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Messier 6 and the H-II region LBN 1117

Description of object:

The open star cluster M 6 is located in the constellation Scorpius at a distance of about 1600 light years. The diameter of the cluster is 12 light years. M 6 appears in the sky with an apparent diameter of 25 arc minutes, almost as large as the Moon. Due to its visual impression in the telescope, it is also called the butterfly cluster. The age of M6 is estimated to 95 million years. The contrast between the hot blue stars of spectral class B and the single orange star BM Scorpii with spectral class K is striking. BM Scorpii is a semiregular variable giant star with brightness fluctuations between 5.5 mag and 7 mag. The right part of the image shows the large reddish HII region LBN 1117. Even though the camera was not modified for astro use in the red spectral range, this weaker emission nebula could be imaged.
With a brightness of 4.2 mag, M 6 can be detected by the eye as a weak nebulous object without stars. Even small binoculars show a large number of the brighter stars and reveal the true nature of the object.

The discovery of the cluster is attributed to the Italian astronomer Giovanni Battista Hodierna in the middle of the 17th century. Hodierna listed the star cluster in a catalogue of celestial objects, which received little attention in the following years. About 100 years later, M 6 is observed by the Swiss Jean-Philippe de Chéseaux and Charles Messier and is listed as number 6 in his famous Messier catalogue.

The image left shows a size comparison with the full moon. Click here or the thumbnail image to load a large version.

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