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Astr212 Project: Light curve of variable star SW AND

SW And Magnitude Difference Animation Contents: The star GCVS SW AND is an RR Lyrae variable in the Andromeda constellation. RR Lyrae variable stars are known for having a short period of variability of less than a day in length. Our star, GCVS SW AND, has a period of 10 and 3/4 hours, or .442 days. The goal of our project was to observe this star over a period of one evening to see the change in brightness throughout the night.

What is an RR Lyrae star? An RR Lyrae star is a variable brightness star. The brightness of these gas giants change as the outer layers of the star expands and contracts. RR Lyrae stars are usually found in globular clusters, which is a cluster of stars from the same source that is gravitationally bound to each other. Each star will have a very distinctive light curve shape. All RR Lyrae stars have a same average absolute magnitude of 0.6 . Using the absolute magnitude of 0.6, and measuring the brightness, the distance to the star (and therefore the distance to the globular cluster) can be calculated.

The movie of our star, GCVS SW AND, shows the visible amount of change over the course of the nine hours that we observed on October 11th, 2002. The variable star is located in the lower right corner of the image. The other stars visible in the image are stars with a fixed brightness, that we could use to determine the change of brightness in our variable star. The time difference is approximately our entire night of observations. The star can be seen to change from being brightest in early evening to the faintest in early morning. ( The five frames in the animation show the largest visible changes. ) Our project successfully measured the magnitude of the star through the course of the night (and several hundred pictures!), and our results fit the known light curve for GCVS SW AND. Using our data where were able to calculate the distance to SW AND using the average magnitude to 635.5 parsecs away. This is very close to the top of the line data available from the Hipparcos satellite, which gives a distance of 667 pc. Unfortunately, we were unable to observe the jump from the star's faintest to brightest point, but instead we observed the full decrease. More information on our observations, processes, and for our final plot of the light curve are below.

SW AND was catalogued by the GCVS survey conducted in 1982. Information about the light curve of this star can be found as far back as 1932. Information about the GCVS survey and catalogue can be found at http://www.sai.msu.su/groups/cluster/gcvs/gcvs/.

External Internet Resources:
    Light Curve from GEOS RR Lyr database
    Star Chart
    General Catalogue of Variable Stars

Observations and Processing:

Observations: Our observations are based from 17 sets of 15 images each for a total of 255 images. Each exposure was 5 seconds in length. Images were taken from 9:15pm on October 11, 2002 to 5:30am on October 12, 2002. The first two sets were 45 minutes apart, each subsequent set was 30 minutes apart. Darks, which were also 5 second exposures, were subtracted from each image. No flats were necessary.

Processing: The images were aligned and combined to bring out the faint reference stars, leaving 17 final images. Each set had a few bad images thrown out before being combined. After getting these 17 final images, they were compared to a star catalogue using CCDSoft 5 in combination with TheSky 5 software to identify each star and find their brightness's. After identifying each astronomical object, we generated a light curve using two reference stars, C and K, of known brightness. The light curve data was read into Microsoft Excel so that a final light curve plot could be produced, which showed how GCVS SW AND varied over the course of the night. The apparent magnitude of our variable star (C-V), and the magnitude of the reference stars (Norm C-K) were plotted against the elapsed time in hours. The standard deviation of the reference stars were used to produce error bars for the light curve of our star. This gave us our final light curve displayed below.

Orientation and scale: North is up and a touch to the left. East is left, and a touch down. The image is 9.6x6.2 arcminutes in size.

The pictures were taken on October 11, 2002 from the Calvin College Observatory.


Name Class Major
Shannon Fogwell 2005 Physics Major
Chris Wieringa 2003 Computer Science Major
Tony Karsten 2003 Computer Science Major