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Acrux kstars.png
Observational Data
Designation Alpha Crucis
HD 108248
Right ascension 12h 26m 35.8958s
Declination -63o 05' 56.73"
Constellation Crux
Type of object star
Magnitude Apparent: 0.77 Absolute:-4.14
Distance from Earth 325 ly
Radial velocity -11 km/s
Proper motion RA: -35.37 mas/yr
Dec.: -14.73 mas/yr
Parallax 10.17 ± 0.67 mas

Acrux, also known as Alpha Crucis, is the southernmost first magnitude star and the twelfth brightest star in the night sky, with an apparent magnitude of 0.77. The star is also the brightest star of the constellation Crux, otherwise commonly known as the Southern Cross. Because of the star's location at 63 degrees south of the celestial equator, it is not visible north of latitude 27°N.[1]

Acrux in Culture and History

Acrux doesn't have an ancient proper name that most brighter stars have, most likely due to its far southern position. The name itself is simply a combination of the letter A in Alpha (the star's Bayer designation) and the name of the constellation it sits in, Crux. Although in China, the star is known as 十字架二 (the Second Star of the Cross).

Because of the star's prominence in the southern hemisphere skies it is represented in the flags of Australia, New Zealand, and Papua New Guinea, all three which display the stars of the Southern Cross. It star is also on the flag of Brazil, representing one of the 26 states of that country, specifically the State of São Paulo.[2]

Star System

Acrux itself is actually a multiple star system of three known stars, located 325 light years away. Two of the stars of the Acrux system make up a visual binary, and are commonly referred to as Alpha-1 and Alpha-2 respectively. The two stars have a separation of at least 430 AU and take a minimum of 1500 years to complete an orbit of each other. Furthermore, Alpha-1 itself is a spectroscopic binary star. From an observer on Earth, there is another star located on 90 arcseconds from the triple stars of the Acrux system and has the same proper motion in space. If this additional star is part of the system, it would have an orbital distance of 9000 AU from the other stars in the system. However it is likely the star is just an optical binary and not part of the system at all, and may be significantly more distant star that is along the same line of sight.

Alpha Crucis A

Alpha Crucis A or Alpha-1 is actually a binary pair (Alpha Crucis Aa and Alpha Crucis Ab respectively), the binary being visible only through its spectrum. The primary star (Alpha Crucis Aa) is blue-white subgiant star of spectral type B0.5 III. Alone the star has an apparent magnitude of 1.4 and would be the 20th brightest star in the night sky.[2] The star has 14 times the Sun's mass and is 25,000 times as visually luminous, even though most of its radiation is in the ultraviolet. The surface temperature is 28,000 K.[1]

Alpha Crucis Aa and its companion orbit every 75.78 days. The orbit is eccentric, varying between 0.5 and 1.5 AU, with an average of only around 1 AU, or about the distance of the Sun from the Earth.[2] The smaller companion itself is spectral class B0, and may be 10 times as massive as the Sun.[3]

Alpha Crucis B

Alpha Crucis A or Alpha-2 is a blue-white dwarf star of spectral type B1 V. The star has an apparent magnitude of 2.09, making it easily visible to the unaided eye if the star was alone. Alpha Crucis A is 13 times as massive as our Sun. Its visual luminosity 16,000 as great as the Sun's, even with most of the radiation in the ultraviolet. The surface temperature is estimated to be 26,000 K.[1]