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.^{, p. 73} It was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851. 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.

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).