DHENEB (Eta Ceti). Oh no, not another "deneb," from the Arabic
meaning "tail." But afraid so. The prime example is Deneb, the first magnitude star that marks
the tail of Cygnus (the Swan). It's
followed by second magnitude Deneb
Kaitos (the tail of Cetus, the Whale
or Sea Monster), third magnitude Deneb
Algedi (at the tail of Capricornus,
the Water Goat), and by several others that we'll not single out.
Adding to the confusion, third magnitude (3.45) Dheneb is ALSO in
the tail of Cetus (to the northeast of
Deneb Kaitos), which gives us two "denebs" in the same constellation! At least "Dheneb" is spelled
differently. More confusing yet, Dheneb is also known as Deneb
Algenubi, which could confuse it with the "real" Algenubi, Epsilon Leonis. If nothing
else, Dheneb illustrates the danger of using proper names except for those of really
prominent stars. We can relieve the problem simply by using
Dheneb's Greek letter name, Eta
Ceti. And oh no, it's yet another orange class K (K1.5) giant, which speaks of a star
with an evolved, quiet, helium fusing core. Such orange giants are
among the most common of naked eye stars. Yet like all stars, even
common ones, it has something to recommend it in addition to the
issue raised by its proper name. Relatively nearby, 124 light
years away (give or take less than one!), with a well-determined
temperature of 4610 Kelvin, Eta Ceti shines with the light of 78
Suns, a large fraction of which is emitted in the infrared part of the spectrum. Temperature and luminosity then
reveal a radius of 13.8 times solar. A slow projected equatorial
rotation speed of 2.6 kilometers per second gives a rotation period
that could be as long as 260 days. Eta Ceti is a "clump giant,"
such stars grouping closely together on the HR
diagram, the graph of absolute magnitude vs. spectral class (effectively
luminosity vs. temperature), which makes it sometimes difficult to
assess mass. From theory, Eta Ceti's mass seems to come in at
around 2 to 2.5 Suns (one study giving it
2.4). The spectral class has a "CN1" extension, which implies
enhanced carbon and nitrogen abundances. Consistently, the
abundance of iron relative to hydrogen is up from the Sun by a
factor of 30 percent, and the star moves relative to us at a speed
of 48 kilometers per second, three times higher than normal for
local stars, all implying that Eta Cet may be a visitor from
somewhere within the inner star_intro.html#galaxy">Galaxy. Eta
Ceti is cited as having an 11th magnitude companion about four minutes of
arc away, but the separation is changing too quickly, and the
neighbor is clearly a line of sight coincidence. A bit farther
afield, Eta Cet seems to be topped by a "crown" of sixth magnitude
stars with Flamsteed numbers 27, 28, and 30 Ceti (Eta itself 31
Ceti), respectively a K0 giant 303 light years away, an A1 dwarf at
590 light years, and an F7 subgiant at 570 light years, none of
them physically related. Eta Cet will eventually slough off its
outer envelope, probably produce a planetary
nebula, and die as a white dwarf with a mass of
around 0.6 Suns.
Written by Jim Kaler 1/24/14. Return to STARS.