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Castor is six stars in one

Image of bright blue-white star Castor against a backdrop of fainter stars.
The eye sees Castor (Alpha Geminorum) as a single star. But there are 3 binary pairs in this single point of light, or 6 stars here. We typically think of Castor along with another bright star, Pollux. Castor and Pollux are sometimes referred to as “twins” in the constellation Gemini, but they’re really very different. To see Castor and Pollux together, check out the image below by Rogelio Bernal Andreo. This image is via Fred Espenak.

To the unaided eye, the star Castor in the constellation Gemini the Twins appears as a bright pinpoint of light. But it’s actually three pairs of binary stars – six stars in all – in a complex dance about a common center of mass.

Even a fairly small telescope will show Castor as two stars. A much-fainter star nearby can be glimpsed, too; it’s also part of the Castor system. Each of these three stars – called Castor A, B and C – is also double. Their doubleness can’t be seen directly in telescopes, but it can be discerned via spectroscopic data, that is, by splitting starlight into its component colors.

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 the sun.

Image showing pair of stars with a very faint star near them. Inset with pair of stars.
An image of Castor’s 3 stars – called A, B, and C – taken at the University of Oregon’s Pine Mountain Observatory. Each star, in turn, has its own stellar companion that can only be detected spectroscopically. The inset image is a shorter exposure of stars A and B so their separation can be accurately measured. Also recorded on the image are two numbers. One is the position angle, which, in this image is the angular position of a faint star with respect to the brightest star A, measured in a north to east direction. The second number is the angular separation between a pair of stars in arc seconds (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.

Castor is the second-brightest star in the constellation Gemini the Twins. It shines with a bright white light in contrast to the golden glow of its celestial “twin” star Pollux, also in Gemini.

Pollux is not one of the 6 stars in the Castor system. When we speak of the six stars in the Castor system, we mean the single point of light we see as Castor is six stars. But Pollux is nearby, and about equally bright as Castor. The early stargazers identified Castor and Pollux as “twins.”

Two very bright stars against a background of many fainter stars. Pollux appears as a light gold star while Castor appears blue-white.
An image of Pollux (left) and Castor (right), showing their color differences. Pollux is more golden. Castor is whiter, with a tinge of blue. Castor by itself is 6 stars! Image via Rogelio Bernal Andreo/ RBA Premium Astrophotography. Used with permission.

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.

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.
Draw an imaginary line diagonally through the Big Dipper’s bowl to locate Castor and Pollux.

Here’s the second way to find Caster and Pollux. Face generally southward in the winter months from a Northern Hemisphere location – or generally overhead in those same months, the summertime months in 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 – passes near Gemini’s twins.

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.
Draw an imaginary line from Rigel through Betelgeuse to star-hop to Castor and Pollux.

At 34 light-years, Pollux is closer to us while Castor lies 51 light-years away. Thus Pollux and Castor aren’t gravitationally bound, but only near each other along our line of sight. Their proximity in our sky is what makes them easy to spot.

The twin stars are close to the moon’s path across our sky. The moon passes no more than 15 degrees south, and sometimes fewer than 5 degrees south, of Castor on some day every month. For this reason, the moon might help you find Castor.

The sun passes closest to Castor on or near July 14, and consequently the star cannot be seen for several weeks before that date to several weeks afterward. Other than for that time period, intrepid observers can find Castor at some time of night for about 10 months of the year.

Marble statue of two nude 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.

History and mythology of Castor. Castor is designated Alpha Geminorum. Most alpha stars are the brightest in their constellations, but Castor is slightly fainter than Pollux.

The reason for the name Castor is unclear, although there appears no specific connection with a beaver, which is what the word means in Latin. It’s also not clear why this star holds the appellation of alpha, a label typically put on a constellation’s brightest star. Castor is clearly secondary in brightness to Pollux, which is officially called Beta Geminorum.

There is much mythology associated with these two stars, typically only in conjunction with each other. They are usually considered to be twins. In Greek mythology Pollux is immortal, the son of Zeus, and Castor is mortal, the son of King Tyndareus of Sparta. Thus they were really half-brothers rather than true twins, with a common mother in Queen Leda. Their conception and birth, however, was a complicated and unlikely affair, with their mother succumbing to both Zeus (disguised as a swan) and King Tyndareus on the same night, with the resulting birth not only of Castor and Pollux, but of their sister Helen of Troy. Castor and Pollux later were among the Argonauts who sailed with Jason in search of the Golden Fleece, and due to their mutual devotion, Zeus placed them both in the heavens on their death, so that they could remain together forever.

Although many cultures saw Castor and Pollux as twins, early Christians sometimes called them David and Jonathan, whereas the Arabs knew them as 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.

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

A photo of a bright star and a fainter one next to it: both appear as a single point of light on Earth.
An image of Castor A and B through a telescope. With the naked eye, they appear as one star. Each of them, in turn, has a faint stellar companion. The third binary pair is not visible in this image. Image via 1CM69/ Flickr.

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 25, 2020
Brightest Stars

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