Acrux (Alpha Crucis) is the brightest star in the constellation Crux, the 13th most luminous star in the entire night sky, and also the southernmost of all first magnitude stars. However, Acrux is actually a multiple star system that can be split relatively easily, although the primary component, designated Alpha-1 Crucis, is itself a close spectroscopic binary star. The secondary component of the primary pair is designated Alpha-2 Crucis.
• Constellation: Crux
• Coordinates: RA: 12h 26m 35s | Dec: -63° 05′ 57″
• Distance: 320 light years
• Star Type: (a1) B0.5IV / (a2) B1V
• Mass: (a1) 17.80 sol / (a2) 15.52 sol
• Apparent Magnitude: +0.76 (Combined)
• Luminosity: (a1) 25,000 sol / (a2) 16,000 sol
• Surface Temperature: (a1) 24 000K / (a2) 28 000K
• Rotational Velocity: (a1) 120 km/sec / (a2) 200 km/sec
• Age: 10.8 million years
• Other Designations: a Crucis, HIP 60718, CPD-62°2745, WDS J12266-6306, CCDM J12266-6306
Acrux is located in the southern constellation of Crux, whose shape is defined by its beautiful Southern Cross asterism. Having a declination of -63°, Acrux is not visible to observers north of latitude 27° N, meaning that it barely rises above the horizon for observers in places like Miami, Florida, and not at all for observers in New Orleans (Louisiana), or in Cairo (Egypt), which are both at about latitude 30°N. Nevertheless, observers from the Southern Hemisphere can view the constellation’s brightest star, Acrux, from April to June.
Due to precession, Acrux was at one time visible to Hindu astronomers in ancient India, who knew the star as Tri-shanku. At about the same time, it was also visible to ancient Greek and Roman observers, who at the time saw the stars of Crux as part of constellation Centaurus.
The main components of the star system are situated 320 light years from our solar system, and shine with a combined magnitude of +0.76. Its primary star, Alpha-1 Crucis, is a blue-white sub-giant (B0.5IV) with 17.80 times the Sun’s mass and 25,000 times its brightness; while Alpha-2 Crucis is a blue dwarf (B1V) that is 15.52 times more massive than the Sun, and 16,000 times brighter.
Multiple Star System
The two stars in the primary system (A1 and A2 Crucis) are separated by a distance of 4 arc seconds, giving the pair an orbital period of about 1,500 years, based on both their 430 AU (astronomical unit) separation, and their observed orbital motions that have barely changed over the past few decades.
Alpha-1 Crucis is itself a spectroscopic binary system, which means that its two components cannot be observed visually as two separate stars. Nonetheless, investigations of the pair’s combined spectrum suggest that they have an orbital period of only 76 days, with a separation of 1 AU
What is making the Crucis multiple system even more complex, is the fact that located about 90 seconds of arc away from the cooler component of the primary pair is another star, designated HR 4729, which shares a common proper motion with a Crucis, thus suggesting that it is gravitationally bound to the primary system. However, HR 4729 is also a close spectroscopic binary star with an orbital period of 29 hours, with another faint companion about 2.1 arc seconds away. Additionally, another seven dim stars, spread out over two arc minutes, are associated with the a Crucis grouping.
Acrux appears on the national flags of New Zealand, Australia, Samoa, and Papua New Guinea as one of the five stars that make up the distinctive Southern Cross asterism. The star also appears on the Brazilian flag (along with 26 others), on which it represents the State of São Paulo. In China, Acrux forms part of an asterism known as “Cross”, which is made up of only four stars, namely Alpha Crucis, Gamma Crucis, Beta Crucis and Delta Crucis, with Acrux itself known as Shí Zì Jià èr, which means “the Second Star of Cross”.
In the Portuguese language, Acrux is known as Estrela de Magalhães, which translates into “Star of Magellan”- no doubt because Portuguese explorer Ferdinand Magellan used it to navigate by during his long voyages in the southern oceans during the 16th century.