Alpha Aquarii - Sadalmelik

Sadalmelik, or α Aquarii, shines at second magnitude, just to the west of Aquarius's "Water Jar". Sadalmelik is slightly fainter than Sadalsuud, the constellation's Beta star. Its name, along with another star in the constellation, means "the lucky stars of the king or kingdom". The origin of the term is lost to history. Sadalmelik is only one of two stars with ancient proper names to lie within a degree of the celestial equator; the other is Mintaka in Orion.

Properties

α Aquarii is unusual in several ways. It is a class G supergiant, with a surface temperature just a bit cooler than that of our 6000-degree-K Sun. Such "yellow supergiants" are relatively rare, however. Most supergiants are either hot and blue or cool and reddish. Oddly, Sadalsuud, quite close to Sadalmelik, is similar. At a distance of 750 light years, we calculate Sadalmelik to have a luminosity 3000 times the Sun's, rendering it about 60 times the Sun's diameter. This is large, but nowhere near as large as the truly great supergiants that would fill the orbit of Jupiter.

Evolution

Sadalmelik's supergiant status tells us that it is highly evolved. Yellow supergiants of this kind are often Cepheid variables. Oddly, however, 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 with sunlike temperatures tend to have magnetic fields that produce hot surrounding coronas. Very luminous stars, however, do not, instead having much cooler winds. Sadalmelik falls in between, with both characteristics.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]