HD 129116 is a binary star in the northeastern part of Centaurus, east of Menkent. It is also known by its Bayer designation of b Centauri, while HD 129116 is the star's identifier in the Henry Draper catalogue. This object has a blue-white hue and is faintly visible to the naked eye with an apparent visual magnitude of +4.01. It is located at a distance of approximately 325 light years (100 parsecs) from the Sun based on parallax, and has an absolute magnitude of −1.07. The primary star is a hot type-B star with a spectral type of B3V and a mass of 5 to 6 times the solar mass. The secondary star is a close companion separated by approximately 1 AU, with up to 4.4 times the solar mass. In 2021, a massive exoplanet was discovered by direct imaging orbiting the pair of stars (a circumbinary planet) at a distance of about 560 AU.
Star system This is a young stellar system, belonging to the Upper Centaurus–Lupus subgroup of the Scorpius–Centaurus association, the nearest OB association to the Sun. This is an association of stars with common origin and movement. The region inside Upper Centaurus–Lupus where b Centauri is located seems to have a uniform age of 15 million years, which is therefore the age of this system (with an uncertainty of about 2 million years). From its stellar parallax measured by the Gaia spacecraft, b Centauri is located at a distance of 325 light years (100 parsecs). It has been noted that the secondary star may interfere with the parallax measurements, so this distance value may not be completely accurate. In any case, b Centauri seems to be located on the closer side of the Scorpius–Centaurus association, as seen from Earth, which is also indicated by its higher proper motion compared to the mean of the association. The primary star is a B-type main-sequence star with a stellar classification of B3V, which indicates it is engaged in core hydrogen fusion to generate energy. The object has been used as a "standard star" in several photometric systems, and it appears to be non-variable. It has a high rate of spin, showing a projected rotational velocity of 129 km/s. The star has 5 to 6 times the mass of the Sun and 2.9 times the Sun's radius. It is radiating 637 times the luminosity of the Sun from its photosphere at an effective temperature of 18,445 K. In 1968 the primary was found to have variable radial velocity, which is evidence of a second star in the system, but no orbit was published. The existence of the secondary star was confirmed in 2010 with an interferometric observation, which revealed it at a separation of 9.22 mas, or 1.0 AU at the system's distance. The difference in magnitude between the stars is 1.06, from which a mass of 4.4 M☉ is calculated for the secondary. However this value for the magnitude difference is uncertain, since it was based on a single observation and the detection is close to the instrument performance limit, the mass of 4.4 M☉ is considered an upper estimate. Given all uncertainties, the total mass of the system is estimated at 6 to 10 M☉.
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