NU CET (Nu Ceti). The roundish head of Cetus, the Sea Monster, is led in brightness by Menkar, which at third magnitude (2.53) is
the Alpha star in spite of it being a
distinct second to second magnitude (2.04) Beta Ceti (Deneb Kaitos). But by its
very name, "Deneb" (from Arabic for "tail") is housed at rear of
the beast, while fainter Menkar is at the front, clearly showing
that Bayer also had position in mind while Greek-lettering the stars. Way down
in drawing Cetus's Head is fifth magnitude (4.70) Lambda Ceti, while barely beating it to
the bottom is our fifth magnitude (4.86) Nu Cet. Though faint to
the eye, it's actually a fairly impressive class G (G8) giant 340 light years away (give
or take just 8). That and the equally secure temperature of 5040
Kelvin (from which we find a small amount of infrared radiation) leads to a
luminosity 128 times that of the Sun and a
radius 15 times solar. Because they rotate so slowly, rotation
speeds (as derived from the broadening of spectrum lines) of giant stars are
often unmeasured. Here, though, we see a projected equatorial spin
rate of 3.7 kilometers per second, which gives a ponderous rotation
period that could be as long as 204 days. Theory tells of a fairly
massive star, one of three or so Suns, that started off life nearly
400 million years ago as a warm class B8 dwarf, and is now either
fusing helium in its core or will do so shortly. Nu Ceti will then
eventually lose its outer envelope and expire as a white dwarf of roughly 0.7
solar mass (all stars dying with much less mass than they started
with, their ejecta becoming fodder for new stars). Among the
better features of Nu Ceti is a faint companion 8.4 seconds of arc
away. Writing from the nineteenth century, Smythe and Chambers
tell us of "A double star in the Whale's eye...A 4 1/2, pale
yellow; B 10, blue. This very delicate object (is) marked
difficult...." Modern observations of Nu Ceti B make it a ninth
magnitude (9.1) class F (F7) star, the apparent blue color of Nu-B
caused by contrast effects. Over the past 180 years, the two have
tracked each other well against the distant background, so Nu Ceti
B most almost certainly belongs to Nu proper. Magnitude, distance,
and an adopted temperature of 6300 Kelvin give a luminosity that is
double solar, a mass of 1.25 Suns, and an age consistent with that
of "A." From its angular separation, "B" must be at least 875
Astronomical Units away from "A" (the foreshortening not known),
and assuming the above masses, takes more than 11,000 years to make
a full circuit. From the orbiting companion, the grand giant would
appear to the naked eye as a brilliant yellow-orange point of light
dozens of times brighter than the full
Moon.
Written by Jim Kaler 12/21/12. Return to STARS.