40 Eridani - Triple Star in Eridanus

40 Eridani finder chart

40 Eridani finder chart

This month, we travel 16.5 light years to the remarkable triple star 40 Eridani (aka Keid and omicron2 Eridani). This system merits must-see status by virtue of the fact that one of its members is the most easily-seen white dwarf in the night sky. Trekkies would add that the primary, a K1 dwarf not unlike our sun, is orbited by the planet Vulcan - homeworld of the starship Enterprise’s first officer Spock.

A small-aperture telescope and magnification of 50-60X shows two stars – the yellowish 4th magnitude primary (designated 40 Eridani A) and a faint 9th magnitude companion (40 Eridani B) some 83 arc-seconds away. This ordinary-looking speck is a white dwarf with half the mass of the sun packed into a body whose diameter is only half again that of the earth. One cubic inch of its matter would weigh 4 tons!

The white dwarf has a companion of its own, located about 9 arc-seconds away. Discovered by William Herschel in 1783, 40 Eridani C is an 11th magnitude main sequence red dwarf which can be glimpsed with a 6-inch telescope and magnification of 150X or more. Viewed with a large-aperture Dob, the colors – yellow for A, white for B, and pale red for C – are amazing!

40 Eridani B and C are separated by an average distance of 35 Astronomical Units (slightly less than the gap between the sun and Neptune) and orbit each other in a 252 year cycle. They lie 400 Astronomical Units (about 37 billion miles) from 40 Eridani A, circling this star in a period estimated to exceed 7000 years.

You may not have the starship Enterprise to transport you to the 40 Eridani star system, but a good backyard telescope will put you in the neighborhood. Would you like to get even closer? A wonderful piece of artwork by Andrew Taylor takes us to the surface of a planet (Vulcan, perhaps?) orbiting 40 Eridani A. .

Glenn Chaple
Glenn Chaple
Entry Date:
Feb 14, 2013
Published Under:
Glenn Chaple's Columns
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