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SS Cygni

SS Cygni is a cataclysmic variable star.

RA 21 42 42.66, Dec +43 35 9.48

SS Cygni is in the constellation Cygnus the Swan. It is a variable star. You can investigate this star, calculate its changing magnitude and chart its cycle.  You will use Hands-On Universe Image Processing (HOU-IP) software to analyze  images taken with three telescopes at Yerkes Observatory.  As you add more data you will begin to see the nature of this popular variable star. Read about SS Cygni.  See the American Association for Variable Star Observers (AAVSO), Star of the Month: June, 2000. http://www.aavso.org/vstar/vsotm/0600.stm

These steps are explained in the following activity.

A.  Open HOU-IP.  .  Open the .fts images of SS Cyg.  Open the images in reverse order, starting with the last image first. If you open with the last image first in reverse order, when you tile, the images will be in chronological order.  Change the palette to IGREY to see the stars and data clearly.  Images are dated 02 for 2002, then the month, day, and Universal Time; y24 for Yerkes 24 inch telescope, the filter and the exposure time in seconds.

When all six images are open, go to Window, Tile. Then adjust the slider bars so you can see the SS Cyg star and the comparison star on every image.  Since these images were taken on different nights with different observing conditions, times, etc., they display with different ranges of brightness.  Adjust the tab for Max to make the comparison stars all look about the same.

Question A.1.  What do you notice about the SS Cyg star?  ____________


B.  Here is the finder chart from AAVSO.  This is a portion of Chart (e). (www.aavso.org). This chart has South up and West to the left.  To use this chart with the HOU images, turn the chart upside down so North is up and East is to the left. We will use the comparison star 109. Magnitudes of the comparison stars do show the decimal.  However, 109 really means 10.9. If you click on the chart it will pop up on a separate page which you can print. 

sscygfinder80.jpg (22462 bytes)

Question B.1.  Find the two comparison stars on this chart that are also on the HOU images of SS Cyg.  How are they labeled on this chart?

(Hint: Remember to imagine or turn this chart upside down to compare it to the HOU images.)

Question B.2. What are the V Magnitudes of the comparison stars?

(We will use the dimmer of these two stars, the one in the middle of the curve of three stars.)

C.  In HOU-IP, use Data Tools, Aperture to find the Counts of SS Cyg and the Comparison Star.  You choose Data Tools Aperture, set the Radius and Sky Radius, then click on SS Cyg.  Repeat for the comparison star.  Suggestion: Set the Radius to 15 and the Sky Radius to 30.  This way the star is always enclosed in the circle made by the aperture tool around the star.  Do this for all the six images of SS Cyg. 


You will see your results on the images and also recorded in the Results Box. This data can be copied to another application for printing or for analysis, such as in Excel.

D.  Calculate the magnitude of SS Cyg by comparing it to other stars of known magnitudes.  Refer to the HOU HS Curriculum Module: Measuring Brightness, SA 14.  Magnitudes are a logarithmic scale. (Our eyes perceive brightness changes logarithmically too.)  The astronomical formula is

m1 - m2 = 2.5 log(B2/B1). 

M is magnitude; m1 is the unknown magnitude of star, SS Cyg in this case and m2 is the magnitude of the comparison star, 10.9, for the star we will use in our images.

Counts are a proportional measure of the star's brightness as recorded by the CCD camera.  We use the aperture tool in HOU-IP to get C, counts, which we use for the B or brightness measure. C is counts as measured with Data Tools, Aperture. C1 is for the counts of SS Cyg and C2 is for the comparison star counts. 

m1 - m2 = 2.5 log(C2/C1)
m1 = 2.5 log(C2/C1) + m2

To solve this formula with calculators on your computer follow the following procedure.

*PC Note: When using the calculator in Windows PC computers, enter the values and push the calculator buttons in this order.

known star counts/unknown star counts = log * 2.5 = + known magnitude =

  1. Find the calculator.  Start; Programs; Accessories; Calculator.
  2. Make sure you are using the scientific view.  View:   Scientific
  3. To find the magnitude of the target star such as SS Cyg, follow the formula by entering data and operations in this order.  In this case the known star counts and magnitudes refer to the comparison star.  The unknown star counts refer to SS Cyg.  We are using the comparison star that has a known magnitude of 10.9.

*MAC Note: When using a graphing calculator as often found on MACs, follow this procedure. Go to the apple in the upper left corner of the computer screen.   Choose Graphing Calculator.

2.5 log   comparison star counts / ss cyg star counts  + known magnitude =

E.  Record and Graph SS Cyg's changing magnitudes.

Now working with HOU image processing software and your downloaded .fts files for SS Cyg, calculate the magnitude of SS Cyg on several dates.  Create a graph showing how SS Cyg changes magnitude.  What is the brightest magnitude you calculated?  The dimmest magnitude?  Can you create a spreadsheet in Excel that will calculate magnitudes and graph your results?

Reference: Spreadsheet in Excel - Sample Format

Download SS Cyg Images  Yerkes 24 inch 2002

F.  Now add more data points with images from other telescopes.  You can even add the analysis to the spreadsheet you worked on for the images from the 24 inch telescope.  Just be sure to note which telescope and image corresponds to which data set. Suggestions:  For the ten inch telescope images, use Aperture settings of Radius 10, Sky Radius 20.  For the eight inch telescope, use aperture settings Radius 7, Sky Radius 14.

More SS Cyg Images: Yerkes Rooftop 10 inch 2002; or Yerkes 8 inch 2002

Other pages.

Images of SS Cyg with the Yerkes 24 inch telescope have a five arc minute field of view.  Here are some samples of images and comparison star data from the Yerkes 24 inch telescope. These images were taken with an Apogee AP7p CCD.

Yerkes Rooftop 10 inch Telescope
Comparison Stars and Calculating Magnitude Reference for an image of SS Cyg taken with the Yerkes Rooftop North 10 inch telescope.  Here is another page with hints for analyzing SS Cyg and its comparison stars on the HOU-ELM 10 inch Yerkes rooftop telescope with an ST 9 CCD, about a quarter degree field of view. 

Yerkes Rooftop 8 inch Telescope
Here is SS Cyg and comparison stars with the HOU 8 inch Yerkes rooftop telescope with an ST 7 CCD, about a half degree field of view.

Link to Previous Version of this Page about SS Cyg.

* HOU™ Explorations *
06/26/2003 08:24