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The Twin Supergiants: Alpha and Beta Centauri

Hadar, also known as Beta Centauri, is the 10th brightest stars (11th as viewed from Earth). Hadar is a blue-white super giant in the constellation Centaurus (Cen). In about 4,000 years, the motion of Alpha Centauri, who’s proper name is Rigel Kentaurus, will carry it close enough to Hadar that they will appear to be a magnificent double star. Because of the distance away from Earth that Alpha and Beta Centauri are (approximately 90 parsecs), they will be an optical double. As they sit today, the two stars look like a pair of eyes, the right one being Hadar and the left being Rigel Kentaurus.

These two stars are considered pointer stars. A “pointer star” is a star that points towards the Southern Cross. Some of the Australian aboriginals call this pair “The two men that once were lions”. Other aboriginals consider them to be the twins that created the world. ” 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.

At a distance of 525 light years, blue class B (B1) Hadar is 130 times farther away than Rigel Kentaurus, and is bright because it is truly and very generously luminous, shining (accounting for the ultraviolet radiated from the 25,500-Kelvin surface) 112,000 times more brightly than the Sun. Hadar, however, is not one star, but two. Sophisticated observations that rely on the interference properties of light show that the single point of light actually consists of a pair of nearly identical stars each some 55,000 times more luminous than the Sun separated (from our perspective) by only 2. stronomical units.

The temperature and luminosity show each to contain 15 solar masses. Spectra suggest an orbital period of not quite a year, this and the masses rendering them an actual 3 astronomical units apart. Twin Hadar also has a fourth magnitude sibling 1. 3 seconds of arc away that, because of the brightness difference, is difficult to see and study. A class B dwarf, Hadar- B is a grand star in its own right, a star of 5 solar masses 1500 times more luminous than the Sun; it only pales by comparison with Hadar (or the Hadars) proper.

Hadar-B orbits the close pair at a minimum distance of 210 astronomical units, taking at least 600 years to make the trip. Conjure a hypothetical planet orbiting Hadar-B. For us to survive, it would have to be as far from the star as Pluto is from the Sun. From there, the distant twins (each 12 solar diameters across) would appear as tiny disks two minutes of arc across separated by half a degree (the angular diameter of the Moon), each shining as much energy on the mythical planet as the Sun does upon us.

The twins of Hadar 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 12 million years old, they will quickly expand to become red super giants and will surely affect each other quite profoundly. Within the next million or so years at least one may explode as a grand supernova. If it were to go off where it is today (which it will not), it would shine in our sky with nearly the brightness of the full Moon.

One of the twins is also a variable of the “Beta Cephei” type (which includes Mirzam), the star subtlety chattering away with multiple periods of less than a day. Here is a nice short piece of information. When are the Hadars going to die? Well that can be answered very easily: now! The Hadars are actually shutting down and dying as we speak! They are already shutting down their internal hydrogen fusion and are beginning to evolve and die.

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