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'''Acrux''', also known as '''Alpha Crucis''', is the southernmost first magnitude star and the twelfth brightest star in the night sky, with an apparent magnitude of 0.77.  The star is also the brightest star of the constellation [[Crux]], otherwise commonly known as the '''Southern Cross'''.  Because of the star's location at 63 degrees south of the [[celestial equator]], it is not visible  north of latitude 27°N. <ref name="david">http://www.daviddarling.info/encyclopedia/A/Acrux.html</ref>

'''Acrux''', also known as '''Alpha Crucis''', is the southernmost first magnitude star and the twelfth brightest star in the night sky, with an apparent magnitude of 0.77.  The star is also the brightest star of the constellation [[Crux]], otherwise commonly known as the '''Southern Cross'''.  Because of the star's location at 63 degrees south of the [[celestial equator]], it is not visible  north of latitude 27°N.<ref name="david">http://www.daviddarling.info/encyclopedia/A/Acrux.html</ref>

==Acrux in Culture and History==

==Acrux in Culture and History==

==Star System==

==Star System==

Acrux itself is actually a multiple star system of three known stars, located 325 light years away.  Two of the stars of the Acrux system make up a [[Binary_stars#Visual_Binaries|visual binary]], and are commonly referred to as '''Alpha-1''' and '''Alpha-2''' respectively.  The two stars have a separation of at least 430 AU and take a minimum of 1500 years to complete an orbit of each other.  Furthermore, Alpha-1 itself is a [[Binary_stars#Spectroscopic_Binaries|spectroscopic binary]] star.  From an observer on [[Earth]], there is another star located on 90 arcseconds from the triple stars of the Acrux system and has the same proper motion in space.  If this additional star is part of the system, it would have an orbital distance of 9000 AU from the other stars in the system.  However it is likely the star is just an optical binary and not part of the system at all, and may be significantly more distant star that is along the same line of sight.

Acrux itself is actually a multiple star system of three known stars, located 325 light years away.  Two of the stars of the Acrux system make up a [[Binary stars#Visual Binaries|visual binary]], and are commonly referred to as '''Alpha-1''' and '''Alpha-2''' respectively.  The two stars have a separation of at least 430 AU and take a minimum of 1500 years to complete an orbit of each other.  Furthermore, Alpha-1 itself is a [[Binary stars#Spectroscopic Binaries|spectroscopic binary]] star.  From an observer on [[Earth]], there is another star located on 90 arcseconds from the triple stars of the Acrux system and has the same proper motion in space.  If this additional star is part of the system, it would have an orbital distance of 9000 AU from the other stars in the system.  However it is likely the star is just an optical binary and not part of the system at all, and may be significantly more distant star that is along the same line of sight.

===Alpha Crucis A===

===Alpha Crucis A===

'''Alpha Crucis A''' or '''Alpha-1''' is actually a binary pair (Alpha Crucis Aa and Alpha Crucis Ab respectively), the binary being visible only through its spectrum.  The primary star (Alpha Crucis Aa) is blue-white subgiant star of spectral type B0.5 III.  Alone the star has an apparent magnitude of 1.4 and would be the 20th brightest star in the night sky.<ref name="illinois">http://stars.astro.illinois.edu/sow/acrux.html</ref>  The star has 14 times the [[Sun|Sun's]] mass and is 25,000 times as visually luminous, even though most of its radiation is in the [[ultraviolet]].  The surface temperature is 28,000 K. <ref name="david">http://www.daviddarling.info/encyclopedia/A/Acrux.html</ref>

'''Alpha Crucis A''' or '''Alpha-1''' is actually a binary pair (Alpha Crucis Aa and Alpha Crucis Ab respectively), the binary being visible only through its spectrum.  The primary star (Alpha Crucis Aa) is blue-white subgiant star of spectral type B0.5 III.  Alone the star has an apparent magnitude of 1.4 and would be the 20th brightest star in the night sky.<ref name="illinois"/>  The star has 14 times the [[Sun|Sun's]] mass and is 25,000 times as visually luminous, even though most of its radiation is in the [[ultraviolet]].  The surface temperature is 28,000 K.<ref name="david"/>

Alpha Crucis Aa and its companion orbit every 75.78 days. The orbit is eccentric, varying between 0.5 and 1.5 AU, with an average of only around 1 AU, or about the distance of the [[Sun]] from the [[Earth]].<ref name="illinois">http://stars.astro.illinois.edu/sow/acrux.html</ref>  The smaller companion itself is spectral class B0, and may be 10 times as massive as the Sun.<ref>http://jumk.de/astronomie/big-stars/acrux.shtml</ref>

Alpha Crucis Aa and its companion orbit every 75.78 days. The orbit is eccentric, varying between 0.5 and 1.5 AU, with an average of only around 1 AU, or about the distance of the [[Sun]] from the [[Earth]].<ref name="illinois"/>  The smaller companion itself is spectral class B0, and may be 10 times as massive as the Sun.<ref>http://jumk.de/astronomie/big-stars/acrux.shtml</ref>

===Alpha Crucis B===

===Alpha Crucis B===

'''Alpha Crucis A''' or '''Alpha-2''' is a blue-white dwarf star of spectral type B1 V.  The star has an apparent magnitude of 2.09, making it easily visible to the unaided eye if the star was alone.  Alpha Crucis A is 13 times as massive as our [[Sun]].  Its visual luminosity 16,000 as great as the Sun's, even with most of the radiation in the [[ultraviolet]].  The surface temperature is estimated to be 26,000 K.<ref name="david">http://www.daviddarling.info/encyclopedia/A/Acrux.html</ref>

'''Alpha Crucis A''' or '''Alpha-2''' is a blue-white dwarf star of spectral type B1 V.  The star has an apparent magnitude of 2.09, making it easily visible to the unaided eye if the star was alone.  Alpha Crucis A is 13 times as massive as our [[Sun]].  Its visual luminosity 16,000 as great as the Sun's, even with most of the radiation in the [[ultraviolet]].  The surface temperature is estimated to be 26,000 K.<ref name="david"/>

[[Category:Astronomy]]

[[Category:Astronomy]]