A Cepheid Variable Laboratory Exercise


You will examine a series of photographic negatives which are used by observers to estimate the visual magnitude of a variable star. After examining the 40 photographic negatives and estimating a visual magnitude, you will plot your estimates in Excel and determine a periodicity. Once you have determined a periodicity of the variable star, you will utilize a simple formula for estimating the absolute magnitude and calculate a distance for the star you have been observing.

PART I of lab

Here are the steps you will be following:
  1. Examine this sample image
  2. Note the actual visual magnitudes of the neighboring stars
  3. Now you will examine 40 sample images of the variable star and utilize the neighboring stars to estimate the apparent magnitude of the variable star on different dates
  4. You will need to make an Excel spreadsheet to log your data and then plot the data to determine a period for the variable star
  5. Examine first image and record data
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  45. Now that you have acquired all your data from the images, you will be making your plot of the periodicity of the variable star. Remember to have the time axis (in day number) as the x-axis and the visual magnitude as the y-axis.
  46. Once you have a plot of the data, print it out and mark on the plot the period of the variable star. It's best to use more than one period.
  47. With the period of the variable determined from your plot, the next step is to calculate the absolute magnitude of the variable star using the formula for Cepheid Variables: absolute magnitude = -0.60081 - 3.51605*log(period)
  48. Now that you have an absolute magnitude for the variable star, calculate the distance to the variable star. Do you remember how? All you need is the absolute and apparent magnitudes. [Hint: Use an average of the maximum and minimum magnitudes you obtained for the visual magnitude.]

PART II of lab

Here are the steps for this part of the lab:
  1. You will be constructing a Period-Luminosity relation curve utilizing data from the David Dunlap Observatory Database of Galactic Classical Cepheids.
  2. Read some of the introduction and description of individual files and then proceed to the "Physical Data" table link.
  3. At the top of the page it says "Cepheids - Physical Data"
  4. The table that comes up is a very big list so it may take a moment to load. When it is finished, continue.
  5. Under the File option of your browser, choose the Save As.. option to save this information.
  6. Put in your Name and save the data as a Text File (*.txt) as yourname.txt
  7. Open the Excel Program from the Start Menu.
  8. Open the file in Excel, and it will ask you what to do with it.
  9. When you try to open this file a Text Import Wizard (1 of 3) [this may vary slightly depending upon the version of Excel you are using] box will come up. It will ask if you want it imported as Delimited or Fixed Width; you need to check the Fixed Width box and hit Next. The Text Import Wizard 2 of 3 page comes up to let you preview the data. Just click Next again. On page 3 of 3 which follows, click Finish. This will put the data into the columns of the Excel spreadsheet. Now the data is within the Excel spreadsheet, however we will cut out most of the extraneous information.
  10. The columns that you want are titled PERIOD and MV and represent the Cepheid's pulsation period and mean visual magnitude, respectively. You need to clear out all other data. Highlight the columns to be deleted, and under the Edit option choose Delete. Do the same for all remaining columns except the PERIOD and MV columns, so that the only two columns left in your spreadsheet are the PERIOD and MV columns.
  11. Now create a colum of absolute magnitudes from the mean visual magnitude (MV) and using the formula for Cepheid Variables which was used in Part I, that is, absolute magnitude = -0.60081 - 3.51605*log(period)
  12. Now we want to plot the absolute magnitude versus the periodicity.
  13. To get a linear view of this relationship, we will need to have a logarithmic scale for the periodicity. You should be able to do this, or get assistance from your instructor.
  14. Remember to turn in your plot properly annotated.
  15. For your conclusion answer the questions that your instructor asks.