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Hoedus II
(Eta Aurigae). Tucked to the south-southwest of Capella in Auriga is a small and beloved triangle of stars, the
"Kids" to Capella's "she-goat." The two at the bottom of the
triangle, Eta and Zeta (Hoedus I), were the
original Kids, while the strange eclipsing variable Epsilon (Almaaz) was added later. The
original two remain the Roman's "Haedi," the western (Zeta) Hoedus
I, the eastern our Hoedus II, which is far more commonly known just
as Eta Aurigae. Though angularly close, the trio have nothing to
do with one another. Shining at third magnitude (3.17), Hoedus II
is in the middle in terms of brightness. At a distance of 220
light years, it is however by far the closest, Zeta (Hoedus II)
lying 850 light years from us, Almaaz close to 2000 light years.
This class B (B3) ordinary (core hydrogen-fusing) dwarf, with a
surface temperature measured at 16,600 Kelvin, is also by far the
hottest of them. Unlike the other Kids, it is a single star, with
no real evidence for a companion. Accounting for considerable
ultraviolet light, Hoedus II radiates a luminosity 760 times that
of the Sun, which yields a radius 3.3 times solar and (from the
theory of stellar structure and evolution) a very considerable mass
5.5 times that of the Sun. With an age of
about 45 million years, the star is half way through its dwarf
lifetime. Spinning with an equatorial velocity of at least 95
kilometers per second, it rotates in under 1.8 days, in contrast to
the 25-day period of the Sun. While Eta Aur is considered very
stable and so ordinary that it is used for a standard against which
to compare others, it is not without some controversy. The
temperature is notably low for the spectral class, which implies a
temperature closer to 18,500 Kelvin. The luminosity has also been
estimated at a higher 955 times that of the Sun. There is
unconfirmed evidence for spectral variations with a 24-day period,
which if they exist probably originate in the stellar atmosphere.
An extremely weak magnetic field has been detected, one only a
couple times that of the Earth. Though not orbited by any other
star that we know of, Hoedus II may still have something of a
family, as it has been considered to be part of the vast (as one
can see from the name) Cassiopeia-Taurus "association" of hot class
O and B stars, an extended group that stretches across 100 degrees
of sky from Taurus to Orion, whose members were born more or less at the same
time and are now separating forever.