Gamma Gruis

Gamma Gruis (Gamma Gru, γ Gruis, γ Gru) is a star in the southern constellation of Grus. It is known as Al Dhanab, "the Tail (of the constellation of the Southern Fish)" and once belonged to the Ptolemaic constellation Piscis Austrinus. With an apparent visual magnitude of 3.0, it is the third-brightest star in the constellation.

In Chinese, 敗臼 (Bài Jiù), meaning Decayed Mortar, refers to an asterism consisting of γ Gruis, λ Gruis, γ Piscis Austrini and 19 Piscis Austrini. Consequently, γ Gruis itself is known as 敗臼一 (Bài Jiù yī, English: the First Star of Decayed Mortar.)

Based upon parallax measurements, this star is located at a distance of roughly 211 light-years (65 parsecs) from Earth. Analysis of the spectrum shows it to match a stellar classification of B8 III, with the luminosity class of III indicating this is a giant star that has exhausted the supply of hydrogen at its core and evolved away from the main sequence. The luminosity of Gamma Gruis is around 390 times that of the Sun, with a significant portion of the energy emission being in the ultraviolet. Its outer envelope has an effective temperature of 12,520 K, which give the star a blue-white hue. Gamma Gruis is rotating relatively rapidly with a projected rotational velocity of 57 km s⁻¹. By way of comparison, the Sun has an azimuthal velocity along its equator of just 2 km s⁻¹.

Based upon analysis of data collected during the Hipparcos mission, this star may have a proper motion companion that is causing gravitational perturbation of Gamma Gruis.