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Location: Chemistry/Physics Building, MUN
Time: 8:00 pm
Present: 19
1. Randy's Introduction. Randy gave a brief spiel about what the Society is all about, membership-wise. He noted our web page is now: http://www.sji.ca/rasc/ . Observing reports: Gary Case has viewed the comet, noting it is awkward right now to view, unless early in the morning. Randy has seen the comet. Garry Dymond had two good nights with the comet and made drawings when it was near the Andromeda Galaxy. Ben Llewellyn has been tracking some planets, particularly Jupiter and its moons. He believes he may have seen a moon of Saturn.
2. Garry Dymond gave an introduction for Dr. David Turner of St. Mary's University. Garry has known Dr. Turner for a number of years. In fact, there are a lot of similarities between them. Both attended the University of Waterloo. When Dr. Turner was later studying at Western Ontario, Garry was there for a wedding. Dr. Turner got his Ph.D. in Astronomy, hmmmm, well, Garry always wanted to do that. Dr. Turner used to run a planetarium, hmmmm, well, Garry always wanted to do that. Considering the huge numbers of papers that are written each year, Garry reckons he must be a full professor.....
Dr. Turner, after some mirth from Garry, presented a speech dealing with Cepheid variables, and a new system for determining magnitudes by eye that can be used by the amateur to determine light curves. Also, dwelt on briefly were his past contributions to the Journal as editor. Currently, he is still involved with the Journal in a capacity as a book review editor. He has also been involved in a strange underground movement, called the RASD. This appears to be a secret society within the RASC?! Some pictures were shown of some pretty shifty-looking characters....
Thanks to the wonders of overhead projectors, we were then transported to the Ukraine. Dr. Turner was finally convinced by a colleague to attend a conference in the extremely beautiful area of Odessa, after many years of prodding. Together, they have decided to work on firming-up our understanding of Cepheid variables, and from there, improving the calibration of the various distance-scales that astronomers have determined over the years.
Cepheids were first discovered in 1784 by Piggot (Eta Aquilae) and later in the same year by Goodricke (Delta Cephei). (The eponymous Delta Cephei is the prototype for this classification.) There are 40 Cepheids that are visible with the eye. With most of them, the variations in brightness can be easily detected by the unaided eye (Polaris is an exception, varying by only about 1%). Theoreticians have determined a number of things about the Cepheids, including the famous Period-Luminosity relationship. Basically, the longer the period of the variable, the greater the peak luminosity will be. Because the Period relates to the Luminosity of the star (total energy emitted by the star per second), we can determine the distances to Cepheids. This relationship comes from theoretical considerations of stellar structure and evolution.
With the eye, you can determine the magnitude of a Cepheid at different times by comparing it with a number of nearby reference stars. The AAVSO has one method for amateurs to use, but Dr. Turner noted that even with a number of data-points from a large number of amateurs, the resulting light curves were pretty poor. He decided to try to come up with a better method for visual observation of Cepheids. He noted that for different combinations of magnification and telescope diameter, it was known that the eye could determine magnitudes to within 0.20 magnitude +/- 0.14 (standard deviation), when the stars in question were near the limiting magnitude for the human eye. Based on this fact, he could draw up charts and use appropriate reference stars in a sequence of magnitudes.
It was felt better magnitude comparisons would be obtained if the eye were forced to work near to the limiting magnitude. Dr. Turner found good curves were obtained when the magnitudes in question were between about 3 and 6 (the limiting magnitude). Observing in twilight also helps, because one can wait for the appropriate stars to transition from invisibility to visibility (averted vision). With this method, he could observe Delta Cephei on successive nights, using at most ten minutes per night. This avoids getting cold, or missing your favourite TV show! This sort of work also has the advantage of almost instant gratification -- light curves can be started after only a couple of weeks. After a season of work, and using combined 5-point means, a light curve was obtained. The quality of this one-man curve with the new method was greatly improved over the results of the AAVSO, which is dependent upon the help of many observers. Quite good, considering there was no effort to record reference stars, etc., etc.!
The talk then moved to a discussion of predicted versus observed maxima of Cepheids, including subtle changes over years in their periods. He then showed how Cepheids evolve through the Main Sequence, including their short-lived crossings of the instability strip. When they pass through this strip, formerly non-pulsating stars become Cepheids (their radius changes by up to 5%). This can happen up to 5 times in a star's lifespan. By using O-C plots, we can determine where an individual Cepheid fits on the Main Sequence, as at each crossing the star will have a different period and rate of change of period, giving the star a distinctive O-C curve.
Unfortunately, most accurate observations (with photomultipliers) date back to the 1950's. It is hoped that by using the Harvard Plates (which date back to the mid-nineteenth century, longer baselines for observations can be gotten. These will help to determine long-term changes in periods, needed to more fully understand the Cepheids. Some unsolved problems still exist in understanding Cepheids, particularly some randomness in O-C curves in mainly cooler Cepheids.
After all this, it is hoped we can understand how some crude observations combined with some mathematics can give us excellent curves that shed some light on not only the evolutionary history of a star, but also upon the various galactic yardsticks that astronomers use. This is work that Dr. Turner hopes amateurs will become involved with. Those interested can contact the Observing Chair and we can set up something with Dr. Turner if there is enough interest. Remember, this is quick and easy to do once you have your chart down-pat!
3. No Handbook Page, or Book Review this month due to lack of time. Randy noted there is an Aurora alert and showed us on the web at http://www.spaceweather.com . There were comet pictures at this site, also. Garry Dymond noted there is a members-only night at the planetarium tomorrow at 8:30. We try to have these every month if Garry is able and willing.
4. Gary Case on Observing. There were two other people out at the site last week (for a while, anyway). The next observing group meeting will try to be on May 7-8. Gary will update the group if this is to go ahead (not sure of the protocol here). Some talk of the "conjunction", and comet F1. Star-B-Que will be going ahead August 9-11. This will be appearing in Sky and Telescope and in SkyNews.
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