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.