Extremely hot O and B class stars release powerful ultraviolet radiation that can strip the electrons from hydrogen atoms. The resulting ionised hydrogen gas glows at several frequencies, including 6563 angstroms, a reddish colour, which astronomers call hydrogen-alpha (Hα). In many cases, hot stars create glowing gas clouds over a wide area. These red patches, called HII regions, can easily be seen in many galaxies and are an important marker of spiral arms.
Astronomers mapping our own Milky Way galaxy have made cataloguing and estimating the distances of HII regions a key priority. The best way to map HII regions is to use a special filter that screens out almost all light except hydrogen-alpha and then to record this light using sensitive photograph plates or (more recently) CCD detectors. Often these images are then compared to images in ordinary light - where there is a difference, then there is a strong hydrogen-alpha source.
The technology to record hydrogen-alpha images became available after World War II. Hydrogen-alpha surveys were part of a larger transition in astronomy created by the realisation that there were many objects in the sky that could not be directly viewed in the lens or mirror of a telescope. This transition was dramatically accelerated by the work of Karl Jansky and Grote Reber in the 1930-40s, who showed that many objects could be detected in radio frequencies. This led to the modern multi-wavelength techniques in which the sky is surveyed across the electromagnetic spectrum all the way from radio waves to gamma rays.
The first major HII region survey for the northern hemisphere was published by Stewart Sharpless in 1953. Sharpless was based at the US Naval Observatory in Flagstaff, Arizona. Unlike the surveys done in the southern hemisphere, Sharpless did not create his own images but instead used existing red-filtered photographs taken using the 48 inch Schmidt telescope on Palomar Mountain, southeast of Los Angeles. The first Sharpless catalog contained 142 nebulae.
Sharpless revised his first catalog using additional plates (also acquired from the 48 inch Schmidt telescope on Palomar Mountain) produced for the National Geographic - Palomar Sky Atlas (usually called POSS-I). He published his second and final catalog of 313 nebulae in 1959.
The story in the southern hemisphere was more complex. Colin Gum at Australia's Mount Stromlo Commonwealth Observatory near Canberra took over an existing nebula photography project begun by Clabon "Cla" Walter Allen. C.W. Allen's involvement was a bit unusual. Mount Stromlo originated as a solar observatory, and Allen was known as a solar astronomer. Initially Gum and Allen were monitoring radio noise from the sun and then (for comparison) other galactic locations. Gum noted in his final survey:
At the Commonwealth Observatory a survey for Hα emission nebulosities in the Southern Milky Way was begun by Dr. C.W. Allen in 1950, and continued and extended by the author since late in 1951. The original aim of the project was to obtain material for the discussion on the origin of galactic radio-noise considered as at least partly due to free-free transitions in the interstellar gas.
Allen left Australia in 1951 to become the director of the University of London Observatory. Ben Gascoigne became Gum's PhD supervisor. Gum decided to write his thesis on the HII regions and continued to photograph them using a 100 mm (4 inch) Schmidt camera. Gascoigne later recounted:
Well, after he had written his thesis he had to go into hospital for medical treatment. During the year he was away, I had to supply all the references, and found that looking them up was a tedious and difficult job – I finished up knowing an awful lot about H-alpha regions myself!.
Gum thanked Gascoigne in his survey paper for "assistance in the preparation of the manuscript for this paper, the publication of which has been delayed owing to unforseen circumstances". Gascoigne provided more than help with the manuscript, however:
When Colin put his thesis in, the two examiners were Woolley [the Stromlo director] and a Professor Plaskett at Oxford. Woolley came in one day and said, 'Gum has failed his PhD.' I think Plaskett was the snag, because he couldn't have known anything about the subject, but I don't think Woolley read the thesis properly, anyway. I don't want to sound too critical of Woolley, because he did a great deal for the Observatory, but he did have these idiosyncrasies. I was most indignant and very distressed that Colin had failed, because I thought he was really good ... I marched in and battled away as best I could until we were interrupted – I was truly thankful for that and went home, the matter quite unresolved. Next day I was back again, batting away, and I thought I made a bit of progress. And on the third day Woolley agreed to appoint a third examiner, Cla Allen, who was by that time in London. And so Colin got his PhD.
