31 Ori (31 Orionis). Also known (erroneously) as CI Orionis. But more about that later. Here and there we find stellar patterns that mimic well-known asterisms. Given the number of stars in the sky, it's inevitable. A good example is that of the "Little Kids" in Auriga, a triangle of fainter stars on the east side of the constellation that is nearly identical to Auriga's prominent "Kids," which lie just to the south-southwest of Capella. Here is something of another example, within Orion, the celestial Hunter. Few asterisms are more celebrated than Orion's three-star Belt, which consists from east to west of Alnitak (Zeta Ori), Alnilam (Epsilon), and Mintaka (Delta). It's a region rich in stars, many of them a part of the two-to-five-million-year-old Orion IB association. Look then just below the Belt to find a set of stars that are more or less parallel to (and shifted a bit southwest of) the Belt, forming a "mini-belt." It's made of glorious and multiple Sigma Ori (near Alnitak), a gaggle of fainter stars near Alnilam, chief of which is fifth magnitude HR 1861 (a hot B1 subgiant) and that includes the eclipser VV Ori, and fifth magnitude (4.71) 31 Orionis (known by its Flamsteed number) to the southwest of Mintaka, the star a rather luminous orange class K (K5) giant.

At a distance of 508 (+/-48) light years, 31 Ori seems to suffer from just a small amount (0.19 magnitudes) of interstellar dust absorption. From these and a temperature of 4050 Kelvin (to account for infrared radiation) we find a total luminosity of 809 times that of the Sun, a radius of 58 solar (roughly 70 percent that of Mercury's orbit), and a mass of 3.5 solar masses. Various listings of angular diameter (the star an important calibrator for interferometric measures) give between 59 and 60 Suns, so the parameters all seem close to the mark. With an age of around 280 million years, the star, now a quiet helium- fuser, gave up core hydrogen fusion some 30 million years ago. The iron content is measured at half that of the Sun, but is unconfirmed. At a separation of 12.7 seconds of arc lies a tenth magnitude (9.70) companion, 31 Ori B. Though little orbital motion is seen, the two have kept the same pace against the background for 175 years, and are thus almost surely mated. From its brightness, the neighbor would be a class A8 dwarf of perhaps 1.7 solar masses that is only some 20 percent or so through its core hydrogen-fusing lifetime. Given the angular separation they must be at least 2000 Astronomical Units apart, which would (from Kepler's Laws) indicate a period of at least 40,000 years. For some reason, probably poor observations, 31 Orionis was noted as variable and given the variable-star name CI Orionis. Further studies have shown the star to be quite stable, yet the name lingers on.
Written by Jim Kaler 2/24/12. Return to STARS.