Brocchi's Cluster (also known as Collinder 399, Cr 399 or Al Sufi's Cluster) is a random grouping of stars located in the constellation Vulpecula near the border with Sagitta. The members of the star cluster form an asterism which has given rise to its name as the Coathanger.
In the 17th century, it was independently rediscovered by the Italian astronomer G. B. Hodierna.
In the 1920s, Dalmero Francis Brocchi, an amateur astronomer and chart maker for the American Association of Variable Star Observers (AAVSO), created a map of this object for use in calibrating photometers.
The status of this group as a star cluster has changed in recent years. The group was considered to be a cluster for most of the 20th century. Looking at a variety of criteria, however, a study in 1970 concluded that only 6 of the brightest stars formed an actual cluster. Several independent studies since 1998 have now determined that this object is not a true cluster at all, but rather just a chance alignment of stars. These recent studies have generally based their findings on improved measurements of parallax and proper motion provided by the Hipparcos satellite which were first published in 1997.
The asterism is made up of 10 stars ranging from 5th to 7th magnitude which form the conspicuous "coathanger", a straight line of 6 stars with a "hook" of 4 stars on the south side. An additional 30 or so fainter stars are sometimes considered to be associated as well.
Under a dark sky, the Coathanger can be seen with the naked eye as an unresolved patch of light; binoculars or a telescope at very low power are usually needed in order to view the "coathanger" asterism. It is best found by slowly sweeping across the Milky Way along an imaginary line from the bright star Altair toward the even brighter star Vega. About one third of the way toward Vega, the Coathanger should be spotted easily against a darker region of the Milky Way. The asterism is best seen in July-August and north of 20° north latitude it is displayed upside down (as in the picture top right of this page) when it is at its highest point. South of this latitude it is shown upright as the 'hanger' is south of the line of 6 stars.
The asterism and its immediate surroundings are a useful gauge for determining the faintest stars visible in a small telescope as there are a wide range of stellar magnitudes within the cluster easily viewed in one small location of the sky.
The following is a list of the 10 stars commonly included as members of the Coathanger, organized by right ascension. They vary widely in distance, with only HD 182422, HD 183261, and 7 Vul being possibly remotely near each other.
At 7 Vul and HD 183261's closest possible distance to each other (1100 light-years), they would be further than 6 light-years apart from one another. At the closest possible distance of all three (1180 light-years), HD 182422 and HD 183261 would be separated by almost 20 light-years, and HD 182422 and 7 Vul would be separated by more than 25. For comparison, the Sun and the closest known star, Proxima Centauri, are slightly more than 4 light-years apart.
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