Saturday 15 August 2020

Astrognome 100 Great Stars No.1 Acrux

 Star 1


Just as Ursa Major is the most famous constellation in the northern hemisphere then Crux or the Southern Cross is the most famous in the southern hemisphere. Yet surprisingly it is also the smallest of the 88 constellations in the night sky.

Acrux is the brightest star in the constellation of Crux, however the southern cross was actually part of the constellation of Centaurus. It is sometimes cited that it was in 1679 that the French astronomer Augustin Royer created Crux, however both the English astronomer Emery Mollineux in 1592 and the Dutch Flemish astronomer Petrus Plancius in 1598 had recorded Crux being a separate constellation.

Acrux is the 13th brightest star in the sky but there is more to the star than meets the eye because there is not one but two stars here. They are both very hot blue / white stars with surface temperatures of around 23,000 degrees centigrade to 25,000 degrees centigrade. It is estimated that they take around 1,500 years to orbit each other. And there is more because Acrux 1 is itself also a double star and Acrux can be referred to as a triple star system.

Best of all Acrux leads us to one of the most glorious regions of our galaxy the Milky Way. To the east of the cross lies the ‘Coalsack’ a dense cloud of star forming interstellar gas and dust that blocks the background starlight. The coalsack is so dark and prominent that the Incas in South America made a constellation of it calling it the ‘Yutu’ a partridge like southern bird.

To the north east of Acrux is one of the finest star clusters the ‘Jewel Box’ it is so bright that it was named after the 10th letter of the Greek alphabet kappa Crucis.

Acrux and the constellation of Crux is one of the few that is honoured in music with the tango by Richard Rogers “Beneath the Southern Cross” from the documentary ‘Victory at Sea’.

Acrux also appears on the national flags of Australia, New Zealand, Samoa and Papua New Guinea.

Once seen Acrux and its surroundings can never be forgotten.

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