Shatabhisha

Naming

The Bayer designation Lambda Aquarii is Latinized from λ Aquarii. In Greek tradition, the star was called Hydor (Ὕδωρ, 'water') by Proclus, and also Ekkhysis (εκχυσις, 'outpouring'); these names originally referred to a group of faint stars in Aquarius and Cetus and were linked to this star by Bayer's Uranometria (1603) and Allen's Star Names (1899). In Indian astronomy, it is the main star of the nakshatra Śatabhiṣa or Shatabhisha, meaning 'a hundred physicians'; some sources instead attribute that name to γ Aquarii, likely a reading or writing error.

The Tibetan name is Mondru. The IAU Working Group on Star Names approved the name Shatabhisha for λ Aquarii on 22 September 2025, and it is now in the IAU Catalog of Star Names. The name Hydor was officially adopted for another star, 2 Ceti. In Chinese astronomy, λ Aquarii is part of the asterism Line of Ramparts (壘壁陣) and is called the Seventh Star of Line of Ramparts.

Properties

Lambda Aquarii has a stellar classification of M2.5 IIIa Fe–1, indicating an aging red giant with an underabundance of iron. It is on the asymptotic giant branch, fusing hydrogen and helium in shells around an inert carbon‑oxygen core. With 3.6 times the Sun's mass, it has expanded to 100 solar radii. The star has a magnetic field and is a slow irregular variable with pulsation periods of 24.5, 32.0, and 49.5 days. It radiates about 1,600 times the Sun's luminosity from a photosphere at 3,702 K. A faint companion, likely an F‑type main‑sequence star, was detected by speckle interferometry during a lunar occultation in 2016 and is probably gravitationally bound.