March 31, 2020

AstroEvent of the Week: 25.05.09: Castor: a Sextuplet Star!

This week’s astro-challenge is a unique multiple star system…Castor, also known as Alpha Geminorum, rides high in the west of the northern hemisphere sky on clear late May evenings. Shining at a brightness of magnitude +1.6, it forms one half of the pair Castor and Pollux, known as the Twin stars of Gemini.

Castor and Pollux crop up in Greek mythology in the tales of the Argonauts and the Trojan War, and are said to manifest themselves to Mariners in the form of St. Elmo’s fire. Here is a neat way to identify the two stars, as they are both nearly the same magnitude: Pollux is on the side of Procyon (both start with a “P”), and “C”astor is located on the side of “C”apella! I learned this one as a kid and never forgot it! Castor is about 49 light-years distant, and is a good target even under moonlit or light polluted skies. Even low magnification will reveal a binary nature of about equal magnitudes. I’ve heard the pair likened to “distant car headlights,” at star parties. But hold on, it gets weirder. Each star is itself a pair of stars, each over 50% more massive than our Sun! Imagine the famous Luke Skywalker on Tatooine sunset scene in the first Star Wars film, times two! Fact is indeed stranger than fiction…after some research, we’re really surprised that no one has ever set a sci-fi drama in a place as bizarre as the Castor star system… can any one prove this wrong? Both systems are spectroscopic binaries and whiz around each other in periods of 9.2 days (Castor A) and 2.9 days (Castor B) The entire system takes about 400 years to revolve, although estimates since its discovery in 1718 vary from 360 (low) to 460 (high). Distance from A to B is about 90 astronomical units, about the width of our solar system. The pair is widening from a minimum separation of less than 2″ arc seconds in 1958 to a current spread of about 5″ arc seconds. A maximum separation of 7.4″ arc seconds will occur around 2085 A.D.; some of you kids reading this might just be around that long!

Still not weird enough? OK, the entire Castor AB star system is orbited by another stellar pair, two red dwarf stars over 1,000 Astronomical Units distant from the main four, in a +10,000 year orbit! Known as YY Geminorum, this pair is an eclipsing variable, with its 19.5 hour orbit tilted across our line of sight. At 9th to 10th magnitude, Castor C is a tough but not impossible challenge, owing mostly to the brightness of nearby Castor AB. Castor C is about 79″ arc seconds of separation from the main pair, and placing Castor AB just out of the field or using an eyepiece equipped with an occulting bar may help. Good luck in your explorations of this unique, sextuplet star system!

This week’s astro-word of the week is Apastron. This is farthest actual point in the orbit of about a star, usually referred to when talking about binary stars. It’s important to note that this may not always coincide with the greatest separation angle, depending on the orientation of the star’s orbit in relation to our Earth-bound view. Such is the case with the Castor system. The A-B orbit is tilted about 25 degrees from our line of sight and thus apastron of Castor B won’t occur until around 2190 A.D., about a century past greatest separation!


  1. [...] the mass of our own Sun. The system is also a complex one comprising no less than 6 stars, tying Castor for the title of most stars in one system. Gamma Velorum is a binocular double, with a blue-white [...]

  2. [...] Castor: I love showing folks this multiple star in the head of Gemini. In the telescope, its possible to just split the 5” arc second pair. Now, [...]

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