HADAR (Beta Centauri). Residents of the southern hemisphere see a grand parade as Crux (the Southern Cross) rises and is followed by the two luminaries of Centaurus, Hadar and Rigil Kentaurus, the pair more commonly known as Beta and Alpha Centauri. The two named appropriately in order of brightness, Hadar (bright first magnitude, 0.61) is the sky's 11th brightest star. Hadar is a proper name of unknown meaning, and has been paired with the name "Wezen," the two applied to the two bright stars in Centaurus as well as to stars in Columba, "Wezen" now commonly used for Delta Canis Majoris. Hadar, less often known as Agena (from the "knee" of the Centaur), is quite the magnificent star: rather, stars. Visually, Alpha Centauri is notably brighter than Hadar, but only because (ignoring Proxima) it is the closest star to the Earth. At a distance of 335 light years, Hadar, a blue class B (B1) giant, is 77 times farther away, and is bright because it is truly luminous. 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).
The graph shows the orbit of one of the twin stars of Hadar (the bright star, Hadar A) as it appears to make its way around the other (which is at the intersection of the crossed lines). In reality, the two orbit a common center of mass that lies between them. The axes are in thousandths of a second of arc, with north toward the bottom, as is traditional in double star astronomy. The star being orbited does not appear at the focus of the observed ellipse because the orbit does not lie flat against the sky, but is tilted through an angle of 67 degrees. The stars were closest on February 25, 2000 and again after successive intervals of the orbital period of 357.00 days. (From an article by J. Davis et al. in Monthly Notices of the Royal Astronomical Society, vol. 356, p. 1362, 2005.)
A temperature of 25,000 Kelvin (needed to account for invisible ultraviolet light) combined with the theory of stellar structure and evolution gives masses close to 11 solar, too high. But drop the temperature to 22,500 (more like that of a B2 star, which Hadar could well be), and the evolutionary masses drop to a more equitable 9.5 solar, which gives luminosities of 15,500 solar and radii of 8 solar. One of the twins, perhaps both, is also a variable of the "Beta Cephei" type, the star subtlety 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. The stars of Hadar A appear to at the edge of shutting down their internal hydrogen fusion (if they have not done so already), and are beginning to evolve and die. Now some 30 million years old, they will quickly expand to become red giants and will surely affect each other quite profoundly. Lower mass stars become white dwarfs, while high mass ones explode as supernovae, the dividing somewhere in the range of 9 to 12 solar. The fate of the twin Hadars is thus unknown. One or both could blow up, or they could make a pair of massive white dwarfs. Only time will tell. If one of them were to go off where it is today (which it will not), it would shine in our sky with the brightness of the full Moon.
Written by Jim Kaler. Return to STARS.