Hadar

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Beta Centauri (β Cen / β Centauri), also known as Hadar or Agena, is the second brightest star in the constellation Centaurus and the tenth brightest star in the night sky. Beta Centauri is a B1 III blue-white giant star that the astrometric Hipparcos satellite calculates as about 161 parsecs or 525 light-years from the solar system. More recent estimates place the distance of the system to closer to 350 light years or 107 parsecs.

The name Hadar comes from the Arabic word for "ground", while the name Agena comes from Latin words for "the knee".

In 1935, J. G. Voute identified Beta Centauri as a double star, giving it the identifier VOU 31. The companion is separated from the primary by 1.3", and has remained so since the discovery, although the position angle has changed slightly. This would indicate that the orbital period is very long, if the pair are actually gravitationally associated. The primary is also a spectroscopic binary, having one companion with an orbital period of 352 days, and a second companion further out, forming a triple star system.

Hadar, however, is not one star, but three. About a second of arc away is a 4th magnitude class B star, Hadar B. More interesting, sophisticated observations that rely on the interference properties of light show that Hadar A (the brighter of the two) consists of a pair of identical stars that orbit each other with a period of 357 days. Direct observation of orbital motion combined with those of spectral Doppler shifts yield an average separation of 2.59 Astronomical Units, a high eccentricity that takes the stars from 4.72 to 0.46 AU apart, identical masses of 9.1 times that of the Sun, and the above distance (smaller than that given by direct parallax, which now appears to be in error).

From accurate measurements of the radial velocity, we can estimate the masses of each component of Hadar, suggested as being between 10-11 solar masses. From the HR diagram, this gives us Luminosities of around 15,500 for each component and a radii of 8 solar.

One of the twins, perhaps both, is also a variable of the "Beta Cephei" type, the star subtly chattering away with a period of less than 4 hours. The close binary is also an X-ray source with a 2 million Kelvin wind. Hadar-B orbits the close pair at a minimum distance of 110 Astronomical Units (AU), taking at least 250 years to make the trip. From there, the distant twins would appear as tiny disks two minutes of arc across typically separated by a bit over a degree.

Beta Centauri is well-known in the Southern Hemisphere as the inner of the two "Pointers" to the Southern Cross. A line made from the other pointer, Alpha Centauri, through Beta Centauri leads to within a few degrees of Gacrux, the star at the top of the cross. Using Gacrux, a navigator can draw a line with Acrux to effectively determine south. Coordinates: 14^h 03^m 49.4^s, −60° 22′ 23″

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