September General Meeting Report
Speaker: Dr Terry Moon, Exchange speaker from the Astronomical Society of South Australia, on Semi-regular Red Variables; old, bloated and wobbling.
Terry graduated in Physics at Monash University in 1974. He was an ASV member in the >60s and >70s and completed his MSc in Astronomy at the University of Melbourne in 1978 and his PhD in Astronomy at Monash University in 1984. Terry has worked at the Defence Science and Technology Organisation (DSTO) in Adelaide since 1986.
With a hop-skip-and-jump introduction to astronomical nomenclature Terry Moon steered us towards his main field of interest in astronomy: Stellar Photometry. Stellar photometry is that branch of astronomy that deals with the accurate measurement of the brightness of stars and the changes in their brightness over time. As a scientific process it probably had its beginnings in the 18th Century with the techniques introduced by Bouger in 1729 and Lambert in 1760. The era of well-determined, reproducible measurements may be considered as starting in 1850 when Podgson defined an increase of 1 magnitude as equivalent to a decrease in brightness of 0.4 (often stated as a magnitude difference of 5 corresponding to a change in brightness of 100). Following this, Zollner constructed the first visual stellar photometer in 1861. Photographic photometry was introduced in 1904 and the international photoelectric system of Johnson and Morgan was established in 1951.
But the basis for brightness classification of astronomical objects goes back a lot further. In the second century BC the Greek astronomer Hipparchus (190-120) introduced a system where the brightest star in each constellation was classified as first magnitude. In 140 CE Ptolemy refined Hipparchus= system by using a brightness scale of 1 to 6 where the brightest stars were called Magnitude 1 and the faintest, that is those just visible to the naked eye, Magnitude 6. This scale is still used today, having been extended to objects fainter than 6th magnitude and brighter than 1st magnitude. Accurate measurement of fractional magnitudes, however, was only made possible in the 20th century through the advent of photoelectric devices.
Photoelectric devices, such as photomultiplier tubes, photodiodes and charge-coupled devices (CCDs), have replaced the photographic plate and visual equipment in professional observatories. In photoelectric photometry an estimate of the quality of the observing can be made by taking a series of consecutive measurements then calculating how much they vary from an average value. Where the variations are 1% or less (corresponding to about 0.01 magnitude), the conditions are considered to be 'photometric. If all this sounds complicated to you, take heart; look on the bright side, as Steve White at the Kitt Peak National Observatory says, the magnitude scale may be unnecessarily complicated and awkward, but once you learn it, you have a valuable tool that turns all those strange astronomical numbers into meaningful information.
Dr Terry Moon=s fascination with minute changes in stellar behaviour, such as his study of the Semi-Regular Red Variables (SRV), the Aold, the wobbly and the bloated@, makes sense when you remember that Dr Moon is working with DSTO, where remote sensing and photometric measurements are the “bread and butter” tools for collecting information. Processing data, and meaningful interpretation of light and shade B colour variation and temperatures, are second nature to him. Concentrating on SRVs serves a double purpose: they are probably the least understood of all variable stars and, by monitoring samples such as Beta Gruis, Gamma Crucis and Epsilon Octantis, the process may shed light on some of the big questions still troubling our understanding of stellar evolution. It may provide insight into the mechanism that balances the incredible thermal forces and convection currents thought to drive these gigantic stars nearing the end of their lives to physically grow and shrink in periods ranging from over one year to sometimes as short as 2 weeks.
It is a long-term program, needing years of observational data. But then, Terry adds, I am trained for this, have the equipment needed, live in a suitable environment and (with an engaging smile) have nothing better to do.