Alpha Fornacis is the brightest star in Fornax, at just magnitude 3.87. It is the only star brighter than 4th magnitude in the constellation. It has the proper name Dalim in Piazzi's Palermo Catalogue of 1814, and was formerly identical with 12 Eridani.
Properties and Components
Alpha Fornacis is a rather nearby binary star, only 46 light years away, and has high proper motion. Its components appear at magnitudes 3.9 and 6.5. Both stars of this 269-year binary appear yellowish; they made their most recent close pass in 1947, will widen to 5.8" by the year 2020, and will be at their maximum separation in 2082. Their average distance apart is 56 AU, but their elliptical orbit takes them from a minimum of 15 apart to a maximum of 97 AU.
α For A is a coolish (6240 K) class F8 IV subgiant with a luminosity four times the Sun's. α For B is a cooler (5500 K) class G7 V dwarf with a luminosity only half of the Sun's. The difference between the two is caused by their mass. The A component contains somewhat over 1.25 solar masses, while the B component only contains 0.75 solar mass. This in itself would make A more luminous than B, since its core temperature is higher as a result of increased gravitational compression. Higher mass stars also evolve faster. As a subgiant, higher-mass A is has nearly finished fusing hydrogen in its core, if it has not done so already. Lower mass B, however, still has a few billion years to go before it follows A's lead.
The pair is close enough for us to watch their orbital motion. In 2004, we finally saw a full orbit since discovery of their binarity in 1835. The orbital characteristics give higher masses than does evolutionary theory, probably as a result of observational error.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]