Like a pair of twins, two stars shine prominently in the evening skies of the Northern Hemisphere spring each year. They are Pollux and Castor in the constellation Gemini the Twins.
Pollux, also known as Beta Geminorum, is slightly brighter than Castor. It shines with a golden glow while Castor appears whiter. Pollux is the 18th brightest star in Earth’s night sky. In 2006, astronomers discovered a planet orbiting Pollux; they named it Thestias.
Pollux and Castor are noticeable for being bright and close together. That’s likely how the early stargazers came to identify them as twins, and it’ll be helpful to you when you’re trying to spot these stars in our sky.
Here are two ways to find Pollux and Castor. From a Northern Hemisphere location face generally northward to find the Big Dipper asterism in the constellation Ursa Major. Draw an imaginary line diagonally through the bowl of the Big Dipper, from the star Megrez through the star Merak. You are going in the direction opposite of the Big Dipper’s handle.
This line will point to Castor and Pollux.
Here’s the second way to find them. Face generally southward in the winter months from a Northern Hemisphere location – or generally overhead in those same months, which are summertime, from the Southern Hemisphere – and look for the very noticeable constellation Orion the Hunter. You’ll spot it easily by looking for the three medium-bright stars that make up Orion’s Belt. A line drawn from Rigel through Betelgeuse in Orion – extending perhaps three times the distance between them – also passes near Gemini’s twins.
Pollux science. Pollux is classified as a “K0 IIIb” star. The K0 means that it is somewhat cooler than then sun, with a surface color that is a light yellowish orange. Keep in mind that when you look at a star, its color depends significantly on the sensitivity of your eyes, and that color is difficult to discern for most point sources. The “III” is a “luminosity” class designator, indicating basically how much energy it is putting out, which is largely dependent on size. A type-III star is considered a “normal” giant or just a giant. Finally, the “b” indicates that Pollux is slightly below the average luminosity for this class.
A relatively close 34 light-years away, Pollux is just under two times the mass of our sun, almost nine times the diameter of the sun, and about 30 times the sun’s brightness in visible light.
Pollux also pumps out a good bit of energy in non-visible infrared radiation. With all forms of radiation counted, Pollux is about 43 times more energetic than our sun.
A large planet, at least 2.3 times the mass of Jupiter, was confirmed in 2006 to be orbiting Pollux. This planet has been named Thestias by the International Astronomical Union. At 34 light-years away, Thestias is one of the nearest of the 4,173 extrasolar planets that have been discovered as of February 2020.
Thestias moves around Pollux with a period of about 590 days in a nearly circular orbit.
History and mythology of Pollux The Greek letter Beta is normally reserved for the second-brightest star in a constellation. But, as with Rigel in Orion, Pollux wears the designation Beta in its constellation, even though it noticeably outshines Castor, which is Gemini’s Alpha star. Being so close together in the sky, Castor and Pollux are easy to compare. If you look, you’ll agree. Pollux is brighter.
It is possible that one or both stars have altered in brightness since German astronomer Johann Bayer assigned the designation about 300 years ago. Another explanation is that Bayer sometimes labeled the stars in their order of rising. Here Castor rises slightly before Pollux, and hence Castor, the dimmer star, received the Alpha label. This explanation also fits for Betelgeuse and Rigel in Orion, as viewed from the latitude of Germany, because the Alpha star, Betelgeuse, rises slightly before the truly brighter star, Rigel. However, there is a geographical dependency here. From some locations south of the equator, both Rigel and Pollux rise first.
The name Pollux is of Greek origin and apparently refers to a boxer. The original Greek word seems at odds with this idea, however, as it apparently means “very sweet,” which may allude to the legendary warm and fraternal relationship between the two brothers.
In Greek mythology, Pollux was one of two brothers who figured prominently among Jason’s Argonauts. By most accounts, they were sons of Leda, Queen of Sparta, but Castor had a mortal father and hence was mortal himself. Pollux was the son of Zeus and immortal. Pollux also had a famous sister, Helen of Troy.
There are many variants to the story of Castor and Pollux, but, by most accounts, Castor was killed in battle and Pollux could not bear to live without him and begged Zeus to let him die too. Zeus could not grant the gift quite as asked, since Pollux was a god’s son and therefore immortal. But Zeus decreed that Pollux would spend every other day in Olympus with the gods, and the rest of the time in the underworld with his brother. To honor Pollux’ devotion, Zeus placed their constellation in the sky as a remembrance.
Pollux and Castor are also sometimes identified with Apollo and Hercules or with the founders of Rome, the brothers Romulus and Remus.
While in many cultures they were the twins, in India they were the Horsemen, and in Phoenicia they were the two gazelles or two kid-goats. It is said that in China they were associated with Yin and Yang, the contrasts and complements of life. In all of these cases, they represent two of something, and you will see why if you gaze upon these two stars in the sky, which are bright and close to each other.
Pollux’s position is RA: 7h 45m 20s, dec: +28° 01′ 35″.
Bottom line: Pollux, also known as Beta Geminorum, is the slightly brighter “twin” of Castor, the other bright star in the constellation Gemini. This golden-hued star, the 18th brightest star in the night sky, has a planet named Thestias that was discovered in 2006.
Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.