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Best Observing Season: any Level: Intermediate to Advanced Learning Goals: The student will find the current magnitude of intrinsic variable stars, and add this information to the database. Terminology: Cepheid variable, eclipsing binary, intrinsic variable, long-period variable Software: MaxIm, a graphing program Archive Image Directory: varstars/intvar Archive Image List: r_uma.fts, z_uma.fts, ry_dra.fts, ru_peg.fts, ss_aur.fts, Finding charts called starname.gif; historic light curves called starnamegr.gif References: Levy, David. 1989, Observing Variable Stars (University Press, Cambridge); the AAVSO homepage ( http://www.aavso.org ); The Astronomical Almanac

Historic light curve of Chi Cyg, an intrinsic variable in the constellation Cygnus

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Background and Theory

For most for recorded history, stars were thought to be absolutely constant in brightness and eternally unchanging. The first variable star to be discovered was Algol, which was found to vary by over one magnitude every 2.8 days. This discovery was made by John Goodricke 1782, an 18 year old living in York, England. Goodricke was totally deaf, but had a friend who encouraged his interest in astronomy. Two years later, he found the 2^nd and 3^rd variable stars (Beta Lyrae and Eta Aquilae), all by extremely careful and diligent visual observations of many hundreds of stars every clear night. Unfortunately, at the age of 21 John Goodricke died.

Goodricke had correctly deduced that the regular variation of Algol was due to the star being in a binary system, and that the companion occulted the light from Algol once per orbit. Such variables are called eclipsing binaries. A second type of variable star is the intrinsic variable. This type of star changes the amount and type of light that it emits. There are many types of intrinsic variables, but among the most common are long period variables. These stars have left the main sequence and become unstable. They vary by ~4-6 magnitudes (a factor of 40-250 in brightness) over the course of a 100-1000 days. The variability is caused by pulsation in the size and temperature of the star. A typical type light curve of the long period variable c Cygni (period 408^d ) is shown in Figure 1.

 

Cepheid variables are also giants but their periods are in the range of days to weeks. The period of variability is relate to their luminosity which makes them very important as distance indicators. Many variable have periods which are semi-regular, irregular or have no period at all. A famous example is SS Cygni shown above. These types of variables are poorly understood and are the subject of much research by professional astronomers.

Procedure

Observing

1.      Choose six stars from the Astronomical Almanac’s list of Variable Stars (Pulsating Variables).  Make certain that these stars are up at night where you live.  Also check the magnitude of the minimum of these stars to be sure that they are above the brightness threshold of the telescope that you are using.  Consult Appendix A to check brightness versus exposure time for the IRO.

2.      Choose a standard star near each variable star from the listing of UBVRI standard stars in the Astronomical Almanac which is up on the night that you would like to observe.

3.      Observe the variable stars at least once each, using a V filter, and an exposure time appropriate to the minimum magnitude.  Observe the standard stars once each also in the V filter.

Image Analysis

1.      Run Megastar.  Create a finding chart of the field around the variable star that you observed.  Locate the variable star.  Print this finding chart for future reference.  This chart will be easier to use if you change the field size to be the same as the field of view of the telescope (about 21 arcminutes for the IRO), and make the magnitude range equal to the magnitude range that you can observe with the telescope you are using (down to 16 for the ATF). Do this for all of the stars you observed (including the standard stars).

2.      Run MaxIm.  Determine the apparent visual magnitudes of the variable stars, using differential photometry.  To do this:

a)      Examine your finding chart from Megastar and the images of your variable and standard stars.  Locate the standard star on the image of the variable star by comparing the fields. 

b)      Load the image of the variable star.

c)      Select View/Information Window from the menu.  Click on Calibrate.  Make sure your target circle is just large enough to fully enclose the larger of the standard or variable star by adjusting size with the right mouse button.

d)      Click on your standard star and enter its V magnitude in the box provided.  Click OK.

e)      Move your cursor over the variable star.  Record its magnitude.

f)        Repeat the above steps for each of the other variable stars. 

3.      Find the historical lightcurve of the variable star at the AAVSO website.  Download the historical lightcurve data and plot it using a program such as Mathcad, Excel or Graphical Analysis. Add your data at the appropriate Julian Date.  Be careful to use the correct Julian Date, as the JD is often given minus an offset such as:  JD-2,000,000.

4.      Discuss your results with your T.A. Together you can decide whether to report your observations to the American Association of Variable Star Observers (AAVSO). For details, see their web page (http://www.aavso.org ).