Pretty Doubles in Orion's Vicinity

32 Eridani is an attractive but little-known pair lurking in the dim expanse of Eridanus due west of Orion’s Belt. Its 5th- and 6th-magnitude components are set 8 arcseconds apart and display lovely pale topaz-yellow and sea-green hues that are evident in a 3-inch telescope and are truly striking in a 6-inch scope at 50x. Italian astronomer Angelo Secchi described this pair’s colors as “magnificent, superb” — and they are! Try cranking the magnification up to 40x per inch of aperture to see this pair’s beautifully tinted diffraction disks.

A line through Orion’s belt stars extended downward points to Beta (b) Monocerotis, also called Herschel’s Wonder Star, which is regarded by many as the finest triple system in the sky. At very low magnifications it appears to contain only two components, but a 3-inch scope at 60x reveals it as three nearly identical 5th-magnitude blue-tinged suns of spectral type B3. Larger apertures and higher powers greatly improve its visual impact.

Sirius, Alpha (a) Canis Majoris, is one of the most fascinating and challenging doubles in the sky. The primary is the brightest and second-closest of all the naked-eye stars, and its faint companion is the nearest white dwarf to Earth. They are opening from their last minimum in 1993, at a separation of less than 3 arcseconds, on their way to the next maximum in 2022, when they will be 11 arcseconds apart.

Sirius B is aptly described by one observer as “frightfully difficult” to see when near minimum, requiring at least 12 inches of aperture and superb seeing conditions. At its current separation of 7 arcseconds, it is visible in an optically excellent 5- or 6-inch scope.

Southeast of Sirius near the eastern border of Canis Major lies the star h3945. Its 5th- and 6th-magnitude K- and F-type suns provide an outstanding color contrast reminiscent of the famous double star Albireo, so I’ve dubbed it the Winter Albireo. Even in a 2-inch scope at 25x, the reddish orange and sapphire tones will surprise and delight you.

Castor, Alpha (a) Geminorum, is the star that convinced William Herschel of the physical reality of binary systems. He plotted the relative motions of the components and concluded that they followed a curved path that could be due only to orbital motion — the first evidence that gravity operated outside the solar system.

Here we find radiant 2nd- and 3rd-magnitude blue-white A-type suns currently 4 arcseconds apart and an easy split in a 3-inch telescope at 100x or more. The pair is opening from its last minimum of 1.8 arcseconds in 1963 to its next maximum of nearly 8 arcseconds in 2120. In a 2.4-inch refractor at high magnification during the winter of 1963, the two stars could actually be seen closing and finally “merging” over a period of just a few months!

Seventy arcseconds south of the AB pair is 9th-magnitude Castor C, also known as YY Geminorum. This M-type reddish orange dwarf is an eclipsing binary that fades from its normal brightness by more than a half magnitude every 20 hours. Spectroscopic studies show that the A and B components are also binaries, with periods of 9 and 3 days, respectively, so Castor A, B, and C actually make up a sextuple system!

If you haven’t explored the world of double stars, this is a great season to start. Once you acquire the habit, you’ll have an endless store of objects to observe, each with its own unique personality.