Hadar is the 11th brightest star visible in the night sky. Due to its visible (if not physical) proximity to Alpha Centauri and Proxima Centauri, it is also known as Beta Centauri. It has also been called Agena, which many sources state is derived from the Latin for "knee" genu - this name being given due to Hadar's position at the knee of the Centaur in the constellation of Centaurus.
Hadar is in fact two stars orbiting each other very closely, at a distance of approximately three au and an orbital period of less than one year. Each star is approximately 15 solar masses, with a combined luminosity (including ultraviolet radiation) of over 100,000 times that of our sun. Each is a blue giant star with a surface temperature of 25000+ kelvin. Another companion, Hadar-B, a class B dwarf 5 times the mass of the sun and 1500 times more luminous, orbits the twin star at a distance of 210 au.
Both stars in Hadar are 12 million years old, and will expand to become red supergiants, probably in less than a million years, being huge, bright and quickly-burning stars. Hadar appears roughly as bright as Alpha Centauri in the night sky, but only because of Alpha Centauri's close proximity to our solar system - Hadar is over five hundred light years away as compared to Alpha Centauri's 4.35.
http://einstein.stcloudstate.edu/Dome/constellns/hadar.html
http://www.astro.uiuc.edu/~kaler/sow/hadar.html