32 ORI (32 Orionis). About one and a half degrees southeast of Bellatrix, the northwestern star of Orion's famed seven-star main figure and at bright second magnitude (1.64) the 26th brightest star in the sky, lies another class B star, this one a fourth magnitude (4.20) B5 dwarf, known commonly by its Flamsteed number as 32 Orionis. Or should we say "stars." Though not as bright, blue, and hot as B2 Bellatrix, 32 Orionis has the merit of consisting of TWO stars, a fourth magnitude (4.5) B5 dwarf joined with a somewhat fainter sixth magnitude (5.8) B7 dwarf, together making an "elegant object" of "white and pale white" components (Smythe and Chambers). The duplicity was discovered by William Herschel, who made the first measure of separation around 1780 and also discovered orbital motion, the two now known to be in a 614 year orbit of which we have observed about a third. Now 1.3 or so seconds of arc apart (about as they were in Herschel's time), the highly inclined orbit (tilted just 7 degrees from the line of sight) can make them devilishly difficult to separate when near their apparent closest. At a distance of 303 light years (give or take 18), the two are on the average 149 Astronomical Units apart, a fairly substantial eccentricity taking them from 182 AU at their farthest extreme to 115 AU at their physical closest. Kepler's Laws then demand a summed mass of 8.8 times that of the Sun, consistent with the spectral classes.

32 Ori "Elegant" 32 Orionis consists of a B7 dwarf in visual orbit about a hotter B5 version, the two at most just a couple seconds of arc (note the scale) apart. In reality they go around a common center of mass. One has to admire the ability of the old astronomers who measured them near their apparent closest. Only about a third of the 614 year orbit has been covered since William Herschel's discovery, but that is enough to fit a good curve. The orbit, seen nearly edge on, is highly distorted by the tilt and orientation, which places 32 Ori A well off the focus of the apparent ellipse. Given the measured distance of 303 light years, the two average 149 Astronomical Units apart, an eccentricity of 0.22 taking them between 182 and 115 AU from each other. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

The other approach to mass is through luminosity, temperature and theory. Substantial amounts of ultraviolet light from surfaces heated to 16,200 Kelvin (for 32 Ori A) and 13,000 Kelvin for the other (as estimated from its class), allowance for a tenth of a magnitude dimming by interstellar dust, and distance then give respective luminosities of 450 and 80 times that of the Sun and radii of 2.7 and 1.8 solar. The brighter, 32 Ori A, is a typically rapid spinner for its class, with an equatorial velocity of at least 170 kilometers per second, which gives it a rotation period under 0.8 days and an apparent oblateness of about 5 percent. The theory of stellar structure and evolution yields respective masses to "A" and "B" of 4.8 and 3.0 times that of the Sun, for a sum of 7.8, short of (but given various uncertainties not bad agreement with) the value of 8.8 derived from the orbit. The two can be reconciled by bringing the orbital size down to 143 AU from its calculated value of 148, which may be simply a matter somewhat too great a distance. Of more significance, both stars are very young, near the "zero age main sequence" of stars just beginning their hydrogen-fusing lives. The significance of some X-ray emission is not known. The core of more massive 32 Ori A will burn out first, in just 100 million years, to be followed by "B" 250 million years later (more massive stars using their internal fuel faster because of higher temperatures). Far enough apart so as not to affect each other much when they evolve into giants and lose their outer envelopes, the system will likely die as a massive white dwarf binary, though it is hard to tell what effect the mass-losing winds of each may have on the other.

Written by Jim Kaler 2/01/13. Return to STARS.