Winter Constellation Spotlight: Gemini
January 30, 2025

With winter in full swing in the Northern Hemisphere, another prominent constellation we'll look at is Gemini, the Twins. Gemini is one of the 12 zodiac constellations and ranks 30th in size of the 88 constellations. Its neighbors in the winter sky include Auriga, Taurus, Orion, Canis Minor, Monoceros, Cancer, and Lynx. Gemini is one of the few constellations with two first-magnitude stars and home to notable objects, including open clusters, planetary nebulae, and even a neutron star! Let's get to know this famous set of twins adorning the winter sky.
Mythology
“Gemini” means “twins” in Latin, and astronomers have long associated the constellation with the mythological half-brothers Castor and Pollux. According to Greek myth, Castor and Pollux are not actually twins but share the same mother, Leda. Castor is mortal, the son of the Spartan king Tyndareus, while Pollux is immortal, the son of Zeus; they are also half-brothers to Helen of Troy. Inseparable and ever-ready for adventure, they join the Argonauts in the quest for the Golden Fleece. During the Battle of Labyrinth, Castor is killed, leaving Pollux distraught. He pleads with Zeus to share his immortality with Castor, and the god grants his wish by placing them both among the stars as the constellation Gemini.
How to Find Gemini
It’s easy to locate Gemini to the northeast of Orion. First, get oriented by finding Orion's three belt stars. From there, you can locate the bright stars Rigel (at Orion’s lower right) and Betelgeuse (at Orion’s upper left). Draw an imaginary line starting at Rigel through Betelgeuse. Keep following the line and it will guide you toward Gemini's two brightest stars, Pollux and Castor, which lay side by side.
Prominent Stars
Pollux
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Pollux, also known as Beta Geminorum, represents the left head of the twin brothers and is the brightest star in Gemini. This orange giant is approximately 34 light-years from Earth. It’s 10 times the diameter of the Sun and about 45 times more luminous. Pollux has an apparent visual magnitude of +1.14, making it the 17th-brightest star in the night sky. Fun fact: In 2006, astronomers discovered an exoplanet named Pollux b, formally named Thestias, orbiting Pollux. |
Castor
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Castor, also known as Alpha Geminorum, represents the right head of the twin brothers. Despite its name, “Alpha,” Castor is not the brightest but the second-brightest star in Gemini after Pollux. This white star is approximately 51 light years from Earth and 2.3 times the diameter of the Sun. Castor has an apparent visual magnitude of +1.58, making it the 23rd-brightest star in the night sky. Although it appears as a single star to the naked eye, Castor is part of a complex sextuple-star system—six individual stars gravitationally bound together and grouped into three binary pairs. |
Star Clusters
Messier 35
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Messier 35, or NGC 2168, is a popular wintertime open cluster located at the right foot of Gemini. This cluster is approximately 2,970 light-years away from Earth and has an apparent visual magnitude of +5.3—within naked-eye viewing from dark skies. The cluster has several hundred stars and is estimated to be 110 million years old. Messier 35 is sometimes called the “Shoe-Buckle Cluster” due to its appearance. |
NGC 2158
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An older and densely compact cluster of stars known as NGC 2158 lies southwest of M35 and is visible in the same field of view through low-power telescopes. It is approximately 11,000 light-years from Earth and is estimated to be 2 billion years old. With an apparent visual magnitude of +8.6, NGC 2158 is observable in binoculars and telescopes from a dark-sky site. |
NGC 2129
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NGC 2129 is a relatively young open cluster of stars estimated to be around 10 million years old. Two close B-type stars, HD 250289 and HD 250290, are likely part of a binary star system. With an apparent visual magnitude of +6.7, the cluster is visible in binoculars and small telescopes. It lies approximately 7,200 light-years away from Earth. |
NGC 2355
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NGC 2355 is a fascinating irregular open star cluster. Astronomers estimate it to be one billion years old, so it’s an ideal object for studying the evolution of stars. Located 5,400 light-years from Earth, it has an apparent visual magnitude of +9.7 and can be seen in binoculars and small telescopes. |
Nebulae
Eskimo Nebula
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William Herschel discovered the Eskimo Nebula, also known as NGC 2392 or Caldwell 39, on January 17, 1787. It is a planetary nebula that resembles a clown or a face surrounded by a fur parka hood. Its unique appearance comes from the intricate structure of gas and dust surrounding its central star. The nebula is currently transforming into a white dwarf star. The Eskimo Nebula is approximately 2,870 light-years from Earth. With an apparent visual magnitude of +9.1, you can see it through a telescope. |
Jellyfish Nebula
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The Jellyfish Nebula, also known as IC 443 or Sharpless 248, is a fascinating remnant from a supernova event that occurred between 3,000 and 30,000 years ago. It is approximately 5,000 light-years away from Earth and has a visual magnitude of +12. Head to a dark-sky site with a large telescope and an OIII filter to reveal its boomerang-shaped shell-like structure. Fun fact: The supernova explosion that created this nebula left behind a neutron star called CXOU J061705.3+222127, or J0617 for short. |
How to See the Geminids Meteor Shower
Geminids Meteor Shower
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The Geminid meteor shower is an annual fall event that is one of the most spectacular and reliable meteor showers, from November 19 to December 24, with its peak activity around December 13-14. Expect to see over 100 meteors per hour under ideal conditions. The Geminids are well known for their bright and colorful displays, often appearing as slow-moving fireballs in blue, white, or yellow colors. Although most meteor showers originate from debris left by comets, the Geminids are remnants of an asteroid, 3200 Phaethon. The radiant point of the Geminids lies within the Gemini constellation, near the stars Pollux and Castor, but you can see meteors across any part of the night sky during this shower. |
Let's Go Observing!
The winter sky is home to some of the most recognizable constellations, rich in mythology and striking star patterns. Among them, Gemini stands out with its brilliant twin stars, Castor and Pollux, serving as a celestial tribute to the legendary brothers and the mysteries that have captivated stargazers for centuries. Whether you're a dedicated astronomer or a casual observer, Gemini offers a wealth of celestial treasures to explore. So bundle up, share the view, and enjoy clear skies!
Clear skies and happy observing!
Other articles you might be interested in: Ultimate Guide to Observing the Universe