Beta Centauri

Lore & Background

The system bears the traditional names Hadar and Agena. Hadar comes from the Arabic root meaning 'to be present' or 'settled area,' while Agena is thought to derive from Latin genua, meaning 'knees,' referring to its position on the left knee of the centaur in the constellation Centaurus. In 2016, the International Astronomical Union approved the name Hadar for the star β Centauri Aa. The Chinese name is 马腹一 (Mandarin: mǎ fù yī, 'the First Star of the Horse's Abdomen'). The Boorong people of Australia named it Bermbermgle (with α Centauri), two brothers who spear and kill Tchingal, 'The Emu' (Coalsack Nebula). The Mursi people of Ethiopia call it Waar.

Reader's Guide

Beta Centauri is significant as a bright navigational aid in the Southern Hemisphere, forming the inner of the two 'Pointers' to the Southern Cross. A line from Alpha Centauri through Beta Centauri leads near Gacrux, helping navigators determine south. The system is also notable for its triple-star nature: a close binary pair (Aa and Ab) with a 357-day orbit and high eccentricity, plus a wider companion (B) separated by 1.3 arcseconds. Both Aa and Ab are giant stars of spectral type B1 III, with masses 12.02 and 10.58 times that of the Sun, respectively. They are massive enough to eventually explode as supernovas, while component B will become a white dwarf. The system's brightness and variability—component Aa is a β Cephei variable—make it a subject of astrophysical study.

Did You Know?

• Beta Centauri is the second-brightest star in Centaurus and the eleventh brightest in the night sky.
• The system's combined magnitude of 0.61 varies by a few hundredths, too small for the naked eye to notice.
• Component Aa rotates much more rapidly than Ab, allowing the two to be distinguished in the spectrum.
• The Boorong people of Australia named Beta Centauri and Alpha Centauri as two brothers who spear the Emu (Coalsack Nebula).