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.
Written by Jim Kaler. Return to STARS.