NU ERI (Nu Eridani). Some stars just stand out in their classes.
Just west of Mu Eridani and the third stepping stone in Eridanus, the River (third stone if you count
fainter Omega), fourth magnitude (3.93,
just a hair brighter than Mu) Nu Eridani
is the champion of its kind. Even by ordinary standards, Nu stands
out as a hot (22,000 Kelvin) blue class B (B2) "giant" (but see below) that lies
a respectable 590 light years away, somewhat farther than Mu's 530
light years. The distance is enough that, even though the star is
well off the plane of the Galaxy, there is sufficient dust
in the line of sight to dim it by 10 percent. (Like Mu, Nu is part
of the local tilted "Gould Belt" of hot stars.) A luminosity of
5200 times that of the Sun (which includes
a whopping amount of ultraviolet light) leads to a mass of 8.5
times solar, and clearly shows the star to be not a giant, but a
hydrogen-fusing dwarf about midway through its projected dwarf life
of 28 million years, when it will indeed give up internal H-fusion
and start to turn into a true giant. Nu Eri's claim to Hall of
Fame status lies in its subtle variability (typical of stars in its
class) as an extraordinary "Beta Cephei"
star, one that chatters away by a few hundredths of a magnitude
with multiple periods all going on at the same time (caused by the
valving of heat below the stellar surface). The largest
oscillation makes the star vary by just 0.04 magnitudes (4 percent)
over a period of 4.16 hours. The clear record holder, Nu is
observed to have 11 more oscillation periods, the shortest being
3.03 hours (0.0012 magnitudes), the longest 4.27 hours (0.025
magnitudes). At the same time, this unique star has two more much
longer oscillations of a few thousandths of a magnitude over 2.31
and 1.63 days, making not just a rather amazing Beta Cep star but
also a "slowly pulsating B" (SPB) star like Mu Eri and the
prototype of the class, 53 Persei. The
multiple periods all interact to produce numerous "beat" periods of
the kind you hear when two just-out-of-tune guitar strings play
against each other. All this action is watched by a distant,
though probable, 13th magnitude companion 51 seconds of away. This
(most likely) K5 dwarf orbits with a radius of at least 9200
Astronomical Units over a period of at least 300,000 years. Study
of Nu's oscillations revealed that both the stars used for
comparison, Mu and Xi Eri, are also variable. While Mu and Nu Eri
are not real a pair, they are not all that far apart, just 60 light
years. From each, the other would shine at the minus first
magnitude, not much different than Canopus, or even Sirius, shines in our sky. And that is
nothing compared with Rigel. Just under
200 light years away, Rigel would brighten Nu Eri's sky at
magnitude -3, about like our Jupiter at opposition to the Sun. At
8.5 solar masses, Nu Eri is right near the edge of being a supernova candidate. If it
does not blow up, it will become a massive white dwarf, maybe one with
a rare neon-oxygen interior. (Oscillation data from M.
Jerzykiewicz et al. in "Astronomy and Astrophysics.")
Written by Jim Kaler 2/9/07. Return to STARS.