Agena, also called Hadar, is Beta Centauri, a brilliant blue giant of spectral type B1 at roughly 390 light years, a hot and massive multiple system and the second-brightest star of Centaurus after Alpha. It is one of the most striking sights of the far southern sky: with Alpha Centauri beside it, Agena forms the pair of bright Pointers that direct the eye to the Southern Cross. Being hot and massive it is short-lived by stellar standards and destined to end in collapse. It lies too far south to be seen from most of the northern hemisphere, and was little known to the classical astronomers of the Mediterranean.
Agena is in fact a triple system, its two brightest components a close, hot binary too tight to resolve without a large telescope, orbited at a greater distance by a third, fainter star; being invisible from most of the Mediterranean world, it was never assigned a nature by Ptolemy, and its entry into the astrological catalogue is entirely a product of the era of southern-hemisphere exploration and mapping from the sixteenth century onward. Agena's designation as Beta Centauri despite its considerable brightness, rivalling many alpha stars elsewhere in the sky, reflects Centaurus having an even more brilliant alpha star as its neighbour, one of the closest and brightest systems in the entire sky.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy