SADALSUUD (Beta Aquarii). Constellations are usually said to be made of stars that have no relation to one another but are just arranged in random patterns. A few, however, defy such a general statement, the most obvious the bright stars of Orion and Scorpius. Here and there are other examples that are not so well known that include a magnificent trio: Sadalsuud, the Beta star of Aquarius, the Water Bearer, Sadalmelik, Aquarius's Alpha star, and just across the border into Pegasus, Enif (Epsilon Pegasi). Sadalmelik (Alpha) and Sadalsuud (Beta) have nearly the same apparent brightness, both mid-third magnitude, Sadalsuud (at 2.91) just barely the brighter and the brightest in the constellation. Oddly, the stars also share a name, both referring to "lucky stars," Sadalsuud coming from an Arabic phrase meaning "the luckiest of all of them," and referring not only to Sadalsuud but to two other fainter stars just to the southeast, one in Capricornus. Sadalsuud is a rare star, a relatively warm class G (G0) supergiant with a temperature (5600 Kelvin) almost the same as that of the Sun. At a distance of 600 light years, it radiates with a luminosity 2200 times solar, from which we infer a size 50 times that of the Sun and a mass of around six times solar. All three, Enif, Sadalmelik, and Sadalsuud, are similar, Sadalsuud the hottest and least luminous, Enif the coolest and most luminous. Sadalsuud is brighter than its constellation-mate Sadalmelik only because it is 140 light years closer to us. All three were born together as hot class B stars not dissimilar from the stars that make the Pleiades of Taurus, though clearly not in as tight a cluster or else they would still be bound together. Instead, they must have been born in a looser "association," rather like the stars that make much of Orion today, their motions of the past tens of millions of years separating them, but not so far that they are not easily in sight of one another. From Sadalsuud, Sadalmelik and Enif would both be "zero magnitude" stars. All three also seem to be moving more or less perpendicular to the plane of our Galaxy, an odd motion that implies they were somehow kicked away from their birthplace. With similar ages and masses, all are now probably fusing helium into carbon in their deep cores, and all will die rather soon as massive white dwarfs rather like Sirius B.
Written by Jim Kaler. Return to STARS.