Brightest StarsSpace

Meet Castor: It’s 6 stars in one

A star map showing Gemini and the Big Dipper, with a line from two stars in the Big Dipper bowl pointing to Castor and Pollux.
From a Northern Hemisphere location face generally northward to find the Big Dipper asterism in the constellation Ursa Major. Look mid-evening or later in February, earlier in March. 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. You’ll see 2 stars noticeable for being bright and close together: Castor and Pollux.

Castor, the less-bright Twin

Castor – in the constellation Gemini the Twins – shines with a bright white light. That’s in contrast to the golden glow of its brother star in Gemini, Pollux. Despite being labeled as twins, Castor and Pollux are not gravitationally bound. Yet Castor is gravitationally bound into a multiple system of its own. It’s six stars in one!

Castor is also known as Alpha Geminorum. And usually an alpha star is the brightest in its constellation. But Castor is 2nd-brightest in Gemini, after Pollux (or Beta Geminorum).

Castor is about 51 light-years away. Pollux is only 34 light-years away. So Pollux is closer to us. And their distances also show Pollux and Castor aren’t gravitationally bound, but only near each other along our line of sight.

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Castor’s complex star system

Castor is three pairs of binary stars – six stars in all – in a complex dance around a common center of mass.

Even a fairly small telescope will show Castor as two stars. You might glimpse a much-fainter star nearby, too; it’s also part of the Castor system. Each of these three stars – called Castor A, B and C – is also double. Telescopes don’t show them as double directly. But a spectroscope – which splits starlight into its component colors – reveals each of the three stars as double.

The two larger visible components in the Castor system are hot A-type stars. The smaller components are cool, M-type red dwarf stars.

The mass of all six stars together is, very roughly, about six times that of our sun.

Visualizing the separation of the stars

Pair of stars with a very faint star near them. Inset with pair of stars.
An image of Castor’s 3 stars – A, B, and C – from the University of Oregon’s Pine Mountain Observatory. Each star has its own stellar companion that you can only see spectroscopically. The inset shows stars A and B. The numbers are for their position angle and the angular separation between the stars in arcseconds (1/3600 of a degree). For example, star C has a position angle of 163.7 degrees from A, and a separation of 70.1 arcseconds. Image via Pine Mountain Observatory/ Journal of Double Star Observations.
Family tree-like diagram showing relationships of six stars.
This figure shows the orbit hierarchy of Castor’s star system, along with each of their orbital periods and separation from each other. Castor Aa and Ba orbit each other, and each have their own stellar companion, Ab and Bb, respectively. Castor C, composed of the binary pair Ca and Cb, is farther away and orbits around Castor Aa/Ab and Ba/Bb. Figure via Wikipedia.

Another way to find the ‘twins’

Northern Hemisphere skywatchers can find Castor and Pollux using the Big Dipper as a guide, as shown on the chart at the top of this post. From anywhere on the globe, you can use the constellation Orion the Hunter (see chart below) to find the the twins.

Star-hop from Orion to the “twin” stars Castor and Pollux by drawing an imaginary line from Orion’s bright star Rigel through its bright star Betelgeuse. The extends about three times the distance between these two stars.

This line will point to Castor and Pollux.

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

A star map showing Orion and Gemini, with a line from Rigel to Betelgeuse pointing towards Castor and Pollux.
A line from Rigel to Betelgeuse in the easy-to-see constellation Orion points to Castor and Pollux.

Greek mythology of Castor and Pollux

The reason for the name Castor is unclear. There appears to be no specific connection with a beaver, which is what the word means in Latin.

However, there is much mythology associated with these two stars, typically 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. Zeus allowed Pollux to spend every other day in Olympus with the gods, and the rest of the time in the underworld with his brother.

To honor the brother’s devotion, Zeus placed their constellation in the sky as a remembrance.

Marble statue of two unclothed young men.
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.

Other stories surrounding the stars

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.

Castor’s position is RA: 07h 34m 36s, Dec: +31° 53′ 19″

Bottom line: The star Castor, which appears as one of two bright stars in the constellation Gemini the Twins, is actually a six-star system.

February 22, 2022
Brightest Stars

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