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 its
much fainter companions. Keid lies a mere 16.4 light years away.
Its closeness makes the brightest member (Keid A) one of the few
class K (K1) ordinary dwarfs visible to the naked eye. 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 near-tenth magnitude (9.50) white dwarf, by far the most
visible of the breed. 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 typical 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 7200 years. More remarkably, the white
dwarf has a companion too, a dim class M (M4) 11th magnitude (11.2)
ordinary hydrogen-fusing red dwarf. The two orbit with a 252 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 35 AU apart, the distance changing from 21 AU
(about Uranus's distance from the Sun) to 49 AU. From the orbit,
the white dwarf, 40 Eridani B, has a mass of just 0.50 times that
of the Sun, the smallest of the classic three white dwarfs, the
other two Procyon B and Sirius B. The little star shines at but 1.3
percent the luminosity of the Sun, its high temperature of 16,700
Kelvin offset by a terribly small radius just 1.48 times that of
Earth (the largest of the classic trio, consistent with lower mass,
which yields a lower gravitational field and less squeezing). The
result is a dead star with an average density of a quarter of a ton
per cubic centimeter. The mass of the red dwarf, 40 Eridani C, is
much smaller than that of 40 Eri B, 0.16 solar. Curiously the
actual luminosities of the two stars are comparable, that of the
cool (about 3500 Kelvin) red dwarf 2.2 percent solar, most of it
coming out in the infrared. As fascinating as the white dwarf may
be, the dim M dwarf is not without its own charm. Like many of its
cousins (such as Proxima Centauri),
it is a "flare star," one with a magnetic field that occasionally
short-circuits, causing the star to suddenly brighten all across
the spectrum. Since higher mass stars live shorter lives than lower
mass stars, the white dwarf must originally have been the most
massive star of the three, with a mass perhaps that of the Sun, to
have evolved first. As a red giant, 40 Eridani B would have quite
dominated the system. The K star will be next to go, while the dim
red one will last for a seeming eternity.