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.
Written by Jim Kaler. Return to STARS.