TABIT (Pi-3 Orionis). Orion, the Hunter, holds a cloak in his left arm
(when he is drawn facing you) made of a near-vertical string of
stars that stand to the west of the main figure. Starting at the
north end, the "Pi's" are numbered Pi-1 through Pi-6 Orionis
(though have nothing physically to do with each other). In the
middle lies the brightest, mid-third magnitude (3.19) Tabit (an
Arabic word probably meaning "the Endurer"), Pi-3 Orionis, a star
that brilliantly illustrates the confusion often surrounding star
names. An earlier list of Pi-1, 2, 3, and 4 (numbered in the
traditional fashion, west to east), was later and oddly changed to
the current north-to-south Pi-3, 2, 4, 1,
so Pi-3 was once Pi-1. Worse, the name "Thabit" (same meaning),
which was most likely applied to Upsilon Orionis (well to the
southeast and below Orion's Belt), was
in more recent times somehow changed to Tabit, and then re-applied
to the current Pi-3. The westernmost of the Pi-stars, Pi-3 is also
(consistently) Flamsteed's
number 1. Physically, this class F (F6) hydrogen-fusing dwarf, the
closest and least luminous of the Pi's, is not all that far from
solar. At 6400 Kelvin, a bit warmer than the Sun, Pi-3 Ori has the minimal distinction of
needing no correction for infrared or ultraviolet light. From its
distance of 26 light years (the farthest of the "Pi's" 1340 light
years off), we find a luminosity of 2.7 times that of the Sun,
which leads to a radius of 1.3 solar, a rotation period (from the
rotation velocity of at least 17 kilometers per second) of less
than 3.9 days, and a mass (from the theory of stellar structure and
evolution) of almost exactly 1.25 solar. With an age of 2.2
billion years, the star is 55 percent of the way through its
hydrogen-fusing lifetime of 4.0 billion years (the higher mass
giving it a lifetime about 40 percent that of the Sun). X-rays
suggest a hot corona, typical of solar-type stars. The well-known
motion shows that the star made a close approach to us 210,000
years ago at a distance of 15 light years, when it would have
appeared over twice as bright. Pi-3 may be a Delta Scuti variable, the brightness
changing by about 5 percent, and then again it may not, the
variation not confirmed.