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.