Beta Centauri, also called Hadar or Agena, second brightest star (after Alpha Centauri) in the southern constellation Centaurus and the 10th brightest star in the sky. Beta Centauri is about 390 light-years from Earth. It is a system of three B-type stars. The two brightest stars orbit each other every 357 days and form a spectroscopic binary—a star that is seen as a binary only through analysis of the velocities seen in its spectrum. The third star is a fourth-magnitude star, and it orbits the other two every 225 years.