NA'IR AL SAIF (Iota Orionis). Dangling from Orion's Belt is the Hunter's mighty three-
star sword, the trio increasing in brightness toward the south.
"Three-star sword" is only a traditional appellation, however, as
a view with any sort of optical aid reveals nests of stars. The
complex "middle star" contains the famed Orion Nebula, which is lit by the tightly compacted
four-star "Trapezium" (Theta-1 Orionis), the quartet dominated by
the hot O (O6) star Theta-1 Orionis C (the Trapezium at the top of
a more-extended "Trapezium Cluster"). The bottom "star" is
dominated by the brightest of the set, bright third magnitude
(2.77) Na'ir al Saif, Arabic for "the Bright One in the Sword," the
name "Saif" (sword) wrongly transferred to Kappa Orionis (Na'ir al Saif receiving the
Iota designation from Bayer). A class O (O9) giant, and one of the
hottest and bluest stars that make Orion's classical figure, Na'ir
al Saif shines with a temperature of 31,500 Kelvin from a quite-
uncertain distance of 1300 light years, giving it a luminosity
(corrected for ultraviolet radiation and a bit of interstellar dust
absorption) of 14,000 times that of the Sun. An O9 star should
have luminosity at least double that, suggesting that the true
distance is closer to 2000 light years. The star is a complex
multiple dominated by the 15-solar-mass O star. At respective
distances of 50 and 11 seconds of arc lie an 11th magnitude class
A or F dwarf and a 7th magnitude B star, with true separations of
at least 4400 and 20,000 Astronomical Units and orbital periods at
least 75,000 and 700,000 years. The brilliant O star also has a
far more interesting hot class B1 spectroscopic companion that
orbits in 29 days at a separation of only half an AU (or so) in an
unusually eccentric path that takes it from 0.8 AU to a mere 0.11
AU. The collision of their strong winds produces powerful X-rays.
Na'ir al Saif and its close companion help reveal the power of
gravity and stellar dynamics. Twenty-six degrees to the south lies
fourth magnitude
Mu Columbae (in Columba,
the Dove); 40 degrees to the north in Auriga lies the variable star
AE Aurigae. The two,
with spectral classes nearly identical to that of Na'ir al Saif,
are hurtling away from each other in opposite directions at 200
kilometers per second. Called "runaway stars," they appear to have
been shot out of Orion. Recent calculations of the movements of
the stars explain the two runaways and Na'ir al Saif as well,
showing that they were all kicked out of the Trapezium Cluster.
Some 2.5 million years ago, the Trapezium region held a pair of
tightly-knit double stars. In a very close encounter between the
two binaries, two of the four stars were ejected, while the
remaining two, instead of being evicted, stayed more or less behind
in highly eccentric embrace. Na'ir al Saif still lingers near the
Orion Nebula, marking the place of the violent event. More violence is
in store, as the star is fated to explode as a supernova.