THETA CAP (Theta Capricorni). The ecliptic cuts right through Capricornus (the "Water Goat"), with the Alpha and Beta stars (Algedi and Dabih) to the north of it, Gamma and Delta (Nashira and Deneb Algedi) to the south. In between, fourth magnitude (4.07) Theta Capricorni (of no proper name) sits practically on the ecliptic itself. Lying a mere half degree to the south, the star nicely marks the solar path. Like Sirius, it is a classic class A (A1) hydrogen-fusing dwarf, though at a distance of 158 light years, it is much farther away and thereby fainter in the sky. Radiating at a rate of 52 times that of the Sun from a 9620 Kelvin surface, Theta Cap is, though, almost exactly twice as luminous as the "Dog Star," the result of a higher mass of 2.5 times solar. Theory shows the star to be in the middle of its hydrogen-fusing "dwarfhood." With an age of 360 million years, it has another 225 million to go before the internal fuel runs out, the core contracts, and the star begins its journey toward becoming a giant. Luminosity and temperature give a radius of 2.6 times that of the Sun. Like many of its kind, Theta Cap is a fairly rapid rotator, spinning equatorially with a speed of at least 114 kilometers per second, which with the radius gives a rotation period of under 1.1 days. The spectrum shows the star to be quite rich in heavy elements (silicon double that in the Sun, sodium more than ten times solar), though deficient in helium, probably the result of gravitational settling. The spectrum also shows the Theta Cap to be double with a very close companion that orbits every 2.3 years. Nothing is known of the companion, but if it carries a solar mass, it orbits at an average separation of 2.3 Astronomical Units, which if local, would place it within our asteroid belt.
Written by Jim Kaler 9/12/08. Return to STARS.