MUPHRID (Eta Bootis). Arcturus, the
orange giant that quite dominates the constellation Bootes, the Herdsman, nicely points the way to a
fainter star almost directly to the west of it. Just over the line
into third magnitude (2.68), Muphrid ranks third in the
constellation, falling right behind Izar.
With the constellation's stars lettered more or less from north to
south (excepting brilliant Arcturus), poor Muphrid got only the
seventh letter of the Greek alphabet, Eta. The name is
mystifyingly obscure. It came originally from a name for Arcturus
that referred to a "lance bearer," and that was later applied to
other stars in the neighborhood, and by other people to Eta Bootis
"alone," which in Arabic is something close to the name "Muphrid."
So the name actually refers not to a property of the star or to its
location, but in circular fashion to another name! The star itself
is of considerable interest. Though a class G (G0) star with a color
and temperature (6100 Kelvin) near that of the Sun, it is classed as a "subgiant," meaning
that it has started to evolve. From its close distance of 37 light
years, we find it to be only 9 times brighter than the Sun
(relatively dim for a naked-eye star), which together with
temperature yields a radius 2.7 times solar, all these again
indicative of an evolving star. With a mass about 1.5 times solar,
Muphrid has just ceased core hydrogen fusion. Its quiet helium
core surrounded by a shell of fusing hydrogen, the star is just
beginning to make the transition to becoming a red giant. Muphrid
is also double, its spectrum revealing a close companion (possibly
a dead white dwarf) with an orbital period of 1.35 years, the two
separated by about the distance between Mars and the Sun. Muphrid
also has a visible "companion," a ninth magnitude star two minutes
of arc away, easily seen in small telescope. The "companion,"
however, is merely a line of sight coincidence. The odd thing is
that absolutely nothing is known about it the star, rather strange
for one that bright. Most interesting, Muphrid is one of a
somewhat unusual breed of "super metal rich" stars. "Metal" to an
astronomer means "everything else" other than hydrogen and helium.
Muphrid has a metal abundance about twice solar. It is surrounded
by a hot X-ray emitting corona with a temperature of 3.5 million
Kelvin, nearly twice as hot as the solar corona.