Star Facts: Hadar

Hadar
Image Credit: © Alan Dyer

Hadar (Beta Centauri) is a triple star system whose combined visual magnitude of +0.61 makes it the second-most luminous star in constellation of Centaurus, and the 11th most luminous stars in the entire sky. It is also the inner of the two stars that form the “Southern Pointer Stars”, the other being Rigil Kent (Alpha Centauri), which point towards a famous asterism called the Southern Cross, and its brightest star Gacrux in the constellation Crux.

Hadar is located very close to the radiant point of the Beta Centaurids meteor shower that usually runs from February 5th to 25th, with a peak on February 8th/9th when between 5-14 meteors per hour can be seen.

Quick Facts

• Constellation: Centaurus
• Coordinates: RA: 14h 03m 49s|Dec: -60° 22′ 23?
• Distance: 390 light years
• Apparent Magnitude: +0.61 (Combined)
• Luminosity: 41,700 sol (Combined)
• Age: 14.1 million years
• Star Type: Beta Cen A1 (B1 III); Beta Cen A2 (B1 III); Beta Cen B (B1V)
• Mass: Beta Cen A1 (10.7 sol); Beta Cen A2 (10.3 sol); Beta Cen B (4.61 sol)
• Surface Temperature: Beta Cen A1 – 25,000K; Beta Cen A2 – 25,000K; Beta Cen B – Undetermined
• Other Designations: Agena, HR 5267, HD 122451, CD-59°5365, LHS 51, SAO 252582, FK5 518 , HIP 68702, GC 18971

Visibility

Hadar in Centaurus can be seen from between +25° and -90° of latitude, meaning that the only observers in the northern hemisphere that can see it are those that live on, or below southern Texas, and parts of Hawaii and Florida. In addition, there are no “pointer” stars to Hadar in the northern hemisphere, but observers that are favourably located may catch a brief glimpse of the star very low on the southern horizon at around 1 AM (local Daylight Savings Time) during the first days of May.

Physical Properties

Although Hadar appears as a single star, it is in fact a trinary system that is made up of two blue giants that are 4 AU apart and orbit each other once every 357 days in a highly eccentric orbit. At a distance of 100 AU is the system’s third component, a B1 classified blue dwarf which orbits the principal pair once every 1,500 years or so. The two principal stars, Beta Centauri Aa and Ab, are each roughly 10 times as massive as the Sun, while the smaller star, Beta Centauri B, has around 4.6 solar-masses.

The small separation between the two stars in the primary pair means that the system is a spectroscopic binary, with similar spectra that reveal both stars to be B1 III-class stars with similar masses, and presumably, similar compositions, since only the profiles of the lines in their spectra differ somewhat. Both stars in the primary have evolved off of the main sequence and are now in their giant phases, and will likely end their lives in supernova events. Both stars are also Beta Cephei variables, which is a class of stars that display several luminosity variations in only a few hours. Typically, the luminosity variations of Beta Cephei stars do not exceed a few hundredths of a magnitude, but in the case of Beta Aa and Ab, the full range of luminosity variations is yet to be determined.

History

Southern Pointer Stars
Image Credit: Starry Night Education

The name Hadar derives from the Arabic phrase for “on the ground”, perhaps in reference to it appearing near to the horizon as viewed from low latitudes. The star is also known as Agena, a word derived from the Latin word for “knees”, presumably belonging to the centaur the constellation depicts. In China, however, Hadar is known as “ma fù yi”, meaning “[the] First Star of the Horse’s Abdomen.”

In Aboriginal folklore, and particularly the Boorong people of northwestern Victoria, Hadar and Alpha Centauri are collectively known as Bermbermgle, two brothers who were known in ages past both for their daring, and their penchant for destroying things. In this instance, the two brothers are commemorated for having speared and killed Tchingal, “The Emu”, represented by the Coalsack Nebula, a prominent dark nebula close to the Southern Cross.

Related Articles