ALSAFI (Sigma Draconis). Poor little fifth magnitude (4.68) Alsufi, stuck in Draco (the Dragon) between Delta and Epsilon Draconis, seems too obscure even to mention. It has several significant features, however, that make it well worth a stop: its name, proximity, motion, and intrinsic luminosity (and associated mass), thus proving that apparent brightness is no indicator of interest. Why some faint stars have proper names and other much brighter ones (witness Gamma Cassiopeiae) is at times a mystery. Here there is some sense, as "Alsufi" erroneously derives from the Arabic "Athafiyy," which (from Allen) refers to a "kitchen tripod" that references a trio of stars, the name falling finally to Bayer's Sigma alone. At a distance of only 18.8 light years, Sigma Dra is one of the nearest stars to the Earth, ranking within the closest 100 star-systems (wherein doubles and multiples are considered as units), Alsufi coming in at number 65. Most naked- eye stars are more massive and luminous than the Sun. Here's a fine chance to see one that is "subsolar," Alsufi one of the few class K (K0) dwarfs readily visible (in a dark sky) to the naked eye. With a temperature of 5315 Kelvin, the star radiates rather feebly at a rate of just 43 percent that of the Sun, which leads to a radius of 0.77 solar and a mass between 0.8 and 0.9 solar. Such stars live as dwarfs enormously long times (as a result of slow hydrogen fusion caused by low interior temperatures related to low mass), as long as the age of the Galaxy, this one estimated to be a bit on the young side, about 3 billion years old. Alsufi's velocity relative to the Sun, 57 kilometers per second, some three times normal, and a bit of a low metal content (2/3 solar), suggest that the star is a something of a visitor from a different part of the Galaxy. Lower mass stars (including the Sun) are magnetically active, spots and the like causing small variations that allow the determination of rotation period, Alsufi spinning once every 29 days, a bit longer than our Sun's 25-day rotation period. Alas, there is probably nobody there to witness it, as Sigma Dra seems to have no planets, or even a disk of debris that might imply them, which is consistent with its low metal content. The star does not even have a companion to accompany it. While listed as a double, the 11th magnitude apparent neighbor is moving away at much too high a pace, showing it to be a mere line of sight coincidence. No matter: if Alsufi lacks planets or companions, we can still admire it from here, the star a metaphorical stone's-throw away.
Written by Jim Kaler 7/18/08. Return to STARS.