Meet Castor, a remarkable double-double-double star

The waxing moon lies directly between Procyon and Pollux tonight. Above Pollux is our featured star Castor. Created with Stellarium

Sure, sure. I know the Superbowl’s on TV this evening, but after downing all that dip, you’ll need to take a walk to shake off your food coma. May I suggest you look up at the moon? It’s in the “egg phase” called gibbous just two days before full. In line directly above and below it, are the bright stars Procyon in Canis Minor the Little Dog and the duo of Pollux and Castor in Gemini the Twins.

In Greek mythology Pollux and Castor were identical twins and the best of friends. Pollux grew up to become a champion boxer; his brother was a horseman and warrior. They used their skills on the perilous journey with Jason and the Argonauts in pursuit of the Golden Fleece, one of the oldest hero quest stories around.

Tonight the twins will join the moon as all three sail high across the southern sky. Pollux is an orange giant star and a little brighter than its brother Castor to the north. While Castor appears single to the naked eye and in binoculars, it’s actually a gravitationally bound family of six stars. Yes, a sextuple!

Castor A and B form a binary or double star, each one orbiting about the pair's common center of gravity. The two are currently far enough apart to easily split in a small telescope. Illustration: Bob King

The two brightest of the six, Castor A and Castor B, revolve around one another over a period of about 445 years. Back in the 1960s and 1970s, they were just 1.8 arc seconds apart, so close you needed a medium-sized telescope to split them apart. (An arc second is 1/60 of an arc minute which is 1/60 of a degree. The full moon is about 30 minutes across.) Today they’re a comfortable 5″ apart and their separation will continue to increase to a maximum of 6.5″ around the year 2100.

The beautiful binary star Castor as it would look in a small telescope. Credit: Frederick Ringwald/California State-Fresno

Even a small telescope magnifying around 60x and higher will show Castor as two brilliant white gems nestled close together. The pair is one of the brightest and prettiest doubles in the northern sky. Castor A, the brighter of the two, is magnitude 1.9, while its companion is 3.0. Both stars are similar to the sun but hotter, heavier and whiter and lie 51 light years from Earth. As stars go, we’re practically neighbors.

You can use this photo to help you find Castor C, located just south of the bright A-B pair.

That’s not all the warrior star has up his sleeve. Turns out both Castor A and B are orbited by a third star called Castor C. It’s 9th magnitude and lies about one arc minute to the south. Many amateur astronomers pass this one up since the brighter pair rivets our attention. Though fainter, Castor C is still easily visible in a small scope. It’s about 1000 times the Earth-sun distance from the bright pair and takes 14,000 years to orbit around them.

Wow - what a star! Single to the eye, a triple in a telescope and six altogether. Illustration not to scale

It gets even more interesting. Each of the three is double again making Castor a sextuple star. Most telescopes won’t show these three additional companions, because they’re much too close to each other to tease apart. We know they’re there thanks to the spectroscope, an instrument that plainly reveals each star’s telltale light “fingerprint” when astronomers examine the star’s spectrum.

Castor C is a pair of red dwarf stars a little more than half the size of the sun that revolve around one another in just under 20 hours. The companions of Castor A and B are also smaller dwarf stars. How perfect that the Twins  should be home to sextuplets! I encourage you to take a look at this fascinating multiple star the next night you’re able.