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Object description
NGC 6231 - an open star cluster in the constellation Scorpion

Object description:

NGC 6231 appears at first as a relatively inconspicuous open star cluster, but this is deceptive, because it is one of the brightest star clusters of the southern sky. According to Trumpler it is assigned to class I 3 p.

The cluster belongs to a larger group of young hot stars surrounded by large regions of H-II emission nebulae, which is known as the SCO OB I association (Gum 55). NGC 6231 forms the center of GUM 55, and the distance to NGC 6231 is about 5700 light-years. A diameter of 15 arcminutes on the sky gives an absolute diameter of 25 lightyears for the brightest cluster part.

The age of the stars in NGC 6231 is estimated to be only a few million years. The number of hot blue stars of spectral class O is given as 15 to 20, whose strong UV radiation excites the whole region to emission. Violent stellar winds apparently provide for the ring-shaped structure of Gum 55.
The stars of NGC 6231 are influenced by interstellar dust. It attenuates the light of the stars at least by a factor of 6 and also leads to a reddening of the starlight. That's why the young, hot stars of the cluster don't appear very blue, but rather whitish, because the blue part is scattered out in the interstellar dust.

NGC 6231 was discovered by Giovanni Battista Hodierna around 1654.

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