The contrast between Sharpless coolly examining plates taken by others at the great Palomar observatory and Gum struggling to have his own 4 inch camera images taken seriously could not be more extreme!
While the drama unfolded in Australia, another team led by Dutch-American astronomer Bart Bok was carrying out its own southern hemisphere survey from Harvard University's Boyden Station in South Africa. Bok and his colleagues published some preliminary results in 1953, in which they mentioned that they were using "a camera with a 3 inch Zeiss Sonnar lens (f/1.5) on loan from Mr. Richard S. Perkin".
Dorrit Hoffleit published a catalog of 69 nebulae in the Carina region, based on the Boyden station data. Another Harvard catalog of 41 items, known as the BBW catalog, was published in 1955. The published catalogs were also able to use images gathered from a much larger 32/36 inch Schmidt telescope that was installed at the Boyden Station in December 1950.
Although the two southern HII projects may have begun separately, they soon began to interact. Hoffleit quoted Gum in her 1953 survey, and Gum said in his own study:
The number of completely new HII regions discovered, over and above those known at the epoch of commencement of the project is 40. A number of these new objects have, however, been independently discovered in a southern Hα survey at the Boyden station of Harvard College Observatory.
The two projects soon became much more intertwined when Bart Bok left Harvard in 1957 to become director of the Stromlo observatory. Bok quickly arranged for a 8 inch Schmidt survey camera, which while small compared to the 48 inch at Palomar, was much larger than the 4 inch camera Gum had used or the 3 inch camera he himself had used in his own preliminary survey in South Africa. The camera was delivered to Mount Stromlo in December 1957 and a new more comprehensive survey of the southern nebulae began.
Led by Stromlo astronomer Alex Rodgers and Bok's graduate students Colin Campbell and John Whiteoak, the survey ran from December 1957 to April 1959. The RCW catalog of 182 entries was published in 1960.
It is interesting to know that both the Gum and RCW HII region surveys, which mapped much of the southern Milky Way, were carried out using equipment that would usually be considered appropriate for amateur astronomers. (Indeed, the well known San Diego based amateur astronomer and telescope maker Harold Lower described an 8 inch Schmidt camera he had constructed in a pamphlet published in 1939.)
Colin Gum went on to write several more important papers on the structure of the Milky Way and spent 10 months as a Carnegie fellow at the Mount Wilson and Palomar Observatories where presumably he finally got to use the giant 48 inch Schmidt camera. Tragically, Gum was killed in a skiing accident near the Matterhorn in the Swiss Alps on 29 April 1960. Bok's grief comes through in his obituary for Gum:
Colin Gum was one of the best liked and most admired of younger Australian astronomers and many of us have a deep sense of personal loss at his too early death. Australia is still very short of first class astronomers trained and educated in the country, and his death is a severe blow to the development of Australian astronomy ...
Jointly with Professor C.W. Allen, he carried out from Mount Stromlo Observatory one of the early sky surveys (1950) of the distribution of radio noise (at 200 MC/s) and he will be remembered for many years to come for his successful search for and discovery of hydrogen-emission regions of the Southern Milky Way. The large and significant complex of nebulosity in Puppis and Vela is generally referred to as "Gum's Nebula"; a finer monument is difficult to imagine. His Survey of Southern H II Regions (Royal Astronomical Society Memoirs, Vol. 67, part 4, 1955) has become a standard reference volume for all workers in the field ...
For a person of his age, Colin Gum leaves behind a solid and impressive bibliography of significant scientific contributions and, in spite of his too-early death, he has left his permanent mark on the development of astronomy.
Bart Bok himself retired to Arizona and died on 5 August 1983. Stewart Sharpless moved from Arizona to Rochester, New York, where he spent many years as an astronomy professor and died on 19 January 2013 at the age of 86.