up to 100,000 times more luminous.
They are usually yellow supergiants belonging to spectral class F6-K2 and exhibit variations in brightness as a result of pulsation periods lasting from a few days to months.
Zeta Geminorum, like other stars in this class, has a regular pulsation frequency, determined by the mass of the star. Its brightness varies over a period of 10,148 days and ranges from magnitude 3.68 to 4.16. The star is about 2,900 times brighter than the Sun.
The star’s proper name, Mekbuda, comes from ancient Arabic and means “the lion’s bent leg.”
τ Geminorum (Tau Geminorum)
Tau Geminorum is an orange giant with the stellar classification K2 III. It has an apparent magnitude of 4.42 and is approximately 321 light-years away from the solar system. The star can be seen without binoculars under good conditions.
Tau Geminorum is twice the mass of the Sun and 27 times its radius. It is about 224 times more luminous.
The star has a brown dwarf companion, Tau Geminorum b, with 18.1 Jupiter masses. Discovered in 2004, the dwarf star takes 305 days to complete one orbit around the primary star.
U Geminorum
U Geminorum is a dwarf nova in Gemini. It is a binary star composed of a white dwarf orbiting closely around a red dwarf star. Approximately every 100 days, the star has an outburst, causing a dramatic increase in brightness. The system was discovered by English astronomer John Russell Hind in 1855, when the star had one of its outbursts.
The two dwarfs have an orbital period of 4 hours and 11 minutes, which is very short and the main reason why U Geminorum is a variable star. The stars eclipse each other with each revolution. The system usually has an apparent magnitude ranging from 14.0 to 15.1, but during an outburst, the system can reach magnitude 9, making it 100 times brighter. The period is very irregular, ranging from 62 to 257 days. U Geminorum is approximately 400 light-years away from the Sun.
Use the moon to find Gemini
As the moon circles through the constellations of the zodiac, it passes through Gemini for a few days each month. Look at the sky chart above to see the ecliptic as a dotted red line crossing Gemini. The ecliptic marks the path of the sun as it travels eastward (from right to left) between the two bright stars of Gemini, Castor and Pollux, and the bright star Procyon in Canis Minor, the lesser dog.
While the sun passes eastward through Gemini for about a month each year, the moon travels eastward through Gemini for a few days each month. Because the moon stays within about 5 or (10 moon diameters) of the ecliptic, the sun’s path through the stars, the moon travels south of Castor and Pollux, and north of Procyon as it moves through Gemini.
In cycles of 18.6 years, the moon reaches its northernmost point on the ecliptic in Taurus and simply loses the occultation, which covers the star Pollux. In ancient times, when Pollux was a little closer to the ecliptic, the moon used to occult, covering this star in the centuries before Christ.
Mythology of the celestial twins
The constellation Gemini represents the twins Castor and Polydeuces in Greek mythology. The constellation Gemini is associated with the Greek mythological characters Castor and Pollux. They were twin brothers born of the same mother but different fathers.
The king of Sparta was Castor’s father, but the Greek god Zeus was Pollux’s father. In the story, their mother, the queen of Sparta, was raped by Zeus while he was disguised as a swan. Castor and Pollux are said to have been born from an egg along with two sisters, one of whom was Helen of Troy. Later, Leda also became pregnant with Castor and Clytemnestra (who would later marry Agamemnon and eventually murder him and be killed by her own son, Orestes). Castor and Clytemnestra were fathered by Tyndareus and, unlike the children of Zeus, were mortal.
Castor and Pollux grew up together and were very close. Castor was an excellent horseman and skilled swordsman, said to have taught Heracles how to fence, and Polydeuces was famous for his boxing skills. The two were part of the Argonauts’ expedition to obtain the Golden Fleece. Polydeuces’ boxing skills came in handy when Amycus, a son of Poseidon who ruled Asia Minor, refused to let the Argonauts leave until one of them fought him in a boxing match. Polydeuces accepted the challenge and won easily.
The twins came to the rescue of the crew on several occasions. They are known as the patron saints of sailors, and it is said that they were given the power to rescue shipwrecked sailors by the sea god Poseidon himself, who also gave them two white horses, which the twins often rode.
The Dioscuri are associated with St. Elmo’s fire, an electrical phenomenon that occurs during thunderstorms, when a coronal discharge from a pointed object in a strong electric field creates a luminous plasma. The phenomenon is named after another patron saint of sailors, Saint Erasmus of Formiae. Saint Elmo’s fire would appear to sailors as a bright ball of light during thunderstorms, and they considered it a sign that their guardian saint was with them.
Castor and Pollux eventually clashed with Idas and Lynceus, who were also twins and former Argonauts, over two women, Phoebe and Hilaira. The other two brothers became engaged to them, and Castor and Pollux took the women away. Idas and Lynceus pursued them, and eventually there was a fight between the four. Lynceus stabbed Castor with a sword, and when Polydeuces saw this, he killed Lynceus. When Idas saw his brother die, he attacked Polydeuces, but Zeus intervened and sent a thunderbolt that saved his son. Polydeuces asked Zeus to share his immortality with his dead brother, and the god placed them both in the sky, where they remain inseparable as the constellation Gemini.
The two brightest stars in the constellation, Alpha and Beta Geminorum, mark the heads of the twins.
Not everyone identified the constellation as Castor and Polydeuces in ancient times. Hyginus and Ptolemy associated the two stars with Apollo and Heracles, who were half-brothers and both sons of Zeus.
