Sadalmelik (Alpha Aquarii)
Sadalmelik (Alpha Aquarii) is a star in the constellation Aquarius, magnitude 2.9.
‘The lucky star of the king’, sitting almost exactly on the celestial equator.
From almost anywhere on Earth, Sadalmelik rises close to due east and sets close to due west. It lies within a degree of the celestial equator — one of only two stars with ancient proper names that do. The name comes from the Arabic sa‘d al-malik, ‘the lucky star of the king’, though how the star earned it is lost to history.
Like Sadalsuud, its neighbour in Aquarius, it is a yellow supergiant: about 53 million years old, swollen to around 70 times the Sun’s radius and shining some 3,900 times as brightly, from a surface cooler than the Sun’s. It sits in the Cepheid instability strip, where stars of its temperature and power tend to pulse like Cepheids, yet it is not classed as a variable star. Instead, a massive wind streams off it, reaching supersonic speed in its outer atmosphere.
Sadalmelik (Alpha Aquarii) is 690 light-years away: the light we see from it set off around 1335, 120 years before Gutenberg printed his Bible.
Did you know?
- First Star of the Rooftop. In Chinese astronomy Sadalmelik joins Theta and Epsilon Pegasi in Wēi, the Rooftop, one of the lunar mansions. It is the Rooftop’s first star.
- A borrowed name. Sadalmelik was also once called Rucbah, a name it shared with Delta Cassiopeiae. The IAU settled the matter in 2016: Delta Cassiopeiae is now Ruchbah.
- Quiet in X-rays. NASA’s Chandra X-ray Observatory found Sadalmelik giving off far fewer X-rays than Sun-like stars do — a shortfall common among early G-type giants.
A pale yellow 3rd-magnitude star almost exactly on the celestial equator, a few degrees west of the little Y-shaped Water Jar of Aquarius. Compare it with Sadalsuud, ten degrees to the south-west: the two are near-twins.
When and where to see it
It is best placed in the evening sky around September, when it crosses the meridian at about 10 pm local time (around midnight in August). It sits on the celestial equator, so it can be seen from almost everywhere on Earth.
At its highest it stands 40° up from 50°N, 55° up from 35°N, 90° up from the equator, 55° up from 35°S.
Facts
- Type
- Star
- Constellation
- Aquarius
- Designations
- Alpha Aquarii, α Aqr, 34 Aqr, HR 8414, HD 209750, HIP 109074
- Magnitude
- 2.9
- Position
- RA 22h 5.8m, Dec −0° 19′ (J2000)
- Spectral type
- G2Ib
- Colour
- orange (B−V 0.96)
- Distance
- 690 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Sadalmelik (Alpha Aquarii) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.