51 ERI (51 Eridani). Hidden in eastern Eridanus, dominated by fourth magnitude, Greek-lettered Mu and Nu Eridani, 51 Eri at first doesn't seem like that much of a star. A seemingly ordinary fifth magnitude (5.23) class F (F0) dwarf 96 light years away (give or take 1), it shines with the luminosity 5.4 Suns from a 7270 Kelvin surface, which is not all that hotter than the Sun itself. Indeed it seems almost sunlike (more super-sunlike) with a radius of 1.5 times solar, and from straightforward theory a mass of 1.5 Suns. One significant feature is that 51 Eri lies above (hotter than) the "rotation break," where stars begin to spin faster, 51 Eri rotating at a minimum speed of 84 kilometers per second (at the equator), which gives it a rotation period of under 0.9 days. Now things get more interesting. 51 Eridani is the northernmost outpost of the "Beta Pictoris Moving Group," a set of stars with common origins that are moving more or less through space together, though slowly separating from one another. "Moving groups" are spread-out, young. low mass versions of the much more spectacular OB associations. Beta Pic itself is a mid class A dwarf with not only a thick infrared-radiating disk but a planet. Among the other brighter members of the moving group are 53 Eri, Zeta Leporis, and d Scorpii (Roman letters occasionally used for star names). The group seems to be related to the vast Ophiuchus-Scorpius-Centaurus association complex. At a separation of 67 seconds of arc, at least 2000 Astronomical Units, lies an 11th magnitude (10.6) M0.5 red dwarf called GJ 3305 that seems to be moving along with 51 Eri and probably belongs to it as a wide binary companion. With a mass of about 0.5 Suns (and a better-determined mass from the literature of 1.75 Suns for 51 Eri itself), the two must take at least 60,000 years to orbit each other. Both are very young, a recent study suggesting that 51 Eri is not just an ordinary dwarf, but is so young as to be a "pre-main sequence star," one still settling into its place as what will eventually be a stable hydrogen fusing dwarf star like the Sun. GJ 3305 makes its own mark by being highly magnetically active, as so many low mass red dwarfs are, especially those still showing off their youthful high rotation speeds, which are responsible for the super-solar magnetic activity. (Thanks to E. D. Feigelson et al. in the Astronomical Journal, vol, 131, p 730, 2006, and also to Paolo Colona for suggesting the star.)

Written by Jim Kaler 12/20/13. Return to STARS.