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.