Collinder 399 is known as Al Sufi's Cluster, Brocchi's Cluster, and the Coathanger. It is a distinctive asterism, but not a true star cluster.

It is located in Vulpecula and is visible to the unaided eye off the head of Cygnus, the Swan. It can be easily resolved in a pair of binoculars or a small telescope. It contains 10 stars with brightness ranges from 5th to 7th magnitude. Cr 399 is 90 x 30 arcminutes in size.

It was first described by Al Sufi, a Persian astronomer, in his Book of Fixed Stars in the year 964, and independently rediscovered by Giovanni Battista Hodierna, an Italian astronomer, and published in a catalog of his in 1654.

In the 1920's, amateur astronomer Dalmero Francis Brocchi used the cluster in a map for calibrating photometers for the American Association of Variable Star Observers (AAVSO), and the cluster took on another unofficial name - Brocchi's Cluster.

In 1931, Per Collinder, a Swedish astronomer, listed it in his catalogue of 471 open clusters. Today its official name is Collinder 399 (Cr 399).

There has been some debate as to whether Cr 399 is a true cluster whose stars formed together, or just an asterism.

In 1970, astronomers Douglas S. Hall and Franklin G. Vanlandingham, published a paper where they concluded that only six of the brightest stars shared a common proper motion, and later in the 1980's, Pavlovskaya and Filipova found that CR 399 shared a proper motion with 10 other clusters, including M45, the Pleiades. These results seemed to indicate that Collinder 399 was a true star cluster.

However, data from the Hipparcos satellite published in 1997 show that Collinder 399 is not a true star cluster based on more accurate measurements of their proper motion and parallax. It is now best classified as an asterism, or chance alignment of stars that happen to lie along the same line of sight.

The small open cluster NGC 6802 lies just to the right of Brocchi's cluster. This magnitude 8.8 cluster is elongated north-south, and is 3.2 arcminutes in size. It contains 50 stars, the brightest of which is magnitude 12.9.

South is to the top in the above image.

Collinder 399 Catalogs: Cr 399, OCL 113.0 Common Name: Al Sufi's Cluster Brocchi's Cluster Coathanger Object Type: Asterism Magnitude: 3.6v Size: 90' x 45' Constellation: Vulpecula Image Field Centered At: RA: 19h 26m 07s Dec: +20° 17' 23"