DENEB ALGEDI (Delta Capricorni). As the Sun leaves the winter solstice and Sagittarius behind, it moves into its next zodiacal station, Capricornus, the "water goat, and in early February crosses between the figure's head and tail, between Algedi (Alpha Capricorni) and our star Deneb Algedi, the Delta star. "Deneb" in Arabic refers to "tail," the name found in several constellations including Deneb itself in Cygnus, Denebola (the Lion's tail), Deneb Kaitos (the Sea Monster's tail), and here the "Kid's tail." The star is a fascinating, confusing wonder, and astronomers seem unsure of just how to classify it. It is a white star with a temperature of 7700 Kelvin, placing it (like Vega) among the "A star" class. Deneb Algedi is a mid-third magnitude (2.87) star 39 light years away, from which we calculate a luminosity 8.5 times that of the Sun. It is not alone, however, but has a companion of unknown type that eclipses it every 1.023 days, causing the apparent brightness to drop by about 0.2 magnitude, just enough to be seen (with care) with the naked eye. (There are two other very faint and more distant companions one and two minutes of arc away.) It has been variously called a "main sequence star" that, like the Sun, lives off the ordinary fusion of hydrogen into helium, a giant star, one that has ceased such fusion and has expanded to larger proportions, and a "subgiant," an intermediate phase. Most likely it is in its last stages of ordinary solar-type life. Its greatest claim to distinction is that it is among the brightest of the "metallic A stars," those hotter stars that seem to be highly enriched in most metals yet have deficiencies in others like calcium. Theories for the odd chemistry include contamination from a companion that, in the process of dying, passed enriched matter to the surviving star. More likely, some process has separated various chemical elements, causing some to drift downward, others to rise upward. On top of all this, Deneb Algedi is suspected of being slightly variable all on its own, of a type called a " Delta Scuti" star, one that begins to pulsate in part as a result of the onset of its impending demise. With all these features, the star has spawned a small industry of research and publication, and a great deal remains to be learned about it.
Written by Jim Kaler. Return to STARS.