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.