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.