ARKAB PRIOR (Beta-1 Sagittarii). "In 1603 Johannes Bayer gave Greek letters to the stars according to the apparent brightnesses," so goes the story. Sagittarius, however, shows how fickle the rule. The constellation's brightest star, Kaus Australis, is Epsilon, while number two, Nunki, is of all things Sigma. The Alpha and Beta stars (Rukbat and Arkab), far to the south, are hardly consequential at all except in that they make a significant part of the Archer ("Arkab" from Arabic meaning "the archer's achilles tendon"). In his "Uranometria," however, Bayer, seems to have been fooled into thinking that the stars are really brighter than they are, perhaps because Arkab is really two stars, the western and brighter called Arkab Prior (the leading one as the pair goes across the sky), or Beta-1, followed by Arkab Posterior (Beta-2), both only fourth magnitude (respectively 4.01 and 4.29). In spite of their proximity (just a third of a degree apart) they are not a real couple, Arkab Prior 378 light years away, Arkab Posterior, at 139 light years, only about a third as far. Both Arkabs are understudied. Arkab Prior's listed temperature of 13,630 Kelvin is way out of line with its B9 dwarf class, which implies more like 11,000 Kelvin. Adopting the lower value (from which we derive the amount of invisible ultraviolet light), its magnitude and distance then giving a luminosity 440 times that of the Sun, a radius nearly 6 times solar, and a mass 3 times solar. Near the end of its hydrogen-fusing lifetime (if indeed the star has not already given it up), Arkab Prior has a dimmer, hydrogen-fusing, seventh magnitude (7.11), class A (A5), 1.8 solar mass dwarf companion, which lies at least 3300 Astronomical Units away (83 times Pluto's distance from the Sun), the pair taking at least 82,000 years to orbit each other. From Arkab-1 A (the brighter), the companion would shine with the brightness of our full Moon, whereas from Arkab-1 B (the companion), the dominating star would appear 17 times brighter.
Written by Jim Kaler 9/28/00. Return to STARS.