, 2 min, 331 words
Tags: physics astrophysics
Castor, one of the twin stars in Gemini (as in the constellation, not a movie star who plays a twin in a TV show called Gemini), is actually six stars. Castor A and B are a visual binary, which means that using a powerful telescope, one can see that there are really two separate stars. Further investigation reveals that Castor A and Castor B are actually both spectroscopic binary systems.* That makes the system a quadruple star system. But wait, there's more! There's another, far dimmer, system called YY Gem that is much further away from the quadruple we've already discussed. And did I mention that it's a binary system too? Basically it is far enough away from the other stars that it responds to Castor A/B as if the collection of four stars is just a large massive object, and is in a 'binary' system with them. And this whole thing manages to stay gravitationally bound. Fun, right?
* Spectroscopic binary systems are detected by examining the Doppler shift of spectral lines as the brighter of the two stars wiggles back and forth in response to the orbit of the dimmer one - as it wiggles towards us, the spectral lines are all slightly blueshifted, and they're redshifted as it wiggles away from us. It's useful for detecting binary systems in which the two stars are very close together, which makes them difficult or impossible to distinguish visually. The really amazing thing about this measurement is that the amount of blue or redshifting that occurs is generally smaller than the natural width of the spectral line. Observational astrophysics is awesome.
Note: A binary star system is one in which two stars orbit their mutual center of mass. There are a variety of ways to detect them (see the Wikipedia page for a fairly comprehensive explanation), and it's believed that something like 60% of stellar systems are actually binary star systems. So our single-star solar system is actually in the minority!