ALPHA FOR (Alpha Fornacis). Many are the quiet areas of the sky, those that have myriad faint stars but lack the luster of the Orions and the Bears and the Argos. To the ancients, these regions were the "unformed." With the explosion of astronomical knowledge that began with Copernicus, however, there was a distinct need to "fill in the blanks" with new constellation patterns. Rather than from myth, many were chosen from the culture of the times. Thus we see a microscope (Microscopium), a telescope (Telescopium), and among many others, Fornax, the Furnace. While not an easy pattern to see, at least its luminary, known but as Alpha Fornacis, or Alpha For, reaches toward the brighter end of fourth magnitude (3.87). Rather nearby, only 46 light years away, it is a pity that the star is so little known, as it harbors a delight. Most stars that make the constellation patterns are far more luminous than the Sun. In Alpha For, however, we see a double star that consists of a pair that spans the solar qualities. The brighter, fourth magnitude (3.9) Alpha For A, is a coolish (6240 Kelvin) class F (F8) subgiant with a luminosity four times that of the Sun. The fainter (Alpha For B), is a near-seventh-magnitude (6.5) cooler (5500 Kelvin) class G (G7) dwarf with a luminosity only half that of the Sun. The difference between the two is caused by mass. The "A" component has a mass somewhat over 1 1/4 that of the Sun, while the "B" member has only 3/4 of the solar mass. This difference in itself will make "A" more luminous than "B" since the core temperature is higher as a result of increased gravitational compression. Higher mass stars also die first. As a subgiant, higher-mass "A" is coming close to ceasing hydrogen fusion in its core if it has not done so already. Lower-mass "B," however, still has a long way to go, a few billion years or so, before it follows "A"s lead. The pair is close enough for us to watch the orbital motion. They loop around each other with a 269 year period at an average distance of 56 Astronomical Units (the AU the distance between the Earth and Sun), or about 4 seconds of arc as seen from Earth. The elliptical orbit takes them from a maximum of 97 AU apart to a minimum of 15; they made their most recent close pass in 1947 and will be at their maximum separation in 2082. In 2004, we will finally have seen a full orbit since discovery of binarity in 1835. The orbital characteristics give higher masses than does evolutionary theory, probably as a result of simple and always-present observational error.
Written by Jim Kaler. Return to STARS.