Gemini Constellation Guide – Twins in the Night Sky

In this article, we’ll be sharing with you everything you need to know about the constellation of Gemini, including how to find it, deep space objects contained within it, and how to use it to find other night sky objects more easily.

The Constellation of Gemini

This constellation was first cataloged by Ptolemy more than 2000 years ago. Gemini means ‘the twins’ in Latin.

In Greek mythology, this constellation represents the twins Castor and Pollux. Castor and Pollux were half-brothers who shared the same mother, Queen Leda of Sparta.

Castor was the mortal son of Tyndareus (king of Sparta) and Pollux was the immortal son of Zeus. In some versions of the myth, Queen Leda and King Tyndareus’ twins were Pollux and Helen (who would become Helen of Troy), while Queen Leda and Zeus’ twins were Castor and Clytemnestra (who would marry Agamemnon).

Castor and Pollux were very close and both were skilled athletes who joined Jason and the Argonauts’ mission to retrieve the Golden Fleece (which is part of the myth behind the constellation of Aries).

Myths vary regarding the exact circumstances, but Castor was killed by another man. Pollux was grief-stricken and asked Zeus to share his immortality with his dead brother and the two were placed in the sky together.

Today, Castor and Pollux are two of the most well-known stars in the northern hemisphere’s night sky.

To help you spot Gemini, here’s what SkySafari 6 shows for the twins Castor and Pollux. Note the stars of the same name in the heads of the twins.

The mythical figures of Castor and Pollux.
An image of Castor and Pollux. Click for full-screen.

Gemini has an area of 514 square degrees making it the 30th largest of the 88 recognized constellations. This constellation resembles two stick figures holding hands.

If you’ve observed the annual Geminid meteor shower, which peaks in mid-December, you may recognize this constellation as the location of its radiant.

In the next section, we’ll discover how to find Gemini.

How To Find Gemini In The Night Sky

Gemini is part of the zodiac family of constellations, which are those constellations that the Sun appears to travel through over a year.

Shows the route the Sun appears to take against the constellations. The ones it passes through are zodiac constellations.
Zodiac constellations are ones the Sun travels through (yellow line), note Gemini at the 3 o’clock position. Click for full-screen

The twins constellation is visible to observers at latitudes between +90° and -60°. Northern Hemisphere observers can see Gemini from November to May, while Southern Hemisphere observers can see it during their summer months.

In the mid-latitudes of the northern hemisphere, Gemini is practically overhead in March evenings.

The constellation of Gemini is bordered by the constellations Auriga, Cancer, Canis Minor, Lynx, Monoceros, Orion, and Taurus. Each of the bordering constellations is shown in the SkySafari 6 image below.

Constellations bordering Gemini
Constellations bordering Gemini. Click for full-screen

To find Gemini, do a naked-eye search for Castor and Pollux, which are 4.5° apart. If you have trouble spotting them, do a naked-eye search for the Big Dipper. Draw an imaginary line from Megrez to Merak (~10°), then extend this line ~45° to Pollux.

Alternatively, do a naked-eye search for Orion. Draw an imaginary line from Mintaka, which is the rightmost star in Orion’s belt, to Betelgeuse (~18°). Extend this line ~33° directly to Castor in Gemini. This method is shown in the image below.

Finding Gemini from Orion
Finding Gemini from Orion. Click for full-screen.

If you’re unsure about measuring distance in the night sky, click the link to discover how to do it with your hand held at arm’s length. The article opens in a new tab.

To find Gemini’s exact position for your location on any night, use software such as Stellarium (free) or SkySafari 6, which is the paid software used to create the star charts on this page.

Gemini’s Brightest Stars

Gemini is a moderately sized constellation containing several stars brighter than magnitude 5.0. See them all on the star map below, with details about the important ones beneath that.

All the stars in the constellation of Gemini brighter than magnitude 5.0
Stars brighter than magnitude 5.0 in Gemini. Click for full-screen

Alpha Geminorum (Castor) – This double star is the second brightest star in Gemini and the twenty-third brightest star in the sky. The 1.58 magnitude, white, main-sequence primary and 2.97 magnitude, white, secondary components are 5.2 arcseconds apart.

Castor is a fascinating system of six individual stars! The primary component (Castor A) is a spectroscopic binary pair of a main-sequence star and a dwarf star. The secondary component (Castor B) is also a spectroscopic binary pair of a main-sequence star and a dwarf star.

Castor A and B are easily separated in a backyard telescope, being 5.4 arcseconds apart from each other. This gap is slowly widening too as they continue their 440-year orbit of each other. See the screengrab from SkySafari 6 below for an indication of what this looks like in the eyepiece.

