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 Photo of starfield (Click to enlarge) Background: Throughout the 20th century, variable stars have played an incredibly important role in astronomy. Variable stars allowed Edwin Hubble to prove that our universe is made up of many galaxies. Today, variable stars remain at the forefront of astronomy. There is still much to be learned about variable stars. Judging from recent studies, astronomers estimate that 2-3% of all stars are variable, but only 10% of all bright variable stars are known (Paczynski 2001). Many fundamental questions, such as why some stars vary the way they do, have yet to be answered. Variable stars can vary for many different reasons. Most of the variable stars known today are pulsating variables (Samus, Durlevich 1998). Pulsation is when a star changes its shape or size rhythmically (Bradley 2001). All pulsating stars have fundamental frequencies at which they can oscillate, and these determine the star's period. Some stars oscillate with more than one frequency. In these stars there are several outer layers, each pulsating at its own rate.  A Hertzsprung-Russel (H-R) diagram (Click to enlarge) The Study of Variable Stars: Astronomers have been studying variable stars since their discovery in the early 16th century. At first, astronomers tracked changes by using the naked eye; later, they used the optical aid of telescopes (Williams 1997). For the study of variable stars, just as with all other stars, astronomers adopted the system of magnitudes. This system was developed by the ancient Greeks and designed so that the brightest stars in the sky are first magnitude and increasingly fainter stars get larger numbers (Berry, Burnell 2000). For every five magnitudes there is a 100-fold difference in brightness. A 10th magnitude star is 100 times fainter than a fifth magnitude star. In the late 1800's and up to about 1970, astronomers used photographic film to discover many new variable stars (Wilson 2001). The photo multiplier tube (PMT) was invented in the early 1900's. This allowed astronomers to get a reading of the photons coming from a variable star quite accurately. By switching back and forth between a comparison star and the variable, very accurate measurements could be achieved. With a large enough telescope, an accuracy of +/- 0.01 magnitude was sometimes possible (Budding 1993). Both methods had problems, though: Photographic measurements were inaccurate, while a PMT could only observe a single star at a time. |
In the 1970's the charge-coupled device (CCD) camera was brought into use. This led to a huge advance in photometric observing. Much fainter stars could be researched, and accuracies of +/- 0.001 magnitudes can be achieved occasionally by professional observatories (Berry, Burnell 2000). Recently, astronomical CCD cameras have come down sufficiently in price to make them available to amateur astronomers, creating the potential to revolutionize the study of variable stars. With a CCD camera, an amateur astronomer can collect data that rivals anything before the invention of the CCD. A CCD camera has the further advantage of being able to collect accurate data on many stars in a single image.
Introduction: Our knowledge of variable stars is incomplete; only a small percentage of variable stars have been identified and studied. Searching for variable stars is a very time-consuming process, and professional observatories can rarely offer large blocks of time to one observing group, as the cost is too prohibitive. The availability of relatively low-cost CCD cameras has made it feasible to study variable stars using amateur astronomy equipment. Over the past two years, the author conducted several investigations into the study of variable stars with amateur telescopes and CCD cameras. First, the light curves of known variable stars were plotted using a CCD attached to various camera lenses. Then the CCD was attached to the telescope for greater photometric accuracy, and a poorly understood variable star named v377 Cas was observed in an attempt to better understand its variation. This research demonstrated that amateur equipment could be extremely effective in collecting very accurate photometric data. The progression of these experiments lead to the goal of this project, which was to set up and use a system of amateur astronomy equipment to search for new variable stars. Several wide field surveys for variable stars are currently being conducted using camera lenses and CCD cameras, such as the Hungarian Automated Telescope and the All Sky Automated Survey (Bakos et al 2002; Pojmanski 2000). These surveys cover huge portions of sky but without much resolution. It was decided to use a telescope to observe a smaller area of sky, but with more detail, and to a fainter limiting magnitude. The system was designed using only hardware that is readily available commercially, so that this setup could serve as a prototype for other variable star searches operated by amateur astronomers. Many amateur astronomers already have all of the hardware needed to set up a system similar to that described here; the hope was that if the system proved effective, it could lead other amateurs to put their equipment to use searching for variable stars. The target fields were selected so as to contain an open cluster. An open cluster is a loose grouping of stars that is held together by their mutual gravity. There are several good reasons to search for variable stars in open clusters. First, there will generally be more stars in an image of an open cluster than in a random star field. Second, and more importantly, the distance and age of open clusters can be calculated from their color-magnitude diagram. This information is very interesting because astronomers normally have no way of knowing the ages of, or the distances to, most stars. These extra pieces of information make the discovery of a variable star in an open cluster far more exciting than the finding of a randomly placed star. For example, if one knows a star's distance from Earth, the absolute magnitude, or true brightness, of a star may be calculated. This gives the star's exact placement on the Hertzsprung-Russel diagram. Furthermore, if one knows a star's age, it is possible to tell what phase of its life cycle a star is in, which is yet another piece of information explaining why the star varies the way it does. The clusters to be observed were selected based on three criteria. First, they had to be well placed for viewing during the fall. Second, only clusters in which there had never been any searches for variable stars were considered. And, finally, the clusters needed to be located so that there was a bright star in the cluster that could be used to guide the telescope during the exposures. Much care was taken in this process to ensure that this research would not duplicate any previous studies. |
 
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