Two More Fine Multiple Stars

February 3, 2009

In the depths of winter, when the sky is dark and the air is cold, you may wish to try shorter observing sessions early in the evening.  Here are two fine multiple stars that, along with the multiple star sigma Orionis are ideal for a quick look on a cold night.


Basic View

• Castor is the second brightest star in the constellation Gemini.  While it’s a little fainter than nearby Pollux, Castor is listed on star maps as alpha Geminorum, traditionally the designation of the brightest star.  If you confuse Castor and Pollux, remember Castor is closest to Capella, the brightest star in Auriga.

• In a small telescope at 80x or so, Castor resolves into two bright stars.  The A component has magnitude 2.0 and lies some 4″ from the slightly fainter B component.  These stars take some 400 years to revolve around each other in an elliptical orbit.  In 100 years, the stars will increase their apparent separation to about 6″.  At their closest, the two stars lie only 1.8″ apart.  Castor A and B are blue-white A-type stars only 45 light-years from the Sun.

• There is a third star in the Castor system: to the south, you’ll see a much fainter red-orange 9th magnitude companion in your field of view.  This third (or “C”) component is a small M-type star that revolves around the two primary stars every 10,000 years or so.

• Each of the three stars in the Castor system is itself a double star… so there are six stars in all.  Castor A and B each have reddish M-type stars that hurtle around their brighter primary stars every 9 days.  And the M-type companion of the C-component lies less than 2 million miles away; the stars revolve around each other in less than 24 hours.  The close companions of Castor A, B, and C are far too close to resolve directly in any telescope.

Deeper Look

• Not too far away, in the constellation Monoceros just south of the celestial equator and east of Orion’s Belt, you’ll find another pretty multiple star, beta Monocerotis, which lies about 450 light-year away.

• At lower power, this star easily splits into two blue-white stars some 7″ apart.  The brighter A component is magnitude 4.6, and the fainter B component is magnitude 5.2

• Now, pop a higher power eyepiece in your scope and you may see a third component just grazing the edge of the B component.  It’s much closer, only 2.8″ away.  Use as much magnification as you can to split these stars.  Winter skies are often unsteady, and on a night of bad seeing when the stars are twinkling like mad, you may have trouble splitting the third component.

Good To Know

The word “castor” is also the Latin root for some types of European and North American beaver.  So is Castor the “Beaver Star”?  Not at all.  The star Castor is named after the one of the twin sons of Zeus and Leda in Greek mythology.  Castor’s twin brother was Pollux.

Personal View

Like fine Scotch or the works of William Shakespeare, double and multiple stars are an acquired taste.  Many beginners pass them up for more glittering objects like stars clusters and galaxies.  But give multiple stars a try… they are a great way to learn more about the variations of stars in the galaxy, and many will test your optics and visual acuity.

Posted in Celestial Objects