Alpha Circini

At magnitude 3.19, Alpha Circini is the brightest star in Circinus, the Compass, a small modern constellation overwhelmed by nearby southern Centaurus.

Properties

From a distance of only 54 light years, the star shines with a luminosity of 11 suns, which implies a radius of 1.7 times solar, and a mass of 1.8 suns. A temperature of 8000 K suggests spectral class A7. But because of its peculiar spectrum, this star's subclass is hard to judge. α Cir is an "ApSrEuCr" hydrogen-fusing main-sequence star, where the "p" stands for "peculiar," and the other letters indicate strong enhancements in strontium, europium, and chromium.

Alpha Circini is the brightest "roAp" star of the sky. "roAp" stands for "rapidly oscillating class A peculiar". The star oscillates and varies quickly, with periods of 6.825 and 6.832 minutes (with respective amplitudes of 0.002 and 0.004 magnitudes). It has other periods, including one of 2.94 hours, that may indicate pulsations like those seen in δ Sct. A four-month period may be present as well.

"Ap" stars are characterized by strong, localized magnetic fields that seem to be arranged in patches on their surfaces, where various chemical elements are enhanced by diffusion/elevation processes. No one really knows how they are generated; the classic case is α2 CVn (Cor Caroli). Variations in α Cir's spectrum and in brightness imply a rotation period of 4.48 days, and an axial tilt of 42 degrees. Its magnetic field strength is about 500 times that of our Sun.

Companion

Some 16" away lies a likely companion, a magnitude 8.47 class K5 V dwarf less massive than the Sun. It orbits at least 260 AU away with a period of at least 2600 years. If at that separation, α Cir proper would see the companion shining twice as bright as our full Moon, while the companion sees α Cir 130 times brighter.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]