Beta Orionis (Rigel)

February 2010  :  Glenn Chaple

You won’t need a finder chart to locate this month’s featured sky object. It’s the first magnitude star β Orionis, better known by its proper name Rigel. Seventh brightest star in the night sky, Rigel dazzles us with a diamond-white color; especially striking when compared with Orion’s other first-magnitude star, the ruddy-hued Betelgeuse.

Many backyard astronomers are unaware that Rigel is a double star. Its companion (Rigel B) lies 9 arc-seconds away – a gap that should be easily breached by the smallest of telescopes. Unfortunately, it shines at magnitude 6.8, 400 times fainter than the primary. As a result, the little star often hides in the glare of its master.

In 1822, the first reliable measure of the Rigel system indicated a separation of 8.9” and a position angle of 201o, the latter meaning that Rigel B lay south and slightly west of the main star. Not much has changed in nearly two centuries. In 2004, the separation and P.A. had increased slightly to 9.4” and 204o. Because Rigel A and B share a common proper motion, astronomers believe they form a physical binary separated by a whopping 2500 AU – a distance over 60 times greater than the gap separating Pluto from the sun. Their orbital period is thought to exceed 25,000 years. The last time Rigel B was in its current orbital position the earth was in the grip of the Ice Age!

Because of the large disparity in brightness between its components, Rigel offers a similar challenge to the one presented by the notoriously difficult Sirius. While Sirius and its white dwarf companion the “Pup” require absolutely steady seeing conditions and an 8-inch or larger telescope, Rigel may be split with a 6-inch under normal sky conditions. Years ago, on an evening of unusually steady skies, I managed to glimpse Rigel B with a 3-inch f/10 Edmund reflector (the classic model sold back in the 50s and 60s) and a magnifying power of 120X. I cheated, first spotting the companion with a 6-inch reflector. Knowing where to look, I had no trouble capturing Rigel B with the 3-inch. It appeared as a tiny bluish speck just outside the brilliant sparkle of the main star.

Next time you turn your telescope skyward to admire the Orion Nebula, take a side trip to Rigel. Unlike the legions of backyard astronomers who have marveled at the great nebula, you’ll be among a much smaller group of observers who have admired Orion’s brightest binary star.

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