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 TwinsΒ  should be home to sextuplets! I encourage you to take a look at this fascinating multiple star the next night you’re able.

19 Responses

  1. thomas s

    Bob,interesting as usual. wonder what it would be like to live on a planet in a six star system. but I doubt that a planet or planets would be possible in such a system. any thoughts on that?

        1. astrobob

          Figure 8s are unstable, so the planet would either orbit one star or the pair. Planet Kepler 16-B orbits a binary star, where each star in turn orbits around their center of gravity.

  2. Steve

    Thanks, Bob! Saw Castors A, B, and C for the first time last night. Showed it to some friends who were looking through the telescope for the first time, but they were much more impressed with the moon πŸ˜€

  3. Nick

    I added Castor to our Christmas Eve star list. Thanks for all of the background information I can share. Will my five inch Newtonian separate them or should I print your photo? I enjoy sharing our skies more than you can ever imagine.


  4. Teamonger

    Hiya Bob, nice page. When I first tried to see Castor as a double back around 1970, it was so close that I couldn’t split it with my little telescope. As you say, now it’s much easier. But something seems off with your numbers. You say in 2012 the separation was 5″, and will be 6.5″ in 2100. But in your diagram, the 2100 distance appears twice as far as in 2012. Me so confused πŸ˜‰

    1. astrobob

      Hi Teamonger,
      I hope you enjoy tea as much as I do. The diagram is accurate and the 6.5″ max. is too. I recall now that I rounded to 5″ arc seconds (from 4.7-.8″) for the 2012 separation because I was having difficulty finding accurate measurements for that particular year. That would help to explain what it appears farther apart on the diagram. Thanks for asking.

      1. Teamonger

        Thanks for the quick reply, Bob. Something still seems amiss though. I googled “Castor double star diagram” and found a diagram like yours which also has a 6″ scale. Measuring out to 2100, I figured a max separation of 8.3″, which seems more reasonable if about 5″ today. Guess we’ll have to wait 86 years and find out who’s right! πŸ˜‰

        Yes, I’m very fond of malty Assam each morning.

        1. astrobob

          Assam – really? That’s my old reliable in the a.m. — CTC variety. I like it dark and strong. That’s very weird about 8.3″ for Castor. Unless several reliable sources are wrong, the max. separation is 6.5″ in 2100. But that would imply a current separation of 3.9″ which it much too narrow. I did a lot of digging and finally found a recent measure in the Washington Double Star Catalog. They give 4.6″ for 2013.

  5. Teamonger

    Okay, on your diagram I measure 31 mm to your 2012 position, and 53 mm to the 2100 position. That figures to 1.71 times as far in 2100. You said 4.8″ in 2012, right? So 1.71 x 4.8″ = 8.2″ in 2100. If it was 4.5″ in 2012, then 7.7″ in 2100.

    Who knows if the diagrams are right? But it’s fun to have a double star controversy. I can see the headline now, “Experts Disagree on Future of Famous Double Star”.

    I buy my Assam from, great stuff! πŸ™‚

    1. astrobob

      I will fight to the death over this! Just kidding. Perhaps the diagram source itself is slightly flawed. There are few available but I think I may have one I can cross-check in a book in my library. I’ll get back to you on it after tonight. Too much eclipse fever right now. Upton’s also my supplier πŸ™‚

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