Epoch J2000 Equinox J2000
|Right ascension||12h 26m 35.89522s|
|Declination||−63° 05′ 56.7343″|
|Apparent magnitude (V)||0.77 (1.33+1.75+4.86+15.00)|
|Spectral type||B0.5IV + B1V|
|Radial velocity (Rv)||−11.2 / −0.6 km/s|
|Proper motion (μ)||RA: −35.83 mas/yr
Dec.: −14.86 mas/yr
|Parallax (π)||10.13 ± 0.50 mas|
|Distance||320 ± 20 ly
(99 ± 5 pc)
|Absolute magnitude (MV)||−4.14|
|Mass||14 / 10 M☉|
|Luminosity||25,000 L☉|
|Temperature||28,000 K|
|Rotational velocity (v sin i)||124 km/s|
|Companion||α Crucis Ab|
|Period (P)||0.208 yr|
|Semi-major axis (a)||1.0 AU"|
|Eccentricity (e)||0.0|
|Inclination (i)||0.0°|
Alpha Crucis, Acrux, HD 108248
Alpha Crucis (α Crucis, abbreviated Alpha Cru, α Cru) is a multiple star system located 321 light-years from the Sun in the constellation of Crux and part of the asterism known as the Southern Cross. With a combined visual magnitude of 0.77, it is the brightest star in Crux and the 13th brightest star in the night sky. It is the southernmost first-magnitude star, just a little more southerly than Alpha Centauri.
Only two components are visually distinguishable; α¹ Crucis (also named Acrux) and α² Crucis.
The two components, α¹ and α² Crucis, are separated by 4 arcseconds. α¹ is magnitude 1.40 and α² is magnitude 2.09, both hot class B (almost class O) stars, with surface temperatures of about 28,000 and 26,000 K respectively. Their luminosities are 25,000 and 16,000 times that of the Sun. α¹ and α² orbit over such a long period that motion is only barely seen. From their minimum separation of 430 astronomical units, the period is at least 1,500 years, and may be much longer.
α¹ is itself a spectroscopic binary star, with its components thought to be around 14 and 10 times the mass of the Sun and orbiting in only 76 days at a separation of about 1 AU. The masses of α² and the brighter component of α¹ suggest that the stars will someday explode as supernovae. The fainter component of α¹ may survive to become a massive white dwarf.
Another class-B4 subgiant, Alpha Crucis C or alpha-3 Crucis, lies 90 arcseconds away from triple Alpha Crucis and shares its motion through space, suggesting it may be gravitationally bound to it, and it is therefore generally assumed to be physically associated with Alpha Crucis. It has also been suggested that Alpha Crucis C is under-luminous for its class, meaning that the system would be an optical double star.
Rizzuto and colleagues determined in 2011 that the Alpha Crucis system was 66% likely to be a member of the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus Association. It was not previously seen to be a member of the group.
The historical name Acrux is an 'Americanism' for Alpha Crucis, coined in the 19th century, but entering common use only by the mid 20th century. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Acrux for α¹ Crucis.
Since Alpha Crucis is at −63° declination, the southernmost first-magnitude star, it is only visible south of latitude 27° North. Therefore, it barely rises from cities such as Miami, Florida, or Karachi, Pakistan (both around 25°N) and not at all from New Orleans, Louisiana, or Cairo, Egypt (both about 30°N). Because of Earth's axial precession, however, the star was visible to ancient Hindu astronomers in India who named it “Tri-shanku”. It was also visible to the ancient Romans and Greeks, who regarded it to be part of the constellation of Centaurus.
In Chinese, 十字架 (Shí Zì Jià, "Cross"), refers to an asterism consisting of Alpha Crucis, Gamma Crucis, Beta Crucis and Delta Crucis. Consequently, α Crucis itself is known as 十字架二 (Shí Zì Jià èr, "the Second Star of Cross".).
Alpha Crucis is represented in the flags of Australia, New Zealand, Samoa, and Papua New Guinea as one of five stars which comprise the Southern Cross. It is also featured in the flag of Brazil, along with 26 other stars, each of which represents a state; Alpha Crucis representing the State of São Paulo.
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