Alpha Caeli is Caelum's brightest star, at magnitude 4.46. Caelum, the Engraving Tool, is among the dimmest of the 38 modern constellations invented between about 1600 and 1800.
Components
A double star, α Cae consists of a somewhat sunlike class F2 V main sequence star, coupled with a much fainter class M0.5 dwarf at magnitude 12.5. The companion's faintness and 6" separation from the primary makes it very difficult to observe in small telescopes. The true separation of α Caeli A and B is not really known. They were 3" apart in 1896, 6.3" in 1933, and have not been measured since! Assuming 6.3" today, the two stars are at least 1000 AU apart, and take at least 130 years to orbit each other. From B, A would shine 360 times brighter than our full Moon, while from A, B would glow redly with 100 times the brightness of Venus.
Properties
The system is fairly nearby, at 66 light years away. At that distance, the brighter F-class component shines with a surface temperate of 7100 K, and the luminosity of 5.2 suns. Its radius is only 1.5 times solar. Rotating with an equatorial speed of at least 52 km/sec, its rotation period is at most 1.4 days - much less than the Sun's. It is typical of hotter F stars that lie near the so-called "rotation break" which divides slower-rotating stars (like the Sun) from faster-rotating, hotter ones.
Though α Cae A has a mass of only 1.5 suns, it is much more massive than its companion, α Cae B, which weighs in at 0.3 suns. The low mass of B gives it a luminosity of only 1% that of the Sun. This red dwarf has a surface temperature of only 3800 K. Both temperatures are averages for stars of these spectral classes; its temperature has never actually been measured. Alpha Caeli B is a flare star, and can unpredictably brighten by a magnitude or more as a result of the release of magnetic energy. Alpha Caeli A may be a subtle Delta Scuti variable star.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]