Hogg Catalogue of Open Clusters (1965)

15 March 2020

The twenty two Hogg open clusters offer an unusually rich observing project – not because the clusters themselves are rich – indeed, many of them are notably obscure – but because they all lie in exceptionally lush and beautiful star fields, with many of them juxtaposed with lovely NGC and Trumpler clusters, thus making it a superb visual treat to track down these tiny little treasures and endeavour to unscramble the more obscure cluster members from their star-rich backgrounds! Indeed, it isn’t often that one embarks on an observing project that is as much about the background as it is about the object!

In 1965 Australian astronomer Arthur Robert Hogg published a comprehensive catalogue and photographic atlas of southern open clusters south of -45°. This was a collection of 98 plates taken on Mount Stromlo’s 74-inch reflector, and a search of the plates revealed the existence of 22 hitherto unknown clusters.

Interestingly, he wrote:

‘Initially, the clusters were recognized from their visual appearance. This procedure is, of course, open to question, for the extent to which a group of stars appears as a cluster is subject to personal bias, to the scale of the plate, and to the nature of the surroundings. As a partial confirmation of the reality of the suspected groups, star counts have been made inside and outside of the suspected cluster area and the number of “members” estimated, using a standard deviation computed as the square root of the mean number of background stars in an area equal to that assumed to be covered by the cluster. In one case, cluster No. 18, a color-magnitude array has been observed to indicate the reality of the cluster.’

Hogg was born at Creswick, Victoria, on 25 November, 1903. He was educated at the Royal Melbourne Institute of Technology and at the University of Melbourne where he earned his B.Sc. in 1923 and M.Sc. in 1925. He was awarded his Ph.D. in 1950 from the University of Melbourne, based on his study of cosmic rays. 

Hogg’s scientific interests covered an unusually wide range. He began his professional life as an industrial research chemist. Upon joining the Commonwealth Solar Observatory (as Mount Stromlo Observatory was then called) he took up the study of a number of atmospheric electrical phenomena, in particular of the conductivity, ionic mobilities and ion balance in the lower atmosphere. He then transitioned into the study of cosmic rays. During the Second World War years (with the Munitions Supply Laboratories and the Chemical Defense Board), his work there focused on the study of respirators.

When he returned to the observatory in 1946 Hogg became an astronomer. The field he chose was photoelectric photometry and he published a long series of papers on eclipsing variables, standard magnitudes and galactic clusters – and in 1965 his Catalogue of open clusters south of -45° declination. (He had played a leading role in establishing the new 74-inch reflecting telescope which was the Observatory’s primary research instrument throughout its lifetime. Alas, it was destroyed in the 2003 firestorm that largely destroyed Mount Stromlo Observatory.)

He was also instrumental in selecting Siding Spring Mountain as the site of an 150-inch telescope, away from Canberra’s increasing light pollution. He was the deputy director of the Mount Stromlo Observatory from 1961 to 1966. He died on 31 March, 1966. He was honoured with a crater on the Moon’s far side.

In the way that an error is perpetuated on the Internet, one often sees Arthur Hogg’s discoveries attributed to Helen Sawyer Hogg with statements along these lines “… the 22 Hogg star clusters were catalogued by Helen Sawyer Hogg during her research into the variable stars in the Large Magellanic Clouds”.