SAIF AL JABBAR (Eta Orionis). While Orion's Belt is the most striking part of the great
Hunter, the area just below it is equally intriguing. Immediately
south (and in the Sword) we find the Orion Nebula. Just southwest of the
left hand Belt star is multiple Sigma
Orionis, while southwest of the right of the right-hand star
is another, lesser-known, multiple, Eta Orionis. The honor of
representing the Sword has gone to several stars, and is now
officially reserved for Kappa Orionis
(Arabic "Saif," for "Sword"). But Iota
Orionis claims it as well with "Na'ir al Saiph," and even our
Eta has held it, R. H. Allen giving us "Saiph al Jabbar," the
"Sword of the Giant." The name is no longer in any official use,
but is given here to show the complexity of the history of star
names. With its brilliant surrounding company, Eta Ori is sadly
overlooked. Glowing at third magnitude (3.36) from a great
distance of 900 light years (though uncertain to about 25 percent,
more or less consistent with the Belt Stars), Eta at first
telescopic look consists of a close pair of hot blue class B (B1
and B2, 27,500 and 23,000 Kelvin, magnitudes 3.63 and 4.91) stars
set 1.7 seconds of arc apart, the duplicity discovered in 1848.
(Together, they have also been classified as B0.5.) While really
the same color, contrast effects have led some to describe the
dimmer as "purplish." The fainter is also a "Be" (for "emission")
star, implying an encircling ring of gas. Accounting for
ultraviolet light gives total respective luminosities 32,000 and
6200 times the of the Sun. The stars are
youthful hydrogen-fusing "dwarfs" (so called in spite of their
being 8 and 5 times the solar radius) that are only about 10
million years old. Both are massive stars, 15 and 9 times solar.
The brighter of the two harbors a wonderful surprise. Spectra show first that it has a
close orbiting stellar companion that circles and eclipses it every
7.989268 days and eclipses it (causing a 0.15 magnitude drop in
brightness) much in style of Sheliak
(Beta Lyrae). Further investigation reveals another companion that
takes 9.5 years to make a circuit. If the orbits are face on, the
closer one is only 0.09 Astronomical Units (AU) from the brilliant
star (25 percent Mercury's distance from the Sun), while the more
distant neighbor lies 12 AU away (a bit farther than Saturn is from
the Sun). Nothing is known about either. The two seen through the
telescope are much farther apart, at least 470 AU, and must take at
least 2000 years to orbit. But we are not yet done. Nearly two
minutes of arc away is a fainter ninth magnitude (9.4) star that is
considered to be part of the system. Again, nothing is actually
known about it. If it belongs to Eta proper, then its brightness
reveals it to be a class A (A8) dwarf, which would have a
temperature of 7800 Kelvin, a luminosity 8 times solar, and a mass
1.7 times solar. Eta Ori is thus quintuple, and a serious rival of
nearby Sigma. From the fainter telescopic companion, the brighter
star would appear triple, with maximum separations of 2/3 minute of
arc (not separable by eye) and 1.5 degrees, while from the ninth
magnitude star, the system would appear quadruple, with maximum
separations of 0.6 seconds of arc, 75 seconds of arc, and 0.8
degrees. From the inner quadruple, the dim outer star would shine
with the light of a couple of our full Moons, this star so far away
that it is almost certain to be torn from the inner set. The
primary of the gang is so massive that it will surely someday
explode, while the "purplish" one may just barely escape that fate
and die as a massive white dwarf, as will the others three members
of the set.