35 PSC (35 Piscium), better known as UU Psc, the double Roman letter telling of
a variable star. Though the variation of just five percent is not
really visible to the eye, the source of the variation is
intriguing indeed. Just over the line into sixth magnitude (5.79),
the star (somewhat hard to spot) lies to the east of the Circlet of Pisces four degrees to the northeast of fourth
magnitude Omega Psc. At a distance of
59 light years, this class F (F0) subgiant (but see below) is not
one star, but two identical stars in very tight orbit with a period
of just 0.842 days, 20.2 hours. There are no accurate parallax measures, so the
distance is derived from the double-star properties and is
probably not all that accurate. But it's pretty much all we've
got. The orbit is not in the line of sight, so the stars do not
eclipse each other. But they are so close that mutual tides
flatten them out. As they revolve around each other, they then
present different-sized faces to us, and thus vary as "ellipsoidal
variables," the most famed of which is Spica. Given their similarity, each star
must be just barely over the line into seventh magnitude (6.54).
From that, distance, and an estimated temperature of 7400 Kelvin,
each star radiates the light of 6.4 Suns,
from which we derive radii of 1.55 Suns. A projected equatorial
rotation velocity of 76 kilometers per second, give a rotation
period of under a day, which fits with rotation that is tidally
locked to orbital revolution. From theory, each star must then
carry a mass of 1.6 times the solar mass. Not subgiants at all,
each is a dwarf about halfway through its 2-billion-year hydrogen-
fusing lifetime. The masses, the orbital period, and
Kepler's Laws show that the stars are separated by a mere
0.0258 Astronomical Units, or 5.6 solar radii, or just 3.5 the
radii of the individual stars. No wonder they are so distorted!
Lunar occultation measures of separation reveal a consistently
small value. For now they are peaceful siblings. But one star
must be slightly more massive than the other, and when its hydrogen
core is used up, will be the first to evolve into a larger giant. One star will then
encroach on the other, and perhaps lose mass to its now-denser
mate. Indeed one star could destroy the other. And we only have
a billion or so years to wait to find out what will happen. The
whole affair is watched from a great distance by a third member of
the system, a cooler class F3 eighth magnitude (7.51) dwarf
separated from the inner pair by 11 seconds of arc, which
translates into an orbital radius of at least 660 AU. Masses (that
of UU Psc C estimated from its class) and Kepler's Laws give an
orbital period of at least 8000 years. From C, the inner pair
would still appear to the eye as one star shining with the light of
15 full Moons, the two at most
just 8 seconds of arc apart.
Written by Jim Kaler 12/03/10. Return to STARS.