Acrux

Ácrux (α Crucis) is the fourteenth brightest star in the sky with apparent magnitude +0.77. It is located 325 light years from the solar system in the direction of the constellation of the Southern Cross.

The alignment of the stars of the Southern Cross «γ-α» points in the direction of the south celestial pole —see celestial sphere—; the imaginary trace of a bisector of the line joining α Centauri with Hadar (β Centauri) intersects the previous line approximately on the south pole. The southern pole star is σ Octantis, also known as Polaris Australis.

Features

Identified as a double star by Jesuit missionaries, Ácrux is composed of two bluish-white stars, separated by 4 seconds of arc. The brightest, called Alpha-1 Crucis (HD 108248 / HR 4730), is cataloged as a subgiant of spectral type B0.5IV and magnitude 1.33, while its companion, Alpha-2 Crucis (HD 108249 /HR 4731), has type B1V and magnitude 1.7 and is still on the main sequence. Together, the pair presents a magnitude of 0.77. Alpha-1 Crucis is, in turn, a spectroscopic binary with a period of 75.86 days.

The main component of Alpha-1 Crucis has a surface temperature of 30,000 K. The two stars in this subsystem are respectively 25,000 and 7,000 times more luminous than the Sun. Alpha-2 Crucis has a temperature of 27,000 K and a luminosity approximately 20,000 times that of the Sun. Its mass is about 13 times greater than the solar mass.

The separation of the binary Alpha-1 Crucis with respect to Alpha-2 Crucis is 400 AU —this being a minimum value— and its orbital period is at least 1300 years. For its part, the average separation between the two components of Alpha-1 Crucis is only 1 AU, but the notable eccentricity of its orbit causes it to vary between 0.5 and 1.5 AU.

90 arcseconds SSO is another star of magnitude 4.9 and spectral type B, visible with binoculars and called Alpha Crucis C. Although it apparently has the same proper motion as Alpha-1 and Alpha-2 Crucis, this star is more than twice as distant and therefore has no physical relationship to the pair.