Gemini Constellation: Facts About the Twins (2024)

The northern-sky constellation of Gemini is one of the original constellations listed by Ptolemy in the second century, as well as one of the 12 zodiac constellations.

The word "gemini" means twins in Latin; the constellation is associated with the Greek mythological twins Castor and Pollux, for whom two of the stars in the constellation are named, according to Britannica. (Castor is not actually a single star, but a system of six stars that are so close together, they appear as one.)

In other ancient astrological traditions, the twins are represented by pairs of goats or peaco*cks, or for other famous mythological twins like Remus and Romulus.

Gemini is also known for lending its name to NASA's Gemini program, so-named for its two-person spacecraft, as well as for its Geminga pulsar, unique for emitting primarily gamma rays instead of X-rays or visible light, according to NASA.

Beyond Castor and Pollux, there are eight other named stars in the Gemini constellation, according to the skywatching site Constellation Guide, and numerous deep-sky objects to observe through telescopes. Plus, the radiant point of the winter Geminid meteor shower is in the constellation.

Gemini observing targets

Gemini is full of targets for observation, both "near" and far. If you're looking to gear up for a stargazing session, read our guides to the best telescopes and best binoculars.

Related: Night sky: What you can see tonight [maps]

Jargon buster

Magnitude: Magnitude is a measure of how bright an object appears from Earth; the lower an object's magnitude, the brighter it is. Magnitude can even be measured in negative numbers, which indicate higher levels of brightness than positive numbers.

Right ascension (RA): Right ascension can be considered the sky's version of longitude on Earth. It covers the east-west direction and is measured in hours, minutes and seconds. It's used in conjunction with declination to help astronomers locate celestial objects.

Declination (Dec): Declination is comparable to latitude on Earth. It measures an object's location in the north-south direction in degrees, arcminutes and arcseconds (there are 60 arcseconds in an arcmin, 60 arcmins in a degree).

Castor

Using a telescope, you can zoom in on Castor to distinctly see at least two, sometimes three, of the six stars that make up the system, according to the Milwaukee Astronomical Society. Impressively, each of these three stars is actually a double star, though the individual stars are difficult to discern without a very high-powered telescope.

Messier 35 (NGC 2168)

Messier 35 is an open cluster of more than 500 stars that are loosely gravitationally bound. The cluster takes up an area of the sky that's about the same size as the full moon, according to the European Southern Observatory.

Medusa Nebula (Abell 21)

The faint Medusa Nebula was discovered by astronomer George O. Abell in 1955, according to Astronomy Trek. The planetary nebula, which formed when a red giant star transitioned into a white dwarf, is approximately four light-years across.

NGC 2392

NGC 2392 has a fantastic spherical shape. The planetary nebula is known by several nicknames, the most famous of which is no longer used due to its discriminatory nature; you can read more about the nickname in a NASA statement.

Jellyfish Nebula (IC 443)

The Jellyfish Nebula is a 70-light-year-wide remnant of a supernova that is estimated to be between 3,000 and 30,000 years old, per Astronomy Trek.

Geminid meteor shower

The Geminid meteor shower is one of the most prolific meteor showers of the year, with a rate of 120 meteors per hour during its peak, according to NASA. The shower occurs between Nov. 19 and Dec. 24 with a peak around Dec. 14. Though the shower originates from the debris trail of asteroid 3200 Phaethon, its radiant point (that is, the point from which the meteors appear to emanate), is in the constellation Gemini.

Mythology

Castor and Polydeuces, or Pollux in Latin, were twin brothers in Greek mythology born to Queen Leda of Sparta and two fathers: King Tyndareus of Sparta and the god Zeus. As such, Castor was mortal and Pollux was immortal, according to the Chandra X-ray Observatory. The inseparable duo was known as the Dioscuri. During a familial dispute (or perhaps in battle), Castor was killed, but Pollux shared his immortality with his brother so that they could be together forever.

Gemini constellation FAQs answered by an expert

We asked Mike Reed, an astronomer and a distinguished professor in the Department of Physics, Astronomy and Materials at Missouri State University, a few frequently asked questions about the Gemini constellation.

Dr. Mike Reed

Distinguished Professor in the Department of Physics, Astronomy and Materials at Missouri State University.

Additional resources

Read the story behind the names of the Gemini twins from the Chandra X-Ray Observatory. Learn about the twin stars Castor and Pollux from NASA. Watch a video about the Geminid meteor shower by the Museum of Science, Boston.

Bibliography

Deep-Sky Objects in Gemini. Astronomy Trek. Retrieved July 9, 2023, from https://www.astronomytrek.com/deep-sky-objects-in-gemini/

Geminga. Britannica. Retrieved July 9, 2023, from https://www.britannica.com/place/Geminga

Gemini. Britannica. Retrieved July 9, 2023, from https://www.britannica.com/place/Gemini-constellation-and-astrological-sign

Gemini Constellation. Constellation Guide. Retrieved July 9, 2023, from https://www.constellation-guide.com/constellation-list/gemini-constellation/

Geminids. NASA. Retrieved July 9, 2023, from https://solarsystem.nasa.gov/asteroids-comets-and-meteors/meteors-and-meteorites/geminids/in-depth/

Messier 35 – A cluster of stars. European Southern Observatory. Retrieved July 9, 2023, from https://www.eso.org/public/italy/images/b10/?lang

NASA to Reexamine Nicknames for Cosmic Objects. NASA. Retrieved July 9, 2023, from https://www.nasa.gov/feature/nasa-to-reexamine-nicknames-for-cosmic-objects

Stargazing Gemini the Twins. Milwaukee Astronomical Society. Retrieved July 9, 2023, from https://milwaukeeastro.org/Stargazing/gemini.asp