MU ERI (Mu Eridani). Eridanus, the River,
winds to the west of Orion. Springing
from bright-third-magnitude Cursa (Beta
Eridani), it flows to the west and then far to the south to end in
first magnitude Achernar (Alpha), the
ninth brightest star in the sky. Excepting rather faint Omega (which is usually not part of the
classic pattern), Mu Eri is the first major stepping stone in the
River's course, Nu Eri the second. Both are hot class B stars, Mu
a B5 subgiant (implying that the star is giving up, or has just
given up, core hydrogen fusion), Nu a B2 giant. Mu Eri, 530 light
years away, has an appropriately high temperature gauged at 14,800
Kelvin (from old measures that could use improvement). A small bit
of interstellar dust dims it by a mere six percent. Temperature
(to account for ultraviolet radiation) and distance give a
substantial luminosity of 1675 Suns and a
radius of 6.2 times solar. In league with most stars of its class,
it is a rather fast rotator, its equatorial spin velocity at least
145 kilometers per second, which yields a rotation period of under
two days -- not fast or short enough apparently to make it a B-
emission star like Zeta Tauri. The star
is indeed an evolutionary subgiant, with a mass between 5.1 and 5.5
times that of the Sun (depending on its exact state) and an age of
between 70 and 90 million years. After ejecting its outer envelope
when it becomes a true giant, it will die as a massive white dwarf. Not alone, Mu
Eri is a spectroscopic double (one
known through velocity shifts in its spectrum) with a companion
that takes only 7.36 days to orbit. Ignoring the mass of the
companion, the two are separated on average by only 0.13
Astronomical Units (33 percent Mercury's distance from the Sun).
While many doubles with this age and separation have circularized
their orbits through their gravitational interactions, this one
still has a fairly high eccentricity that takes the two between
0.10 and 0.16 AU apart. In a study of Nu Eri, wherein Mu was used
as a comparison, Mu was discovered to be very subtle "slowly
pulsating B star" (like 53 Persei) with a
principal period of 1.63 days (as well as several other periods).
It was also found to be an eclipsing variable in which
the small companion gets in front of its larger mate every 7.4 days
to produce a dip of a couple hundredths of a magnitude, showing the
orbital plane to be nearly edge-on (and the above rotation speed
likely to be the true value). The hottest stars, those of classes
O and B, are supposed to stick closely to the Milky Way, the disk of our Galaxy, where stars are
born. Mu Eri, Nu Eri, Rigel, those of Orion's Belt and Sword, and many others of the kind,
though, are rather far off it (Mu some 300 light years south of the
Galaxy's plane). It and the
others are part of the famous "Gould Belt," named after B. A.
Gould, in which the distribution of massive stars has been locally
bent and distorted as compared with their more distant brethren in
the Milky Way. The distortion was also noted by John Herschel from
his observations in the southern hemisphere. It is caused by a
huge region of sequential star formation that the Sun just now
happens to be passing through.
Written by Jim Kaler 2/02/07. Return to STARS.