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.