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Published online by Cambridge University Press: 05 March 2013
Editor's note: This paper was presented as an Invited Talk at the 1997 Annual Scientific Meeting of the Astronomical Society of Australia held at the University of New South Wales. Although it is not in the usual format for a research article submitted to PASA, the Editorial Board decided that the paper is of suficient interest to astronomers that it should be published in PASA. It has not been refereed.
* Editor's note: This paper was presented as an Invited Talk at the 1997 Annual Scientific Meeting of the Astronomical Society of Australia held at the University of New South Wales. Although it is not in the usual format for a research article submitted to PASA, the Editorial Board decided that the paper is of suficient interest to astronomers that it should be published in PASA. It has not been refereed.