Hadar (Beta Centauri)

Hadar (r) and Alpha Centauri (l)

Hadar (r) and Alpha Centauri (l). Photo: Southern Astronomical Society.

Hadar (Beta Centauri) is the second brightest star in the constellation Centaurus and the eleventh brightest star in the sky; the origin of its name is unclear, though its alternative name Agena means the "knee" of the Centaur.


Hadar consists of a pair of almost identical, giant B stars, each some 55,000 times more luminous and 15 times more massive than the Sun separated by about 3 AU with an orbital period of just under a year. A fourth magnitude sibling, Hadar-B, 1.3 arcseconds away, is difficult to see and study because of the brightness difference but, as a B-type dwarf, is an impressive star in its own right, with a mass of 5 solar masses and a luminosity 1,500 times that of the Sun. Hadar-B orbits the main pair at a minimum distance of 210 AU with a period of at least 600 years. The twins that make up Hadar-A are some 12 million years old and due to quickly expand to become red supergiants prior to exploding as supernovae. One of the twins is also a Beta Cephei star, subtly varying with multiple periods of less than a day.


visual magnitude 0.61
absolute magnitude -5.43
spectral type B1III
surface temperature 25,500 K
luminosity 112,00 Lsun
distance 525 light-years
position RA 14h 03m 49.4s,
Dec -60° 22' 23"