MU CET (Mu Ceti). Only fourth magnitude (4.27), and generally
un-noticed, Mu Ceti (of no proper name) holds a special place in great
Cetus at the very top of the skewed
circlet that makes the Sea Monster's head. It's also quite an
amazing quadruple star, but one whose structure remains obscure.
All the components are too close together for direct resolution at
the telescope, and require more indirect means for study. The main
member, Mu Ceti A, is a middle-of-the-road class F (F0) subgiant
(but see below) 84 light years away that is 7 to 11 times the
brightness of the lesser members of the crowd. After subtracting
their visual luminosities, Mu-A itself shrinks to fifth magnitude
(4.60). With a surface temperature of 7300 Kelvin, it shines to us
with a total luminosity of 7.5 Suns, from
which we derive a radius of 1.7 solar, a rotation period (from a
projected equatorial velocity of 59 kilometers per second) of under
1.5 days (vastly faster than the Sun, but appropriate for its
class), and a mass of just 1.6 solar. We also see then see that
the star is really a dwarf about two-thirds of the way through its
hydrogen-fusing lifetime of five billion years. Mu Ceti A has a
reputation of being a modest Delta
Scuti variable that also has a modest circumstellar dust
excess. Little else is known. The first hint of multiplicity
comes from the spectrum, the star long known to be a spectroscopic binary with a period
of 1202 days (3.29 years). Mu Ceti is also not that far from the
ecliptic, and is occasionally
occulted by the Moon, which then revealed a companion only 0.1
seconds of arc away (later seen to be 0.3 seconds of arc). The
brightness, from another occultation, suggests that this companion
is a class G3 dwarf very much like the Sun that should orbit at a
distance of 3 Astronomical Units, which is not consistent with
later measures of actual distance (3 AU too small). The later
occultation also revealed two more stars much closer in, which
(again from their brightnesses) appear to be G2 and G6 dwarfs.
Perhaps it is one of these that causes the spectroscopic variation.
The whole situation is mightily confused, as we really do not know
how the quartet is orbitally structured. Good luck to later
researchers!
Written by Jim Kaler 12/21/07. Return to STARS.