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KEID
(Omicron-2 Eridani, or 40 Eridani). An unassuming star, Keid (Omicron-2) of Eridanus (and more commonly known by its Flamsteed number, 40 Eridani) seems to play second fiddle to its somewhat brighter neighbor Beid, Keid the Arabic "egg shells" of Beid, "the eggs." The two stars are not a true binary, Beid seven times farther than Keid. Keid, however, is by far the more interesting star. A modest faint-fourth magnitude (4.43) to the eye, Keid is a triple star, famed not for Keid itself but for the much fainter companions. The 67th closest star and the 50th closest star system, Keid lies a mere 16.5 light years away. Much farther and the cool class K (K1) ordinary dwarf (Keid-A, one of the very few visible to the naked eye) would not be visible. Other than proximity, Keid A has little to offer but a cool temperature of 5100 Kelvin, a low luminosity of 0.4 times that of the Sun, and a mass around three-fourths solar. Such stars abound in space, but they are so faint that few can be seen without a telescope. A little over a minute of arc (83 seconds) away, however, and easily seen with only a small instrument, lies the prize of the system, Keid-B (rather, 40 Eridani B), a tenth magnitude white dwarf, by far the most visible white dwarf of all (though Sirius-B is more famed), its luminosity a mere 0.008 that of the Sun. White dwarfs are the final products of ordinary solar-like evolution, and are the spent cinders of the original stars' cores. Ordinary dwarfs (like the Sun and Keid-A) fuse hydrogen to helium in their cores. When the hydrogen is gone, the stars become giants and fuse the helium to carbon and oxygen. The outer envelope is ejected, and all that remains is the low-mass ultradense carbon-oxygen white dwarf. Typical white dwarfs are only about the size of Earth and have extraordinary average densities of a ton per cubic centimeter. The distance between the white dwarf and the K star is at least 400 Astronomical Units, the orbital period at least 7500 years. More remarkably, the white dwarf has a companion too, a dim class M (M4) 11th magnitude ordinary (hydrogen-fusing) red dwarf. The two orbit with a 248 year period, and are now about as far in angle from each other as they get (around 9 seconds of arc). Analysis of the orbit shows the pair to average 34 AU apart, the distance changing from 20 AU (about Uranus's distance from the Sun) to 48 AU. From the orbit, the white dwarf, 40 Eridani B, has a mass a bit over half that of the Sun, the red dwarf (40 Eridani C) much smaller, 0.16 solar. Curiously the actual luminosities of the two stars are almost the same, as the cool red dwarf (3300 Kelvin) produces most of its radiation in the invisible infrared, making it seem dimmer than it actually is. The white dwarf is much hotter (14,000 Kelvin, causing it to radiate much of its energy in the ultraviolet). The white dwarf must originally have been the most massive star of the three, with a mass about that of the Sun, to have evolved first. (Higher mass stars live shorter lives; as a red giant, 40 Eridani B would have quite dominated the system.) The K star will be next to go. As fascinating as the white dwarf may be, the dim M dwarf is not without its own charm. Like many of its cousins, it is a "flare star," one with a magnetic field that occasionally short-circuits, causing the star to suddenly brighten all across the spectrum.