Nunki

Nunki is the name of the star σ Sagittarii, the second brightest in the constellation Sagittarius after Kaus Australis (ε Sagittarii). Of apparent magnitude +2.05, it is located in the central region of the constellation, in the belt of the Milky Way. It is 220 light years away from the solar system.

Name

Unlike many star names that derive from Arabic, Nunki is a name of Babylonian origin and seems to be derived from the Babylonian Table of the Thirty Stars, where it was identified with the Star of the Proclamation of the Sea. The meaning of the name is not clear, although it could come from Enki, a Mesopotamian god generally associated with the waters. Other names for the star are Sadira and Pelagus, the latter from the Latin word for "sea".

Physical characteristics

Nunki is a bluish-white star of spectral type B2.5V. Like the Sun, it is a main sequence star whose energy comes from the nuclear fusion of hydrogen into helium. But unlike this one, it is much hotter. With a surface temperature of 20,000 K it emits a large amount of ultraviolet radiation. Taking the latter into account, it is 3,300 times more luminous than the Sun and 2,200 times more luminous if only the radiation emitted as visible light is considered. Its mass is 7.8 ± 0.2 times that of the Sun and its radius 5 times larger than the solar radius. The duration of its life is estimated at around 31 million years, before ending its days as a white dwarf.

Nunki is a double star, with a possible companion of magnitude +9.5 at 309 arcseconds; studies carried out using interferometry suggest that a closer companion may exist.

Observation

The star is close enough to the ecliptic that it can be occulted by the Moon, and more rarely by the planets. The last occultation of Nunki by a planet took place on November 17, 1981, in this case by Venus.

On the other hand, Nunki is the brightest star that can be occulted by an outer planet, although only Mars can, and very occasionally. The last time this happened was on September 3, 423.