Acrux
Brightest star in Crux and southernmost first-magnitude star.
Wikipedia / Wikimedia Commons
Acrux is a multiple star system and the brightest star in the southern constellation of Crux. With a combined visual magnitude of +0.76, it is the 13th-brightest star in the night sky and the southernmost first-magnitude star. It is the most southerly star of the asterism known as the Southern Cross and lies at a distance of 348 light-years from the Sun.
- Bayer designation
- α Crucis
- Apparent magnitude
- +0.76
- Distance
- 348 light-years (106.7 parsecs)
- Constellation
- Crux
- Number of components
- 7
- Spectral types
- B0.5 IV, B1 V, B5 V, M0V
- Declination
- −63°
Lore & Background
To the naked eye Acrux appears as a single star, but it is actually a multiple star system containing seven components. Through optical telescopes, it appears as a triple star, with the two brightest components, α Cru A and α Cru B, separated by about 4 arcseconds. Both are B-type stars, many times more massive and luminous than the Sun. This system was the second ever to be recognized as a binary, in 1685 by a Jesuit priest. The two main components are themselves close binaries, with components designated α Crucis Aa, Ab, Ba, and Bb. A more distant companion, α Cru C (HR 4729), forms a triple star through small telescopes and is itself a spectroscopic binary with a faint companion, bringing the total to seven stars.
Reader's Guide
Acrux holds significance as the brightest star in Crux and the southernmost first-magnitude star, visible only south of latitude 27° North. It is a key navigational star and appears on the flags of Australia, New Zealand, Samoa, and Papua New Guinea as part of the Southern Cross. It also represents the state of São Paulo on the flag of Brazil. The system's complex multiplicity—seven components in hierarchical orbits—makes it an important object for studying stellar evolution and binary dynamics. Its two main components are hot B-type stars with masses up to 17 times that of the Sun, and one is predicted to eventually explode as a supernova. The system is likely a member of the Lower Centaurus–Crux sub-group of the Scorpius–Centaurus association. A bow shock around Acrux is visible in the infrared, though not aligned with the star itself.
Did You Know?
- Acrux was the second star system ever recognized as a binary, in 1685 by a Jesuit priest.
- The name Acrux was coined in the 19th century by American astronomer Elijah Hinsdale Burritt as a contraction of its Bayer designation.
- Acrux is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri.
- The system contains seven components, including a red dwarf star (α Cru D) first identified in 1999 using adaptive optics.
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