Butterfly Cluster

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The Butterfly Cluster (also known as Messier Object 6 or NGC 6405) is an open cluster in the constellation Scorpius. It was discovered by Giovanni Batista Hodierna before 1654 and catalogued by Charles Messier in 1764 as number 6 in his catalogue of comet-like objects.

Image:NGC6405.jpg

Estimates of the cluster's distance have varied over the years with a mean value of around 1,600 light years, giving it a spatial dimension of some 12 light years. Most of the bright stars in this cluster are hot, blue B type stars but the brightest cluster member is a K type orange giant star. This star, known as BM Scorpii, is classed as a semiregular variable star, its brightness varying from magnitude +5.5 to magnitude +7.0. In color photographs of the Butterfly Cluster this orange star contrasts sharply with its blue neighbours in the cluster.

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