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.