The Jewel on the Fox's Flank - C399, a non-cluster

by Phil Beastall

It has to be said that the constellation of Vulpecula is not the most prominent in the sky. Those who try to make pictures from the lines between the brighter stars often overlook the shapes traced out by the dimmer ones. After all, this is not how the ancients played the pattern game. Nonetheless, I have to admit that I have only seen a clear fox on two occasions in our light polluted skies. The thought that some of our forebears insisted on having Ansa, the goose, hanging from the jaws of Vulpecula fills us with amazement – I for one have never been able to see either the goose itself nor the point of inventing it. All this said, the constellation of Vulpecula plays host to such well-known objects as the Dumbell Nebula and that glorious group of stars colloquially known as "The Coat Hanger".
Occasionally people complain that groupings of stars have strange names that mean nothing to them and never look like what they are supposed to be. There are a number of reasons for this. As stated earlier, it is partly because they think that the very brightest stars should make stick figures. Partly it is because they do not know the stories that gave rise to some of the sequences in the sky. Mainly it is because they lack imagination and romance. They are probably not the best people to go around an art gallery with. It is with some glee that I suggest to them that "The Coat Hhanger" is hardly the stuff of ancient myth, and that it looks exactly like a coat hanger albeit upside-down.
You may have noticed that up until now, the word "cluster" has been studiously avoided. This is for a good reason. A Galactic Cluster, also known as an Open Cluster, is a group of stars which originated in the same nebula and which are travelling through space together. These are important in our understanding of star formation. A grouping of stars that do not have these characteristics are simply a pretty shape. It could cynically be said that this is the distinction between "stargazing" and astronomy, but that would be too simplistic. There are the issues about the way that people perceive patterns in the sky and how they impose order upon random distributions. There is the question of things that appear so obvious but need to be open to question.
The Coat Hanger, or more historically Brocchi’s Cluster, was first noted by the Persian astronomer Abd al-Rahman Al-Sufi in 964 AD. It was independently rediscovered by Giovanni Batista Hodierna (1597-1660), astronomer at the court of the Duke of Montechiaro, and included in his catalogue of 40 objects published in Palermo in 1654. Hodierna was a great observer in the generation after Galileo. Aged thirteen when the telescope was first pointed at the heavens, he became an astronomer and made his observations with a similar instrument with a magnification of x20. Apart from independently rediscovering the Andromeda Galaxy and the Orion Nebula, among others, he made nine or ten actual discoveries – as listed by Kenneth Glyn Jones, these are M6 M36, M37, M38, M41, M47, NGC2362, NGC6231, NGC6530 and possibly NGC2451. We may marvel that he saw so much with such a small telescope, but perhaps the greatest testimony to his observational skill is the drawing he made of the Orion Nebula in which three stars of the Trapezium may be seen. Those wishing to look further into the achievements of this remarkable man will find much of interest.
The Coat Hanger was not listed by Messier, probably because it would be difficult to mistake for a comet, nor by Herschel. It is perhaps a little surprising that it made its way into the mainstream catalogues as late as 1931 by courtesy of the Swedish astronomer Per Collinder. Collinder wrote much about the history of astronomy including "Swedish astronomers 1477-1900 " (Uppsala, 1970) and various papers. He was interested in the possible influence of the Phoenicians, Egyptians and Babylonians on Greek astronomy and tried to use the statistics of their geographical distribution to determine where their ideas may have spread.. Ironically, he is best remembered now for his inclusion of the Coat Hanger as C399 in his catalogue.
Of course, the Coat Hanger just looks like a cluster. It was not until Douglas Hall and Franklin van Landingham made their study in 1970 that anyone had tried to prove it one way or the other. Using astrometric, photometric and spectrometric data on the member stars of the group they sought to find out if any or all were linked with each other. The trouble is that at the distances involved, stellar parallaxes are small and the uncertainties are large. They concluded that about six of the brighter stars in the field may be part of a cluster.