SADALMELIK (Alpha Aquarii). Though possessed of a rather obscure name, Sadalmelik, the Alpha star of the constellation Aquarius (the Water Carrier), is hardly without its intrigue. Shining in northern autumn skies at mid-third magnitude (2.96) just to the west of Aquarius's "Water Jar", Sadalmelik is just slightly fainter than the constellation's Beta star, Sadalsuud. The name, along with another star in the constellation, means "the lucky stars of the king or kingdom," the origin of the term quite lost to history. Sadalmelik is only one of two stars with ancient proper names to lie within a degree of the celestial equator (the circle in the sky that runs from exactly east to west and lies directly above the equator of the Earth), the other Mintaka in Orion (Orion's right-hand belt star), although Porrima Virgo comes close. The star is unusual in several ways. It is classed as a type G (G2) modest supergiant, its surface temperature just a bit cooler than that of our 6000 Kelvin Sun. Such "yellow supergiants" are relatively rare, however, most supergiants being either hot and blue or cooler and reddish. (Oddly, Sadalsuud, quite close to Sadalmelik, is similar). At a distance of 760 light years, we calculate Sadalmelik to have a luminosity 3000 times that of the Sun, rendering it about 60 times the solar diameter, large, but nowhere near as large as the truly great supergiants that would fill the orbit of Jupiter. Its supergiant status tells us that it is highly evolved and dying. Yellow supergiants of this kind are commonly highly variable in brightness, of a type called a "Cepheid," named after the prototype, Delta Cephei. (Such stars indicate their luminosities by their periods of variability and are used to tell the distances of distant galaxies). Oddly, however (rather like Sadr, Gamma Cygni), Sadalmelik is NOT a Cepheid. No one knows why some stars like this one are Cepheids and others are not. Sadalmelik is also a rare "hybrid star." Less luminous stars of solar temperature tend to have magnetic fields that produce hot surrounding "coronas," the Sun's seen at the time of a solar eclipse. Very luminous stars, however, do not, instead having much cooler winds. Sadalmelik falls in between, with both characteristics. The star points to paths in which we can advance the stellar science, but ones that are still covered by underbrush.
Written by Jim Kaler 9/16/98. Return to STARS.