on the Castor system

Spend enough time looking at any subject, and you’ll eventually come to have your favorites.

Since my time working on the RECONS 25 parsec database, I have a few systems that I look for (and look forward to).  One of them is Castor, one of the stars visible to the unaided eye and brightest star of the constellation Gemini*. Castor is just fun.

I think I’ve talked about how some star systems have more than one or two stars in them- Castor has SIX. Busy place. Actually, though, once you figure it out, Castor is an elegantly simple arrangement: There’s Castor A and Castor B orbiting each other every roughly 300 years, and in a much wider orbit (which probably takes a couple thousand years) going around the other two, a star sometimes called YY Gem. And each of those aforementioned objects are actually two.  The two parts of Castor A orbit each other every 9 days, the two parts of Castor B orbit each other every 2 days, and YY Gem’s two stars orbit each other in less than a day (!).

If it’s not clear from that paragraph, it goes Aa-Ab —– Ba-Bb—————————-Ca-Cb.  That’s not actually so uncommon, it seems.  Nature likes twos. The next closest sextuplet is Alcor & Mizar, in the big dipper, and they’re remarkably similar. Mizar is two sets of doubles, and Alcor is another double.

In fact, pretty much every star system you can find out there, if it’s not a single star or a binary, is in some kind of setup where there are two stars paired up. Alpha Centauri is a triple, but you quite clearly have a binary (AB) and a very far flung companion, C. This can occasionally get confusing, particularly if someone identified something that isn’t real (I once spent several hours determining that the real companions to the star Capella were the ones labelled H, L, and P). This is another minor reason I like Castor: it’s famous enough that you can find complete and accurate accounts of what’s going on. It’s a very well studied system.

Castor is also enormous. Given that four of its six stars are spectral type A, we’re almost definitely talking about the most massive star system within 25 parsecs of the Sun. By that logic, it was once claimed that Castor was at the core of an immense cloud of stars that had all formed together and were still roughly near each other.

Alcor and Mizar actually are members of a group: the Ursa Major moving group. That Ursa Major moving group consists of most of the stars of the Big Dipper surrounded by a cloud of other stars, which means Ursa Major is perhaps the only constellation that is an actual physical thing. Actually, the Sun is passing THROUGH the Ursa Major moving group right now, which means the members are all over the sky.

Unfortunately, the same isn’t true of Castor. People thought they had a Castor moving group for a while, but it turns out that the stars they found had actually formed hundreds of parsecs apart from each other, and haven’t been anywhere near each other until now.

So, Castor has no extended family, but it’s got a pretty big crowd on its own.

*The other bright star in Gemini, Pollux, is also cool: it has a planet!

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