Acrux is the southern most star of the constellation Crux, the Southern Cross. Although the picture below probably gives the impression it is the Northern most of the stars, the picture is the wrong way up. Acrux is one of the brightest stars in the night sky, coming in at position thirteen and being one of the most in the Southern hemisphere.
Alpha Crucis (Alf01 Cru) is the Bayer Classification for the star. The Bayer Classification was created by Johann Bayer in the early nineteenth century. The brightest star in the constellation is normally given the Alpha designation although there are exceptions such as Pollux which is Beta Geminorum.
The Id of the star in the Yale Bright Star Catalogue is HR4730. HIP60718 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD108248.
More details on objects' alternative names can be found at Star Names .
The location of the subgiant star in the night sky is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the object is compared to the celestial equator and is expressed in degrees. For Acrux, the location is 12h 26m 35.94 and -63° 05` 56.6 .
All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving -14.86 ± 0.36 milliarcseconds/year towards the north and -35.83 ± 0.50 milliarcseconds/year east if we saw them in the horizon.
The Radial Velocity, that is the speed at which the star is moving away/towards the Sun is 11.90 km/s with an error of about 2.40 km/s . When the value is negative then the star and the Sun are getting closer to one another, likewise, a positive number means that two stars are moving away. Its nothing to fear as the stars are so far apart, they won't collide in our life-time, if ever.
Luminosity is the amount of energy that a star pumps out and its relative to the amount that our star, the Sun gives out. The figure of 30,302.96 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.
Acrux has a spectral type of B0.5IV. This means the star is a blue subgiant star. The star has a B-V Colour Index of -0.24 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 21,255 Kelvin.
Radius has been calculated as being 4.74 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 3,299,945.35.km. If you need the diameter of the star, you just need to multiple the radius by 2. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 4.76. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.
Acrux has an apparent magnitude of 0.77 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of -4.19 If you used the 2007 Parallax value, you would get an absolute magnitude of -4.20. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.
Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 10.17 which gave the calculated distance to Acrux as 320.71 light years away from Earth or 98.33 parsecs. It would take a spaceship travelling at the speed of light, 320.71 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.
In 2007, Hipparcos data was revised with a new parallax of 10.13 which put Acrux at a distance of 321.98 light years or 98.72 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated.
Using the 2007 distance, the star is roughly 20,362,319.79 Astronomical Units from the Earth/Sun give or take a few. An Astronomical Unit is the distance between Earth and the Sun. The number of A.U. is the number of times that the star is from the Earth compared to the Sun.
The star's Galacto-Centric Distance is 7,351.00 Parsecs or 23,976.27 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.
The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.
|Primary / Proper / Traditional Name||Acrux|
|Alternative Names||Alpha Crucis, Alf01 Cru, HD 108248, HIP 60718, HR 4730|
|Constellation's Main Star||Yes|
|Multiple Star System||Yes|
|Star Type||Subgiant Star|
|Absolute Magnitude||-4.19 / -4.20|
|Visual / Apparent Magnitude||0.77|
|Naked Eye Visible||Yes - Magnitudes|
|Right Ascension (R.A.)||12h 26m 35.94|
|Declination (Dec.)||-63° 05` 56.6|
|Galactic Latitude||-0.36 degrees|
|Galactic Longitude||300.13 degrees|
|1997 Distance from Earth||10.17 Parallax (milliarcseconds)|
|320.71 Light Years|
|2007 Distance from Earth||10.13 Parallax (milliarcseconds)|
|321.98 Light Years|
|20,362,319.79 Astronomical Units|
|Galacto-Centric Distance||23,976.27 Light Years / 7,351.00 Parsecs|
|Proper Motion Dec.||-14.86 ± 0.36 milliarcseconds/year|
|Proper Motion RA.||-35.83 ± 0.50 milliarcseconds/year|
|Radial Velocity||11.90 ± 2.40 km/s|
|Stellar Luminosity (Lsun)||30,302.96|
|Brightest in Night Sky||13th|
|Associated / Clustered Stars||Acrux B|
|Radius (x the Sun)||4.76|
|Effective Temperature||21,255 Kelvin|
The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.
|Proper Motion mas/yr|
|H.D. Id||B.D. Id||Star Code||Magnitude||R.A.||Dec.||Spectrum||Colour||Year|
The map was generated using Night Vision, an awesome free application by Brian Simpson.
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