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NGC 6231, the dark cloud Sandqvist 19 and Zeta 1+2 Scorpii

Description of object: The open star cluster NGC 6231 can already be seen with the naked eye next to the bright star zeta Scorpii in the constellation Scorpio. The cluster belongs to a larger group of young and hot stars known as the Scorpius OB I Association. The distance to NGC 6231 is about 5700 light years. With a diameter of 15 arcminutes in the sky, the absolute diameter of the brightest part of the cluster is 25 lightyears. Zeta Scorpii consists of two stars, which however form only one optical double star system. The brighter component, Zeta 2 Scorpii, is an ordinary orange giant star 150 light-years away. The apparently slightly fainter component Zeta 1 Scorpii is one of the most luminous stars in the Milky Way and is nearly 40 times as far away. It belongs to the Scorpius OB I association and possibly also to the star cluster NGC 6231, which is about 1 million times more luminous, 60 times more massive than our Sun and belongs to the class of hypergiants, which includes the most massive and luminous stars in existence.

NGC 6231 and Zeta1 Scorpii are equally affected by interstellar dust. It attenuates the light of the stars by at least a factor of 6 and also leads to a reddening of the starlight. This is why the young, hot stars of the cluster and Zeta1 Scorpii do not appear particularly blue, but rather whitish, since the blue part is scattered out in the interstellar dust. In the left half of the image the very dense dark cloud Sandqvist 19 appears as a dark spot in front of the Milky Way background. Next to it is a brighter piece of emission nebula, which covers the whole region in Scorpio. « In the labeled version, the cluster can be compared with the size of the moon. To load the large version click here or on the preview image. top of page

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