Beta Centauri is a bluish star of magnitude 0.61, only 4 degrees west of Alpha Centauri. β Cen is the sky's 10th brightest star, and is also called Hadar, which comes from the Arabic word for "ground". The Arabs called α and β Centauri "ground" and "weight" ("Hada" and "Wazn"); they saw the pair skimming along the southern horizon, and thought they could not lift themselves into the sky.
Hadar is also less often known as "Agena", meaning the "knee" of the Centaur. The ancient Chinese called α and β Centauri "Nan Men", which means the "South Gate" of the sky. These two stars are well-known in the Southern Hemisphere as the two "Pointers" to the Southern Cross, from which a navigator can effectively determine south.
Properties
Visually, Alpha Centauri is notably brighter than Hadar, but only because it is the closest star to the Earth. At a distance of 390 light years, Hadar is 90 times farther away, and is bright because it is truly luminous.
Hadar is a blue class B giant, and is not one star, but three. Interferometric observations show that Hadar A consists of a pair of identical stars, which orbit each other with a period of 357 days. Direct observation of their orbital motion, combined with their spectral Doppler shifts, yield an average separation of 3.0 AU, and a high eccentricity that takes the stars from 0.53 to 5.5 AU apart. They have identical masses of 14.7 times that the Sun's. A temperature of 25,000 K, typical of class B2 stars, gives the components of Hadar A luminosities of 15,500 suns and radii 8 times solar. One or both of the twins is a β Cepheid variable, with a period of less than 4 hours. The close binary is also an X-ray source, with a 2 million degree K stellar wind.
Evolution
The stars of Hadar A appear to be at the edge of shutting down their internal hydrogen fusion, if they have not done so already, and are beginning to evolve off the main sequence. Now some 30 million years old, they will quickly expand to become red giants, and will surely affect each other profoundly.
Lower mass stars become white dwarfs, while high mass stars explode as supernovae; the dividing is somewhere in the range of 9 to 12 suns. A detailed study of the orbit gives a smaller distance of 350 light years, and stellar masses of 10.7 and 10.3 Suns. The fate of the twins of Hadar is thus unknown. One or both could blow up, or they could become a pair of massive white dwarfs.
Hadar B is also a class B star, of 4th magnitude, about 1.3" away. It was identified in 1935 by J.G. Voute, and has remained at that separation since the discovery. It orbits the close pair at a minimum distance of 120 AU, taking at least 225 years to make one trip around them. From Hadar B, the distant twins of Hadar A would appear as tiny disks 2' across, typically separated by a bit over a degree.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]