Physical Science 206L

Dark Sky Star Counting Assignment

You are very fortunate to be able to enjoy the exceptionally dark skies that we have here in the Four Corners.  The area just outside of Durango has some of the darkest skies in the country.  One measure of how dark the skies in a certain area actually are, is called the "limiting magnitude."  The limiting magnitude is simply the magnitude of the dimmest star that you can observe with your naked eye.  

In this lab exercise, you will be determining your limiting magnitude in at least two locations.  One of your locations must be very dark - away from any significant light sources, such as buildings or street lights.  You cannot make this "dark" observation when there is any moon at all in the sky.  Your second observation should be in an area that has some light pollution.  Or another option is to make this "light" count when the moon is out.  The goal is to compare the limiting magnitude in those two locations, and to estimate the total number of stars that can be seen in those locations (or conditions).  

The procedure for determining your limiting magnitude is to do a "star count."   That is, count the total number of stars visible in a well-defined small region of the sky.  A link to these regions is http://nineplanets.org/lm/rjm.html.  You should chose regions that will be near your zenith point during the time of your observations.  An example region found in the constellation Gemini is shown below.  Below the figure is the table for this region (and only this region).  You must look up the table for other regions on-line.  If you count 10 stars in this example region (including the corners) then 5.7 is your limiting magnitude.  To get the best limiting magnitude measurement, you should chose three different regions that will be nearest your zenith and average the limiting magnitude.  For each region, each member of your group should count stars.  It sometimes helps for group members to point out the dim stars to each other.  With practice and repetition, you will be able to observe dimmer stars.

Once you have determined your limiting magnitude, you can estimate the total number of stars you can see.  The most convenient link to find the number of stars for a given magnitude is found here:   http://www.stargazing.net/david/constel/howmanystars.html  It may be necessary to interpolate between values on this table, since your limiting magnitude is likely not a whole integer number.  An interpolation is just a way to get values between two points on a graph.  The "cumulative stars" column of  numbers in the table represents how many stars there are down to the lower magnitude listed in the "magnitude range" column of the table.  For example, there are 2822 total number of stars down to a magnitude of 5.49 (You can round this to magnitude 5.5).  The value 5.7 is between 5.5 and 6.5.  That means there will be more than 2822 stars visible.  To find out how many more, you can interpolate.  There are ways to get accurate interpolations on graphs that are increasing exponentially as this one is.  However, you don't need to do that for this lab.  A simple linear interpolation will suffice.  For this example, note that 5.7 is 20% of the way between 5.5 and 6.5, which are both values listed on the table.  (There are 8769 total stars down to magnitude 6.5).  At the table shows, there are 5946 stars between magnitude 5.5 and 6.5.  20% of 5946 is 1189.  So magnitude 5.7 should have 1189 more stars visible than magnitude 5.5.  The total number of stars is then estimated to be 2282+1189 = 3471.  But the values in the table are for the total number of stars in the entire sky - this includes both hemispheres!  So you need to divide the number by 2 to find how many you can see from Durango.   So in this example, where your limiting magnitude is 5.7, there would be 3471 / 2 = 1735.5, or 1736 stars visible.

Here are some helpful hints:

1. Decide on your counting regions before you go outside.  Use your star finder to determine what region will be overhead when you will be doing your count.
2. Dress warmly! If you are not dressed appropriately, you will probably not want to stay outside and finish the observations.

3. Work with a partner. Besides being safer, you can compare counts in several regions.  For each region, each member of your group should count stars.  It sometimes helps for group members to point out the dim stars to each other.

4. Wait for your eyes to get adapted to the dark.  This will take 15-30 minutes depending on the person, but everyone should wait at least 15 minutes before doing any "official" counting.  After your eyes are dark adapted, you should avoid all white light.  Use only a dim red flashlight.  For better results, you should chose three regions that will be nearest your zenith and average the limiting magnitude.