Alpha Crucis

This article is about the star. For the Christian college, see Alphacrucis.

Acrux AB

The position of Acrux.
Observation data Epoch J2000      Equinox J2000
Constellation	Crux
Right ascension	12h 26m 35.8958s
Declination	-63° 05' 56.73"
Apparent magnitude (V)	0.77
Characteristics
Spectral type	B1V
Astrometry
Proper motion (μ)	RA: -35.37 mas/yr Dec.: -14.73 mas/yr
Parallax (π)	10.17 ± 0.67 mas
Distance	320 ± 20 ly (98 ± 6 pc)
Absolute magnitude (MV)	-4.14
Details
Mass	14 / 10 M☉
Luminosity	25,000 L☉
Temperature	28,000 K
Orbit
Companion	α Crucis Ab
Period (P)	0.208 yr
Semimajor axis (a)	1.0 AU"
Eccentricity (e)	0.0
Inclination (i)	0.0°
Database references
SIMBAD	data

Acrux (α Cru, α Crucis, Alpha Crucis, HD 108248) is the brightest star in the constellation Crux, the Southern Cross, and, at a combined visual magnitude 0.77, is the twelfth brightest star in the night sky. Acrux is the southernmost first-magnitude star, just a bit more southerly than Alpha Centauri.

Physical properties

Acrux is a multiple star located 321 light years from the solar system. Only two components are visually distinguishable, α¹ and α², 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 Acrux and shares Acrux's motion through space, suggesting it may be gravitationally bound to Acrux. However, if it is indeed located near Acrux, Alpha Crucis C is under-luminous for its class. It is probably just an optical double star, lying beyond the gravitationally bound stars of Acrux in the background—most probably at double the distance i.e. 640 light-years (200 pc). This star will move further away from the others with time and proper motion and will probably end up as a white dwarf star.

Etymology

"Acrux" is simply a contraction of the A in Alpha plus Crux.

Since Acrux is at −63° declination, the southermost first-magnitude star, it is only visible south of latitude 27°N. 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”.

In Chinese, 十字架 (Shí Zì Jià), meaning Cross, refers to an asterism consisting of α Crucis, γ Crucis, β Crucis and δ Crucis. Consequently, α Crucis itself is known as 十字架二 (Shí Zì Jià èr, English: the Second Star of Cross.).

This star is become Estrela de Magalhães (Star of Magalhães) in Portuguese.

Acrux is represented in the flags of Australia, New Zealand 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. Acrux represents the State of São Paulo.

Notable observations

On 2008 October 2, the Cassini–Huygens spacecraft resolved three of the components (A, B and C) of the multiple star system as Saturn's disk occulted it.

References

• http://jumk.de/astronomie/big-stars/acrux.shtml
• http://www.daviddarling.info/encyclopedia/A/Acrux.html
• Kaler – Acrux page : http://stars.astro.illinois.edu/sow/acrux.html