Despite its “beta” designation, the star Sadalsuud, also known as Beta Aquarii, is the most luminous star in the constellation Aquarius with an apparent visual magnitude of +2.78. Interestingly, Sadalsuud, and two other intermediate mass stars, Alpha Aquarii and Eta Pegasi, share a similar proper motion that is currently carrying them in a direction that is perpendicular to the plane of the Milky Way galaxy.
• Constellation: Aquarius
• Coordinates: RA 21h 31m 33.53171s |Dec. –05° 34′ 16.2320″
• Distance: 540 light years
• Star Type: G0 Ib
• Mass: 6.0–6.5 sol
• Radius: 50 sol
• Apparent Magnitude: +2.87
• Luminosity: 2,300 sol
• Surface Temperature: 5,700K
• Rotational Velocity: 6.3 km/sec
• Age: 60 million years
• Other Designations: Sadalsuud, Saad el Sund, ß Aqr, 22 Aqr, ADS 15050 A, BD–06 5770, CCDM J21316-0534A., GC 30137, FK5 808, HD 204867, HIP 106278, HR 8232, SAO 145457
Inexperienced observers might experience some difficulty in locating this star within the constellation, hence the illustration below that identifies Sadalsuud almost directly below the star cluster M2, which is located close to the border between the constellations Aquarius and Equuleus. The constellation can be observed from between latitudes +65° and -90, although it is best seen from the northern hemisphere during October at around 10 PM (Local Time), and then again during early November at around 8 PM (Local Time). Add one hour to these times if Daylight Savings Time is in effect.
Since 1943, when stellar classifications were more or less finalized, the spectrum of this G0 Ib yellow super giant star has served as one of the stable reference points against which other stars are classified. Given its mass of between 6 and 6.5 times that of the Sun, Beta Aquarii’s 60-million-year age has been long enough to allow it to evolve into the giant phase. As a result, the star’s effective radius is now about 50 times that of the Sun, which translates into a luminosity of around 2,300 solar luminosities.
Sadalsuud is also the first G-type supergiant star in whose corona x-ray emissions had been detected, in this case, by the Chandra X-ray Observatory. Although a second X-ray source has been found close to the star, the general consensus among investigators is that this source is more likely to have an extra-galactic origin, as opposed to being intrinsic to the stellar body.
To the naked eye, Sadalsuud appears as a single-point light source, but it does in fact have two faint optical companions that can be seen with medium-aperture telescopes. As of 1947, the closest companion (magnitude 11.0) had a position angle of 321 degrees, which then translated into a separation from Sadalsuud of 35.4 seconds of arc. The other companion, a magnitude 11.6 star, had a position angle in 1947 of 186 degrees, and a separation of 57.2 seconds of arc. However, studies conducted in 2008 suggest that the companion stars are almost certainly line-of-sight companions, since no direct evidence could be found that the three stars are physically related in any way.
The stars’ traditional name, Sadalsuud, derives from the Arabic term “sa‘d al-su‘ud”, which means “luck of lucks”. In the Calendarium, a star catalogue complied by Egyptian astronomer Al Achsasi Al Mouakket around 1650 AD, Sadalsuud was listed as “Nir Saad al Saaoud “, which was later Latinized into “Lucida Fortunae Fortunarum”, which means “[the] brightest of luck of lucks.” However, in 2016, the International Astronomical Union accepted the name “Sadalsuud” for the star, and it is now included in the IAU Catalog of Star Names under this name.
The connotation with luck probably predates the Arab name, since in earlier Egyptian and Persian cultures, Sadalsuud was linked to the rising of the Sun after winter had passed, and the wet season had started. The advent of the wet season was seen as “lucky” or “fortunate”, and it was closely linked to spring being a symbol of the renewal of life, which in agrarian societies, is the basis of “good fortune” and hence, prosperity for all.
Somewhat contrary to this view was the association of this star by ancient Chinese astronomers with an asterism made up of Beta Aquarii and Alpha Equulei, known as “Xu Sù yi”. Due to its position in this asterism, Chinese observers referred to Sadalsuud as “[the] First Star of Emptiness”.