Lambda Aquarii, informally known as Hydor, is a variable star in the equatorial constellation of Aquarius. The name is Latinized from the Bayer designation λ Aquarii. The apparent visual magnitude of this star ranges from 3.57 down to 3.80, which is bright enough to be visible with the naked eye. It lies just 0.39 degrees south of the ecliptic and so is subject to lunar occultations. The star is eclipsed by the sun from about 1-4 March; thus the star can be viewed the whole night, crossing the sky, in early September, in the current epoch. Lambda Aquarii is located at a distance of 365ly from the Sun based on parallax, but is drifting closer with a radial velocity of −10.5 km/s.
The stellar classification of Lambda Aquarii is, indicating this is an aging red giant star with an underabundance of iron showing in its spectrum. This star is on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen. With 3.6 times the mass of the Sun, it has expanded to 100 times the Sun's radius. It is classified as slow irregular variable and pulsation periods of 24.5, 32.0, and 49.5 days have been identified. On average, it is radiating nearly 1,600 times the luminosity of the Sun from the photosphere at an effective temperature of 3,702 K.
Hydor is from Greek Ὕδωρ "water", a name given by Proclus, according to Richard Hinckley Allen. Another Greek name for the star is Ekkhysis, from εκχυσις "outpouring".
In Chinese, Chinese: 壘壁陣 (Chinese: Lěi Bì Zhèn), meaning Line of Ramparts, refers to an asterism consisting of λ Aquarii, κ Capricorni, ε Capricorni, γ Capricorni, δ Capricorni, ι Aquarii, σ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium. Consequently, λ Aquarii itself is Chinese: 壘壁陣七 (Chinese: Lěi Bì Zhèn qī, English: the Seventh Star of Line of Ramparts.)