Astro Bob: Let the moon whisk you to Castor, Pollux and exoplanet Thestias

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

The moon forms a line with Gemini’s Pollux and Castor tonight, May 26. The lies about 5° to the left of the pair. Fainter stars outline each of the twins’ arms and legs. Stellarium

If you look at the crescent moon this evening (May 26) you’ll notice two bright stars to its right. They’re Castor and Pollux the mythical Greek twins. I’ve also read they were half-brothers so it depends on the story. Either way they’re bonded together for eternity in the constellation Gemini the twins . The two were said to get along famously. Castor was a horseman and Pollux an accomplished boxer. Both loved adventure and accompanied Jason and the Argonauts in search of the golden fleece, a gold-haired, winged ram.

Mythological view of the Gemini twins. (Urania’s Mirror)

Gemini is a prominent winter constellation now slowly sinking in the western sky. Thanks to tonight’s moon we can easily find its two most prominent stars. Stars appear as so many pinpricks of light and would forever be so were it not for astrophysics, the science of determining their nature using chemistry, physics and computer modeling.

Astrophysics began in the 19th century with the invention of the spectroscope , a device that used a prism to spread apart the colors of a star’s light. Within that rainbow of light called a spectrum were narrow, dark lines that matched the bright lines visible in the spectra of elements and chemicals when they were burned in a hot flame in the laboratory. With the spectroscope astronomers now could determine the composition of a star remotely without having to send a rocket there and wait a hundreds of thousands of years for it to return.


A spectroscope spreads the light of stars (including the sun, top) and nebulae into rainbows or spectra. Dark lines represent chemical elements and compounds that absorb the object’s light. Spectra of hydrogen, nitrogen and carbon (below) were made in the lab and serve as comparisons. Public domain / Wikipedia

Using the spectroscope and other instruments of the trade we know that despite their similar appearance Pollux and Castor are anything but twins. Pollux is an orange giant star 9-10 times the size of the sun located 34 light years away while Castor is actually six stars in one 51 light years away. Although Pollux is cooler than the sun it radiates 46 times as much energy because it’s so enormous. More noteworthy, Pollux is a the brightest star in the sky orbited by another planet. Named Thestias , the planet is about 3 times as massive as Jupiter and circles the star a little farther than Mars does from our sun with a period of 1.6 years. From Thestias Pollux would appear almost 3° across or nearly six times bigger than the sun does to us.

Pollux, while large compared to the sun, is dwarfed by the bright spring star Arcturus.

Astronomers measure the diameter of the large stars like Pollux using a technique called interferometry . Light from two widely separated mirrors aimed at the same star are beamed together through a pair of narrow slits. Each produces a set of fringes overlapping the other. The distance between the mirrors and slits can be varied until the overlapping fringes disappear. Using the known separations between the mirrors and slits, a simple mathematical formula will give the diameter of the star in fractions of a degree. When combined with its distance astronomers then calculate its diameter.

Castor’s an amazing star or should I say stars? Single to the eye, triple in a telescope but six altogether. Illustration not to scale. (Bob King for the News Tribune)


Castor appears single but it’s actually a bright triple star in a small telescope consisting of two white, Vega-like suns a few arc-seconds apart and a fainter red dwarf star orbiting farther away. Each one of these is in turn double again, comprising a “family” of three pairs orbiting about the other. What an amazing sight we’d see from a potential planet in the Castor system. Six suns of varying size and brightness parading across the sky day and night.

Castor A and B — the brightest two — make a stunning sight in a small telescope magnifying around 70-100x. Castor A is 2.4 times as massive as the sun; Castor B weighs in a 1.9 solar masses. Why are all the stars we talk about here typically so much bigger and brighter than the sun? They’re bright and easy to see, that’s why. And to be bright you either have to be big or close or both. Modest stars like the sun struggle to be seen from planets around other stars. But to us it’s the brightest thing in the sky and in so many ways the center of our lives.

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