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Meet Pollux: The brighter twin star

Star chart showing Orion and Gemini, with a line from Rigel to Betelgeuse pointing towards Castor and Pollux.
Draw an imaginary line from 2 bright stars in the easy-to-see constellation Orion the Hunter to star-hop to the “twin” stars Castor and Pollux. The line goes from Orion’s bright star Rigel through its bright star Betelgeuse and extends about 3 times the distance between them. Castor and Pollux are noticeable for being bright and close together on the sky’s dome. Pollux is brighter than Castor.

Like a pair of twins, two stars shine prominently in the evening skies in February 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.

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, too, when you’re trying to spot these two stars in our night sky.

Pollux is relatively close to us at 34 light-years away.

You can use the easy-to-see constellation Orion to find Castor and Pollux, as shown on the chart above.

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Two big bright stars side by side against a background of black space sprinkled with many fainter stars.
An image of Pollux (left) and Castor (right), showing their color differences. Pollux is more golden. Castor is whiter, with a tinge of blue. Image via Rogelio Bernal Andreo/ RBA Premium Astrophotography. Used with permission.

Another way to find them

There are two good 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.

Want the view from your specific location at a specific time of year? Try Stellarium.

Star chart: Gemini and Big Dipper, with line from two stars in the Big Dipper bowl pointing to Castor and Pollux.
Draw an imaginary line diagonally through the Big Dipper’s bowl to locate Castor and Pollux.

Science of Pollux

Pollux is classified as a “K0 IIIb” star. The K0 means that it is somewhat cooler than the 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.

Pollux is just under two times the mass of our sun. It’s almost nine times the diameter of our sun. And it’s 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 times the mass of Jupiter, was confirmed for Pollux in 2006. The the International Astronomical Union announced a proper name for this planet in 2015: Thestias. At 34 light-years away, Thestias is one of the nearest of the more than 4,000 known exoplanets discovered so far.

Thestias moves around Pollux with a period of about 590 Earth-days, which is reminiscent of Mars’ orbital period of 687 days. Thestias moves in a nearly circular orbit around its star.

Sun tiny like a BB, Pollux like a baseball, Arcturus like a basketball, each labeled.
The sizes of the sun and Pollux compared, along with a couple other stars. Image via slideplayer.

Why Pollux is Beta

As mentioned above, Pollux is also known as Beta Geminorum. And the Greek letter Beta is normally reserved for the 2nd-brightest star in a constellation. But Pollux is brighter than its brother star 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’s possible that one or both stars have altered in brightness since German astronomer Johann Bayer assigned the designation about 300 years ago. Or maybe Bayer sometimes labeled stars in their order of rising times? Castor rises earlier than Pollux, as seen from Bayer’s location in Germany. But there’s a geographical dependency here. From some locations south of the equator, Pollux rises first.

Marble statue of two unclothed young men, one with a laurel wreath on his curly hair.
Copy of an ancient Roman statue of Castor and Pollux by Joseph Nollekens (1737-1823), at the Victoria & Albert Museum. Image via ketrin1407/ Wikimedia Commons.

Mythology of Pollux and Castor

There is much mythology associated with these two stars, typically only in conjunction with each other. Generally in mythology they are twins. In Greek mythology, Pollux is immortal, the son of Zeus, and Castor is mortal, the son of King Tyndareus of Sparta.

So they were really half-brothers rather than true twins, with a common mother in Queen Leda. Their conception and birth was a complicated and unlikely affair, though, with their mother succumbing to both Zeus (disguised as a swan) and King Tyndareus on the same night. The resulting birth gave us not only Castor and Pollux, but also their sister, Helen of Troy.

Castor and Pollux are later to have sailed among the Argonauts with Jason in search of the Golden Fleece. By most accounts, Castor was killed in battle and Pollux could not bear to live without him. Pollux 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 the brothers’ devotion, Zeus placed their constellation in the sky as a remembrance.

The twin stars in other cultures

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. Early Christians sometimes called them David and Jonathan, while the early Arabian stargazers knew them as two peacocks.

Perhaps the most unexpected connotation for the twins (along with the rest of Gemini) was as a “pile of bricks” as reported by Richard Hinckley Allen. Apparently the pile of bricks stood for the foundation of Rome, and in that context Castor and Pollux were associated with Romulus and Remus, the city’s legendary twin founders.

It’s 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.

You’ll see why if you find these two stars in the night sky.

Pollux’s position is RA: 7h 45m 20s, dec: +28° 01′ 35″.

Antique star map with etching of twin boys among many stars in black on white.
Pollux and Castor in Johann Bayer’s star atlas Uranometria Omnium Asterismorum, first published in 1603. It was the first atlas to cover the entire celestial sphere. In it, Bayer gave Pollux the label Beta in Gemini, even though today we see Pollux as brighter than Castor, the Alpha star. Image via Cartography Associates.

Bottom line: Pollux, aka Beta Geminorum, is the slightly brighter “twin” of Castor in the constellation Gemini the Twins.

Posted 
February 20, 2022
 in 
Brightest Stars

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