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40 Eridani
40 Eridani is a triple star system less than 16.5 light years away from Earth, in the constellation Eridanus. It is also known as Omicron2 Eridani for all collective components. Component A is also known as Keid, from the Arabic word ''qayd'', ''(egg) shells''.
The primary star of the system, 40 Eridani A, is easily visible to the naked eye. The pair 40 Eridani B/C was discovered on January 31, 1783, by William Herschel.〔(Catalogue of Double Stars ), William Herschel, ''Philosophical Transactions of the Royal Society of London'' 75 (1785), pp. 40–126〕, p. 73 It was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851.〔(Astrometric study of four visual binaries ), W. D. Heintz, ''Astronomical Journal'' 79, #7 (July 1974), pp. 819–825.〕〔( The orbit and the masses of 40 Eridani BC ), W. H. van den Bos, ''Bulletin of the Astronomical Institutes of the Netherlands'' 3, #98 (July 8, 1926), pp. 128–132.〕 In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a white dwarf, the first discovered.〔''White Dwarfs'', E. Schatzman, Amsterdam: North-Holland, 1958. , p. 1〕
== General information ==
40 Eridani A, a.k.a. Keid, is a main-sequence dwarf of spectral type K1. Its two companion stars, 40 Eridani B and 40 Eridani C, are a 9th magnitude white dwarf (spectral type DA4) and an 11th magnitude red dwarf flare star (spectral type M4.5e) which has the variable star designation DY Eridani. Presumably, while B was a main-sequence star, it was the most massive member of the system, but ejected most of its mass before it became a white dwarf. B and C orbit each other approximately 400 astronomical units from the primary star, A.〔 Their orbit has a semimajor axis of 35 AU (which is the approximate average distance between B and C) and is rather elliptical (eccentricity 0.410).〔
Although 40 Eridani B is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than Van Maanen's Star (the nearest solitary white dwarf), and unlike the companions of Procyon and Sirius it is not swamped in the glare of a much brighter primary.〔(Keid ), Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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