Gemini is one of the constellations of the zodiac. The 2nd century AD astronomer Ptolemy had listed Gemini as one of his 48 constellations. Gemini comes from the Latin word for “twins”, which the constellation represents, and is associated with the twins of Greek mythology, Castor and Pollux. The two most prominent stars in the constellation are also named for the twins. Locating the constellation is most easily done by finding Castor and Pollux east of the V shaped asterism of Taurus (the Hyades) and Orion’s belt.
Mythology and History
Castor and Pollux were known as the Great Twins to the ancient Babylonians. The twins were almost godlike in this mythology. They were known as ‘the one who has arisen from the underworld’ and ‘might king’ respectively.
Greek mythology also has Castor and Pollux as mighty twins. In some accounts Zeus seduces the Spartan queen Leda and she falls pregnant with Pollux. Leda simultaneously becomes pregnant with Castor by her husband Tyndareus, the Spartan king. It is uncertain who fathered the twins in different accounts.
The myth goes that after Castor is killed (because he was mortal), Pollux seeks Zeus’ hand in keeping them together in immortality, and they are transformed into the constellation Gemini. Castor and Pollux are also regarded as patron saints of sailors, appearing as St. Elmo’ fire to the seaman.
Historically, the constellation has been a place of scientific interest. William Herschel discovered Uranus on 13 March 1781, it was located near η Gem. This was not the last time the constellation was the location of a planetary discovery. Clyde Tombaugh discovered Pluto in 1930 while exposing a series of photographic plates centred on δ Gem. The constellation was also mentioned by Aristotle who observed Jupiter first in conjunction with and then occulting a star in Gemini. The observation is the earliest-known of its kind. The star is identified as 1 Geminorum, and the event dated as December 337 BC according to the suggestions of a study published in 1990.
There are many bright, naked eye stars in the constellation. However, there are fewer deep sky objects as Gemini lies away from the direction of the Milky Way.
When the German uranographer Johann Bayer assigned his stellar designations to the Gemini’s stars in 1603, he was uncertain of which was brighter between Castor and Pollux. He designated Alpha to Castor and Beta to Pollux. Bayer’s designations remain even though we know now that Pollux is the brightest star in Gemini.
- α Gem, more commonly called Castor, is the second brightest star in the constellation. It is part of a sextuple star system that appears to the naked eye as a magnitude –1.6 blue-white star. It lies 52 light years from Earth and includes binary stars. The system hosts a spectroscopic binary with stars of magnitude 1.9 and 3.0 and a period of 470 years. A spectroscopic binary is detected by the Doppler Effect on the light the stars emit. Scientists can see this effect by studying the spectral lines of the stars, and observing the stars’ spectral lines shift from blue to red as the stars move toward and away from us respectively.
The sextuplet system also contains an Algol type eclipsing binary: an early-type main sequence primary star paired with a cooler, less massive giant star.
- β Gem (Beta Gem) is more commonly called Pollux. It is the brightest star in Gemini at magnitude 1.14. The star is 32 light years away and has an extrasolar planet in its orbit. Two other stars in the constellation that host exo-planets are HD 50554 dnd HD 59686.
- γ Gem, or Alhena, lies 105 light years from Earth. It is a blue-white star of magnitude 1.9.
- δ Gem, also called Wasat, is part of a binary system 59 light years away. The primary is a white magnitude 3.5 star accompanied by a dim orange dwarf star of magnitude 8.2. A telescope of medium aperture will resolve the pair.
- ε Gem is also called Mebsuta. This double star’s primary lies 900 light years from Earth; a supergiant of magnitude 3.1. Its optical companion is magnitude 9.6 and is visible through binoculars.
- Ζ Gem is also known as Mekbuda. It is another double star within Gemini’s borders. The primary is a Cepheid variable star, with a minimum and maximum magnitude of 4.2 and 3.6 respectively, and a period of 10.2 days. Mekbuda, a yellow supergiant 220 000 times the size of the Sun, lies 1200 light years away from our planet. Its companion, a magnitude 7.6 star, is easily visible in binoculars.
