The Steavenson Award

 

Variable Stars: Why Professionals Need Our Help

Mr Pickard explained that variable stars were of particular scientific interest because they often give us clues about the processes which underlie stellar evolution, and also because some classes of variable stars, such as Cepheids, are useful as distance indicators. Professional astronomers have to rely upon the assistance of the amateur community, however, for the simple reason that there are vastly more variable stars in the sky than they have the resources to monitor themselves. The BAA's own data archive included results dating back as far as 1890, and contained a total of around 2 million observations, 1.5 million of which were now stored in easily accessible electronic format.

The speaker outlined a few of the marked changes which the Section had seen since its foundation. Firstly, the number of active members had increased sharply during the 1950s and 60s, peaking at well over 100 in the mid 1970s. Sadly there had been a slight decrease since then, and the speaker was keen to see this latest trend reversed. The types of object being observed had also changed significantly throughout the Section's history. In its early, pre-1925, era, Mira-type stars had made up the vast majority of those studied. By 1975, however, a much wider variety of objects were being examined, with semi-regular stars now being the most-observed category.

More recent data from 1997 indicated that the trend had changed again, with cataclysmic variables and dwarf novae now the most observed objects, though Mira-type stars and semi-regulars were still receiving significant attention. Cataclysmic variables are binary systems with very rapid rotation periods, typically of order six hours. Extreme astrophysical phenomena occur under these gravitational conditions, and it was thought that material was often gravitationally accreted from one star onto its more massive companion, giving rise to the observed chaotic variability. The rôle of magnetic processes within this accretion remained poorly understood, with various speculative theories. The speaker believed that the latest trend towards increased interest in cataclysmic objects was possibly the result of the relative ease with which papers could be written about them. Their rapid rotation allowed papers to be written making use of only a modest number of nights of observation.

The preparation of charts and sequences was a major area in which the Section could benefit from additional assistance, and the speaker was keen to hear from anyone who was interested in helping out. In the past, many visual observers had been hampered by poor charts, and despite the strenuous efforts of the Section's Chart Secretary, John Toone, and Mike Simonsen in the US, the task remained substantial. In particular, charts had historically only included stars down to mag 13, but now there was an increasing demand from observers with modern equipment for charts extending down to mag 16. The speaker expressed his gratitude to Chris Jones and Richard Hunt, who had recently joined the team.

The Section's interaction with the professional community had initiated in around 1990, when Dr Paul Roche had requested observations of X Per from the Association. Since then, professionals had been enlisting the assistance of the BAA's observers with ever-increasing frequency. Visual observations were in as much demand as CCD images, and the speaker urged members not to abstain from submitting contributions in the belief that they would not be worthwhile if their equipment was modest.

Karen Holland had instigated a CCD programme within the Section, and was planning to complement the presently available charts, which were primarily aimed at visual observers, with similar resources for all stars on the CCD target list. This included some objects which were simply too faint for binocular observation, whilst others were bright enough for visual identification, but exhibited very small changes in magnitude which only CCDs could detect accurately. Karen Holland had also set up a mentoring scheme for new members of the Section, both CCD and visual astronomers, whereby less-experienced observers were allocated an experienced mentor, who they could contact to seek advice. This was presently running smoothly, and widely thought to be a success.

Mr Pickard explained that the Section operated by producing lists of interesting objects, and observers were urged to check these stars periodically to check for outbursts or other interesting activity. He urged members to get involved, because presently there was a substantial number of objects for which professionals had requested data from the Section, but for which it did not have an adequate number of observations. It was not just CCD observers who were in demand: there was currently a greater shortage of visual observers. The Section was also in need of help with the electronic data entry of historical records, as there were 500,000 observations still needing to be transferred into electronic format. Roger Dymock was presently involved in a project to make the archive available via an online database.

The speaker closed his talk with three sample magnitude-variation plots, for dwarf novae AB Dra and SS Vir, and for the circumpolar Mira variable T UMa. Following the applause, the President then welcomed Jonathan Shanklin to give the third short talk, focusing on the Association's activities in the comet scene.

Fairfield

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