Star of the month

Sadalmelik, a bright gem of Aquarius

It’s fair to say that the constellation of Aquarius, the Water-Bearer, is ill-defined. Apart from a small pattern of four stars arranged as a triangle with one in the centre, known as the Water Jar asterism, the rest is sprawling and not the easiest to pick out. The two brightest stars are Sadalmelik (Alpha (a) Aquarii) and Sadalsuud (Beta (b) Aquarii). Sadalmelik sits west of the Water Jar.

Shining at mag. +2.93, Sadalmelik is the second brightest star in Aquarius after mag. +2.87 Sadalsuud. It’s a distant star, 520 lightyears from the Sun, closer than Sadalsuud’s 540 lightyears. Their meanings are similar.

Sadalmelik is derived from the Arabic meaning “luck of the king” while Sadalsuud means “luck of lucks”. As an aside, Sadalmelik is one of only a handful of stars with proper names which are located within a degree of the celestial equator. The others are Mintaka (Delta (d) Orionis), Zaniah (Eta (e) Virginis) and Heze (Zeta (z) Virginis).

Estimated to be 53 million years old, Sadalmelik is a supergiant star of spectral classification G2 Ib. G2 is the same spectral type as our Sun, and the star’s temperature is close to our Sun’s 5,505˚C at 5,110˚C. Sadalmelik has 5.1 times the mass, is 53 times larger and has a luminosity 2,120 times greater than the Sun. Its supergiant status indicates it is a highly evolved object that is now dying.

Despite appearing 10˚ apart both stars share similarities; they are close in brightness and are placed at a similar distance from Earth.

Sadalsuud is also a supergiant with a spectral type of G0 Ib. It has a similar mass, radius and luminosity to Sadalmelik; compared to the Sun the values being 5.0, 48 and 2,046 times respectively. The major difference is age; Sadalsuud is twice as old as Sadalmelik.