- η Gem, common name Propus, is a binary with a variable component as the primary star. The system is only divisible in larger telescopes. Propus is located 350 light years away from Earth. The primary has a minimum magnitude of 3.9 and a maximum magnitude of 3.1. The secondary is a magnitude 6.0 star.
- The binary stars in Gemini keep coming. κ Gem is another such system which lies 143 light years from Earth. The primary has a magnitude of 3.6 and the secondary is magnitude 8. The pair can be separated with larger telescopes.
- Amateurs with binoculars or telescopes with small to modest apertures will appreciate ν Gem. The binary is easy to observe with small instruments. It consists of a magnitude 4.1 blue giant primary star, paired with a magnitude 8 secondary. Stargazers can also sweep over to 38 Gem, another binary easily separated in smaller instruments. It is located 84 light years away from Earth.
- U Gem is a cataclysmic variable star which is just as chaotic as it sounds. This is a close binary star system where one component is a white dwarf that steadily grows in mass as it steals the matter of its donor companion. The tumultuous relationship can trigger a Type Ia supernova explosion which totally annihilates the white dwarf.
- Mu Gem makes up Castors foot, so its traditional name is Tejat Posterior which translates to “back foot”.
Compared to other constellation in the zodiac, Gemini’s deep sky objects are sparse as its location along the zodiac is directed away from the Milky Way.
- M35 (NGC 2168) is a magnitude 5 open cluster easily visible to the naked eye under dark skies, It is also a great binocular target. M35 was first discovered by Swiss astronomer Philippe Loys de Chéseaux in 1745. Its elongated body covers approximately 0.2 square degrees of the sky, the same as the full Moon. The 200 star cluster is located 2800 light years from Earth. The Medusa Nebula is another planetary nebula in Gemini.
- Amateurs with large aperture telescopes can get a rich view of open cluster NGC 2158, 12000 light years from Earth.
- NGC 2392, also called the Eskimo Nebula or Clown Face Nebula because of its blue-green elliptical disk, is a planetary nebula with an overall magnitude of 9.2. The 10th magnitude central star is visible through a modest telescope.
- Other open clusters in the constellation are NGC 2129, NGC2266, NGC2331, NGC2355, and NGC2395.
The Epsilon Geminids peak between the 18th and 29th of October. They overlap with the Orionids and are difficult to detect visually. They were only recently confirmed due to these factors. Worry not: one of the best meteor showers of the year is the Geminids which peak on 13th – 14th December for 2018.
The 2018 Geminids also fall under ideal conditions, so barring unfortunate weather, they may be of the best yet and have the potential to rain down a flurry of meteors. The waxing crescent Moon also will not interfere with the overall peak, especially after it sets.
The Geminids typically have a maximum rate of 100 meteors hourly, setting it apart as one of the richest showers annually. The Geminids have been growing in intensity every year, with recent showers hitting a maximum of 120 – 160 under perfect conditions. The shower is reliable and spectacular, sometimes even beating out August’s Perseids. North America and the Pacific Basin are prime locations for this year’s Geminids, where observers can expect 100 meteors an hour on both dates.
The peak of shower appears later and less intense in southern hemisphere, and Gemini lies north of the celestial equator. Though the meteors may appear from all over the sky, the majority appear to originate from the constellation Gemini, and this is where they are most concentrated. The meteors travel at a medium speed of 35 kilometre per hour (22 miles per hour) and usually appear yellow.
All but two meteor showers originate from comets. Gemini is one these showers; believed to originate from the object 3200 Phaethon, an odd asteroid-comet hybrid.
Despite lying away from the Milky Way and its indistinct pattern, Gemini has always been a recognized constellation, featuring celestial treasures and having a marked place in ancient astronomy and modern day discoveries alike.
Keep following our constellation series for an intriguing and deeper look at the zodiac.