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.