Castor B's orbit path and position compared to A
Castor B’s orbit path and position compared to A

A tertiary component (Castor C) is an eclipsing binary pair of red dwarf stars.

Castor is 51 light-years away and its Arabic name means ‘the head of the foremost twin.

Beta Geminorum (Pollux) – This variable double star is the brightest star in Gemini and the seventeenth brightest star in the sky. The fact that Pollux has the ‘beta’ designation, meaning it should be the second-brightest star in Gemini, suggests that either it or Castor (with its ‘alpha’ designation) have changed brightness since the labels were given in the 1600s. However, there is no evidence for this.

Pollux is a 1.22 magnitude, yellow-orange giant star with a magnitude 13.70 magnitude secondary component. The two are 40 arcseconds apart and relatively near Earth at 40 light-years distant.

Pollux’s Arabic name means ‘the head of the second twin’.

Gamma Geminorum (Alhena, Almeisan) – This double star is the third brightest star in Gemini.

The primary component shines at magnitude 2.01 and is a sub-giant, meaning hydrogen fusion has come to an end, which is the first stage of a star’s death.

The secondary component is itself a spectroscopic binary shining at magnitude 13.00, putting it at the limit of smaller telescopes.

The two components are 140.5 arcseconds apart and 109 light-years away from Earth. The name Alhena is derived from the Arabic phrase for ‘the brand on the neck of a camel’, while Almeisan is derived from the Arabic for ‘the shining one’.

Delta Geminorum (Wasat) – This double star is the seventh brightest star in Gemini and is 59 light-years away. It’s useful for effectively marking the ecliptic and occasionally is occulted by the Moon. If you drew a line to join it to the star Regulus, in Leo, you’d pretty much get the path of the Sun across our sky.

The primary is a 3.53 magnitude, yellow-white subgiant. The secondary shines at magnitude 8.18 and is an orange dwarf 5.4 arcseconds from the primary.

The name Wasat is derived from the Arabic word for ‘middle’.

Epsilon Geminorum (Mebsuta) – This variable double star is the fifth brightest star in Gemini and 900 light-years away from us.

The magnitude 3.00 primary is an orange supergiant, while the secondary component shines at magnitude 9.64 110 arcseconds away. The primary is an eruptive variable whose magnitude ranges from 2.97 to 3.09.

Mebsuta is near the ecliptic and can be occulted by the Moon. Mebsuta is derived from the Arabic phrase referring to the ‘lion’s outstretched paw’.

Zeta Geminorum (Mekbuda) – This variable double star is 1,200 light-years away.

The magnitude 4.01 primary is a yellow-orange, supergiant. The secondary star shines at magnitude 11.46 and is 87.4 arcseconds away from the primary. The primary is a pulsating variable whose magnitude ranges from 3.62 to 4.17 over a 10-day period.

The name Mekbuda is derived from the Arabic phrase referring to the ‘lion’s folded paw’.

Eta Geminorum (Propus) – This variable double star is the sixth brightest star in Gemini and is 350 light-years away. Like other stars we’ve seen so far, Propus is close enough to the ecliptic to be occasionally occulted by the Moon.

The 3.29 magnitude, orange-red giant primary and its 6.15 magnitude, secondary component are 1.6 arcseconds apart. The primary is a pulsating variable whose magnitude ranges from 3.15 to 3.90 over a period of 233 days.

Propus is Greek for ‘forward foot’.

Theta Geminorum – This double star is 189 light-years away from our solar system.

The primary star is a magnitude 3.60 white giant, while the secondary is magnitude 12.60. The two components are 80.8 arcseconds apart.

Iota Geminorum – This double star is 120 light-years away. The 3.78 magnitude orange giant primary and its 4.69 magnitude secondary component are 0.1 arcseconds apart, and so can’t be split in a backyard telescope.

Kappa Geminorum (Al Kirkab) – This double star is 140 light-years away. The magnitude 3.56 yellow-orange primary and bluish magnitude 10.00 secondary components are 7.2 arcseconds apart.

Its name means ‘the vineyard husbandman’ in Arabic.

Lambda Geminorum (Alkibash) – This double star is 95 light-years away. The two components are 9.3 arcseconds apart and respectively shine at magnitude 3.57 and 10.70.

Its name Alkibash means ‘The Rams’ in Arabic.

Mu Geminorum (Tejat Posterior) – This variable double star is the fourth brightest star in Gemini and is 232 light-years away.

