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 are named with Greek letters appropriately in order of brightness, Hadar (bright first magnitude, 0.61) the sky's 10th brightest star, Rigil-Kent number 3. 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. Why the luminaries of the Southern Cross, Acrux and Mimosa, of similar depth below the equator, received no ancient names is not known except that they are fainter than these two (atmospheric absorption from northern latitudes making them fainter yet). 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 392 light years (second Hipparcos reduction), Hadar, a blue class B (B1) giant (but see below), is 90 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, the spectrum (as well as direct observation) shows 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. Observation of orbital motion yields an average separation of 3.0 Astronomical Units, a high eccentricity that takes the stars from 5.5 to 0.53 AU apart, and nearly identical masses of 14.7 times that of the Sun (an independent study showing that one is just four percent more massive than the other).
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 days. (From an article by J. Davis et al. in Monthly Notices of the Royal Astronomical Society, vol. 356, p. 1362, 2005.)
A rough temperature of 25,000 Kelvin (needed to account for invisible ultraviolet light) combined with the theory of stellar structure gives masses close to 14 solar, very close to those derived from the binary orbit. A spin velocity of at least 140 kilometers per second coupled with a radius of 9 solar gives a rotation period for at least one of the stars of under 3 days. One of the-near twins, perhaps both, is also a variable of the "Beta Cephei" type, the star subtlety chattering away with a dual periods of 3.2 and 5.3 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 120 Astronomical Units (AU), taking at least 225 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. Not giants at all, the twin stars of Hadar A are both still hydrogen-fusing dwarfs, though they probably do not have that far to go before meeting their 11- million-year projected dwarf lifetime. 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, the dividing somewhere in the range of 8 to 12 solar, suggesting that both will eventually blow up as supernovae. All this said, Hadar instructs us that not all is secure. A detailed study of the orbit that combines velocity measure (through Doppler shift) with precise measures of the orbit through sophisticated interferometry give a shorter distance of 350 light years and stellar masses of 10.7 and 10.3 solar (but which do not agree so well with those derived from luminosity and temperature). If so, then the stars may escape destruction to become massive white dwarfs. Only time, or more study, will tell.
Written by Jim Kaler 5/07/00; revised 7/10/09. Return to STARS.