PSI CAP (Psi Capricorni). The classical figure of Capricornus, the Water Goat, bottoms out to the south with the fourth magnitude red giant Omega Capricorni. Just two degrees to the northwest of it lies just-barely-fainter (magnitude 4.14) Psi Cap, whose name lies almost as far down the Greek alphabet. Other than line-of-sight, there is no relation between the two. Psi Cap is a rather local Sunlike class F (F4) dwarf with a distance of only 47.9 light years, while Omega is more than 10 times farther away, testimony to the relative faintness of Psi and the grandeur of Omega. With a temperature of 6620 Kelvin, Psi Cap is just 840 degrees warmer than our own Sun. With little infrared or ultraviolet to account for, the luminosity comes in at just 3.8 times solar, which leads to a mass of 1.35 times that of the Sun. Still fairly youthful, the star is about a third to half of the way through its four billion year hydrogen-fusing lifetime. At first EVERYTHING seems almost solar, even the radius of just 1.5 times that of the Sun and the near-solar metal content. But then come the differences. With a projected equatorial spin-speed of 39 kilometers per second, Psi Cap lies right at the so-called "rotation break," the point at which (going from cool to hot) rotation speeds suddenly increase as a result of a the drop-off in the magnetic fields caused by convective circulation. (The winds from cooler stars drag out the stellar magnetic fields that then act like ropes that drag back and act like stellar brakes.) Given the radius, the star then rotates in under 1.9 days. Rotation indeed seems to be Psi Cap's strong point, as it is the first star whose "differential rotation" has been observed through the appearance of its spectrum, the star (like the Sun) taking longer to rotate at higher latitudes than lower. Such rotation is in part responsible for magnetic activity, and indeed Psi Cap shows it. Mid-temperature solar-type dwarfs are of course known for their planets. Psi Cap seems not to have any, rather it does not have the kind of debris disk that is left over after their creation and that is fueled by planetary collisions. Nor does it have any recognized companion, the star seemingly quite alone.
Written by Jim Kaler 9/25/09. Return to STARS.