PSI ORI (Psi Orionis = Psi-2 Orionis). Great Orion is filled with hot blue stars, many of which are vaguely related through birth within various subgroups of the vast Orion OB1 association. The two most prominent blue-white class B stars in the constellation are brilliant zero magnitude Rigel at the southwestern corner of the figure and Bellatrix (fourth brightest of the second magnitude set) at the northwestern corner. About three degrees almost due south of Bellatrix lies another hot star with the same class B2 flavor, though listed as a subgiant, fifth magnitude (4.59) Psi Orionis, which is dimmed a hair (under 0.2 magnitudes) by interstellar dust but mostly by its great distance of 1140 light years (with a rather large uncertainty of 270). The name is a bit of a problem. Bayer gave us a single "Psi" (the next to last letter of the Greek alphabet), whereas Flamsteed later called it Psi-2 (Flamsteed's 30 Ori), Psi-1 ( 25 Ori) being an unrelated fifth magnitude class B dwarf a bit over a degree to the south. (The numbers refer to west-east position, not brightness.) There has never been an agreement on the naming, so Psi Ori is still sometimes called Psi-2. We'll stick with "Psi," which would be dimmer except for it being not one star, but two in a very tight orbit that takes only 2.526 days to complete. Psi Ori is sometimes called an ellipsoidal variable that changes by a few hundredths of a magnitude. In such stars, the components are so close together that they raise mutual tides in each other and thus present different cross sections to us as they orbit. But others say there are indeed true mutual eclipses. Stars as close together as these are difficult to analyze. The dimmer companion to the B2 subgiant seems to be an uncertain B0, maybe B2, dwarf. Since we do not know, we loosely adopt a temperature of 21,200 for both, assuming that to be appropriate for calculating the amount of ultraviolet light from the system. We then find a total luminosity of at least 10,500 Suns. A suggested luminosity ratio (brighter to dimmer) of 4.5 gives respective individual luminosities of 8600 and 1900 Suns. Masses then come out to be close to 10 and 7 times solar, making the brighter (which is most likely really a hydrogen fusing dwarf) a possible supernova candidate. Kepler's laws then require a mean separation of just a tenth of an Astronomical Unit or so (20 solar radii), and given stellar diameters of a few solar radii, it is no wonder the stars are out of round. While the numbers may be way off, they at least give something of the flavor of the system. Adding to the taste, the brighter of the pair seems to be a Beta Cephei type variable. Such stars typically chatter by a tenth of a magnitude or so over multiple periods measured in hours. Psi Ori (or, if you like, Psi-2) varies subtly with periods of 2.29 and 2.24 hours, making it the nearest Beta Cep binary star with close components to Earth. Just short of three seconds of arc away lies a ninth magnitude companion, that if real would have to be a class A dwarf. With a mass of double solar, it would be at least 1000 Astronomical Units from the inner binary and take at least 7500 years to make a full circuit. At an angular separation of 88 seconds of arc is 12th magnitude Psi Ori C, but that one is just in the line of sight, leaving the system triple with plenty enough problems to keep an astronomer well occupied.

Written by Jim Kaler 1/18/13. Return to STARS.