Ji Aquarii or Chi Aquarii (χ Aqr / 92 Aquarii / HD 219576) is a star in the constellation of Aquarius of apparent magnitude +4.93. It is about 640 light years from the Solar System.
Ji Aquarii is a red giant of spectral type M3III with a surface temperature of 3670 K and a luminosity 1500 times greater than the solar luminosity. From these values one can calculate its radius, 0.45 AU, equivalent to 96 times the solar radius, although direct measurements by interferometry result in a radius greater than 0.60 AU. Located to the south of the ecliptic, Ji Aquarii is periodically hidden by the Moon, which allows, depending on the time it uses the lunar disk to cover the star, calculate its radius by this technique, obtaining a value of 0.66 AU. The discrepancies probably come from a combination of errors in the distance and in the estimation of the temperature, affecting the amount of infrared radiation emitted and, ultimately, the value of its luminosity. Although difficult to estimate, its mass is around twice that of the Sun.
Ji Aquarii is an irregular variable, whose brightness fluctuates between magnitude +4.90 and +5.06, reason why it is thought that it is a giant in an advanced state of evolution. Probably the star is increasing in brightness and size for the second time, now with an inert core of oxygen and carbon. This phase is characterized by an instability before the star detaches itself from its outer layers to become a white dwarf.
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