The 2.90 magnitude, orange-red giant primary and its 11.29 magnitude secondary component are 108.1 arcseconds apart. It’s believed that this is not a true double star because the secondary seems to be a line of sight coincidence.

The primary is a pulsating variable whose magnitude ranges from 2.75 to 3.02 over an unknown period.

Tejat Posterior means ‘back foot’ in Arabic, which is fitting because this star forms part of Castor’s foot.

Nu Geminorum – This double star is 545 light-years away. It is a blue-white giant star, shining at magnitude 4.13. Its secondary component is 113 arcseconds away and shines at magnitude 8.01.

Xi Geminorum (Alzirr) – This magnitude 3.33 yellow-white giant is 59 light-years away.

Omicron Geminorum – This 4.88 magnitude, yellow-white giant is 166 light-years away. Its mass is 1.5 Solar masses, its diameter is 2.9 Solar diameters and it burns 1.4 times hotter than the Sun.

Rho Geminorum – This double star is 59 light-years away and composed of a yellow-white magnitude 4.17 primary and a magnitude 12.50 secondary component which are 1.2 arcseconds apart.

Sigma Geminorum – This variable double star is 125 light-years away. The 4.25 magnitude, orange giant, and 10.80 magnitude, secondary components are 195.0 arcseconds apart. The primary is an eruptive variable ranging in magnitude from 4.13 to 4.29 over 19.4 days.

Tau Geminorum – This double star is 321 light-years away. It’s composed of a magnitude 4.38 orange giant primary and, 1.8 arcseconds away, a magnitude 11.00 secondary component.

Upsilon Geminorum – This double star is 271 light-years away. The 4.07 magnitude orange-red giant and its magnitude 13.20 secondary component are 55.2 arcseconds apart.

Chi Geminorum – This double star is 256 light-years away. The 4.94 magnitude orange giant primary and magnitude 12.23 secondary components are 59.5 arcseconds apart.

Phi Geminorum – This magnitude 4.98 white main-sequence star is 222 light-years away. Its mass is 2.3 Solar masses, its diameter is 2.9 Solar diameters and it is 1.5 times hotter than the Sun.

Star Hopping From Gemini

Gemini is the starting point for two well-known star hops (read our guide to star hopping, which opens in a new tab).

M35, Open Cluster in Gemini – Identify Castor. Hop ~13° west to Mebsuta, ~5° west to Tejat Posterior, ~2° south to Propus, then 2° northwest to M35.

Star hopping to M35 open cluster from Propus
Star hopping to M35 from Propus

NGC 2392 (Eskimo Nebula in Gemini) – Identify Pollux. Hop ~8° southwest to Delta Geminorum (Wasat), then 2° east-southeast to a triangle of magnitude 5, 6, and 7 stars, then 0.66° south to NGC 2392.

Star hopping to the Eskimo Nebula (NGC 2392) from Wasat
Star hopping to the Eskimo Nebula (NGC 2392) from Wasat

Objects To See Within Gemini

Gemini contains one Messier object and some other deep sky objects suitable for small telescopes.

M35 (NGC 2168) – This open cluster has a magnitude of 5.09 and an apparent size of 40.0 arcminutes. It’s 2,800 light-years away and located at right ascension 06h 08m 54s and declination +24° 20’ 00” (2000.0)

NGC 2158  – Another open cluster, this one has a magnitude of 8.60 and an apparent size of 5.0 arcminutes. It’s 16,500 light-years away from us and located at right ascension 06h 07m 25s and declination +24° 05’ 00” (2000.0).

NGC 2129 – Our third of four open clusters in Gemini has a magnitude of 6.69 and an apparent size of 5.0 arcminutes. It is about 7,200 light-years away and can be found at right ascension 06h 01m 07s and declination +23° 19’ 00” (2000.0).

NGC 2355 – This fourth open cluster has a magnitude of 9.69 and an apparent size of 7.0 arcminutes. It’s 5,400 light-years away and is at right ascension 07h 16m 59s and declination +13° 45’ 00” (2000.0).

NGC 2392 (Eskimo Nebula or Clown Face Nebula) – This planetary nebula has a magnitude of 9.19 and an apparent size of 0.8 x 0.7 arcminutes. It’s 3,000 light-years away and is at right ascension 07h 29m 11s and declination +20° 54’ 37” (2000.0).

Summary

Gemini is a medium-sized constellation with many bright, notable stars. It’s an easy constellation to find in the winter and spring and highlights the location of the ecliptic.

Within its boundaries are a handful of enjoyable deep sky objects.

Altogether, there is a lot to look out for with the twins overhead.


Written by Tanya C. Forde