Alpha Crucis

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Acrux AB

The position of Acrux.
Observation data
Epoch J2000
Constellation
(pronunciation)		 Crux
Right ascension 12h 26m 35.8958s
Declination -63° 05' 56.73"
Apparent magnitude (V) 0.81
Characteristics
Spectral type B1V
Variable type ?
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) 0.56
Details
Mass 14 / 10 M☉
Radius ? R☉
Luminosity 25.000 L☉
Temperature 28.000 K
Metallicity ?
Rotation ?
Age ? years
Visual binary orbit
Companion Acrux B
Period (P) 0.208 yr
Semimajor axis (a) 1.0 AU"
Eccentricity (e) 0.0
Inclination (i) 0.0°
Database references
SIMBAD data

Contents

[edit] Physical Properties

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

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 kelvins 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.

Acrux CA

System Acrux.
Observation data
Epoch J2000
Constellation
(pronunciation)		 Crux
Right ascension 12h 26m 35.871s
Declination -63° 05' 56.58"
Apparent magnitude (V) 1.4
Characteristics
Spectral type B0.5IV
U-B color index −1.03
B-V color index −0.08
Variable type ?
Astrometry
Radial velocity (Rv) -11.2 km/s
Proper motion (μ) RA: -35.3 mas/yr
Dec.: -12.0 mas/yr
Parallax (π) 10.17 ± 0.67 mas
Distance 320 ± 20 ly
(98 ± 6 pc)
Absolute magnitude (MV) 1.32
Details
Mass 13 M☉
Radius ? R☉
Luminosity 16.000 L☉
Temperature 26.000 K
Metallicity ?
Rotation ?
Age ? years
Visual binary orbit
Companion Acrux CA
Period (P) 1.500 yr
Semimajor axis (a) 430.0 AU"
Eccentricity (e) 0.0
Inclination (i) 0.0°
Other designations
α1 Crucis/α2 Crusis, HR 4730/4731, CD -62°2745, HD 108248/108249, SAO 251904, FK5 462, HIP 60718.
Database references
SIMBAD data

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-B subgiant 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, it is under-luminous for its class. It is probably just an optical double star, most likely several hundred light years beyond Acrux.

[edit] System Acrux

[edit] Etymology

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

Since Acrux is at roughly −60° declination, it is only visible south of the Tropic of Cancer. Because of Earth's axial precession, however, the star was visible to ancient Hindu astronomers in India who named it “Tri-shanku”.

It is known as 十字架二 (the Second Star of the Cross) in Chinese.

Acrux is represented in the flags of Australia and New Zealand 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.

[edit] Reference

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