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Collinder 399, the Coathanger Cluster

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At magnitude 3.6, the Coathanger Cluster is not difficult to spot with the naked eye. It is also known as Brocchi's Cluster or Al Sufi's Cluster. Because of its large size, it is best viewed through binoculars or a telescope at low power. This grouping is not a true cluster but rather an asterism, or a chance alignment of stars of greatly varying distances.
Evening visibility: July-November
Best viewed with: binoculars, telescope at low power
  Printable chart (pdf) View larger image
Start by finding the Summer Triangle, which consists of the three of the brightest stars in the sky--Vega, Deneb, and Altair. The Summer Triangle is high overhead throughout the summer, and it sinks lower in the west as fall progresses.

Look inside the Summer Triangle, to the north of Altair, to find the small constellation Sagitta, the arrow. Its four brightest stars do indeed form the shape of an arrow.

From the two stars that form the tail fins of the arrow, move your telescope 3 degrees to the northwest to find the Coathanger Cluster, as shown in the chart below. It might be visible to the naked eye, but if not it will be easily seen in binoculars or a finderscope.
Star charts created with Cartes du Ciel