October 25, 2013
The Butterfly Cluster, an open cluster in Scorpius
Image Credit: N.A.Sharp, Mark Hanna, REU program/NOAO/AURA/NSF
The Butterfly Cluster (also known as Messier 6 or NGC 6405) is a bright open star cluster of some 12 light-years across, located around 1,600 light-years away from Earth in the southern constellation of Scorpius (the Scorpion). Its name derives from the vague resemblance of its shape to a butterfly.
The cluster lies near the border of the constellation of Sagittarius, what makes it the Messier Object that is the closest to the center of the Milky Way. Members of this cluster were formed in the same giant molecular cloud and are still loosely gravitationally bound to each other.
The Butterfly Cluster contains probably a little more than 300 stars, although only 80 have been identified. Most of them are young, hot blue stars but the brightest member (on the edge of the butterfly’s left wing) is an orange giant star, called BM Scorpii (HD 160371), which contrasts sharply with its blue neighbors in photographs. BM Scorpii, is classed as a semi-regular variable star, its brightness varying from magnitude +5.5 to magnitude +7.0 in a cycle of roughly two years. The cluster is estimated to be about 100 million years old.
You can find this cluster about 4 degrees north of the bright star Shaula in the Scorpion’s tail, and just five degrees southeast of Messier 7, another open cluster. A glance through a small telescope reveals why it is called the Butterfly Cluster: at 40-50x, the cluster has 3 bright stars running through the center (the body of the butterfly), with two irregular loops of stars on either side (the wings). A little imagination reveals the butterfly’s “antennae” to the northeast. You’ll see just a few dozen stars in binoculars, and perhaps 100 stars in a 6-inch scope.
This photo is created by images taken in June 1995 at the Burrell Schmidt telescope of Case Western Reserve University’s Warner and Swasey Observatory, during the Research Experiences for Undergraduates (REU) program operated at the Kitt Peak National Observatory and supported by the National Science Foundation.