Estimate the distance to a cluster of stars

Michael Richmond
Jan 9, 2009

Your goal for today is to measure properties of stars in a pair of images of a stellar cluster. Using your meausurements, plus a few pieces of information I will provide, you must estimate the distance to the cluster.

Part I: measure instrumental magnitudes of stars in images

1. Log into computers in the CIS lab.
2. Start the "Terminal" program
3. Type the following:

               ssh -Y -l student spiff.rit.edu
   

   When asked for a password, supply the value I will tell you.
You should now be connected to a computer in my office, which has some images of a cluster of stars.
4. Type the following commands:

               propinit
   
               cd $dd
   
               ls
   

You should now be in the directory with the images you'll need. The two images you'll use are

           cluster_a_b.fit             a B-band (blue) image of the cluster
           cluster_a_v.fit             a V-band (green) image of the cluster

You can display one of the image by using the tv command, like so:

           tv cluster_a_b.fit  

A new window should pop up like this:

If you move your cursor into the image, you should see the coordinates (bottom of the window) change as you move the cursor. If the coordinates DON'T change, then click on the window's bar at the top. If they STILL don't change, left-click once while pointing anywhere in the image.

There are three keys you can use to measure stellar properties.

• a press this when you are pointing at a star and you'll see a line of information appear in your terminal window, like this:

   ( 499.69  563.62) flux   9108.6 npix  49.5 mag 15.101  sky   32.0  [ 4.0  8 15]
     

  The number after "mag" will give you the instrumental magnitude of the star in this image.
• r press this when you are pointing at a star and you should see a radial profile, like this:

  

• z press this, and you will create a new, smaller window which is zoomed-in at the location of the cursor.

To destroy an image window, you can right-click while the cursor is inside it, or use the Mac red-color-button at the top left of the window.

Convert instrumental magnitudes to standard apparent magnitudes

The star marked "A" in the picture below has apparent magnitudes

             m(B)  =   11.25

             m(V)  =   10.39

             m(R)  =    9.85

If you find the difference

              delta  =  (instrumental mag) - (apparent mag)

Make a color-magnitude diagram of the cluster

You should measure the magnitudes of 25-100 stars in the image. Perhaps you could cooperate with other students and combine your results ...

The goal is to make a color-magnitude diagram, in which you put

• (B-V) on the horizontal axis
• m(V) on the vertical axis

It might look something like this:

Once you have a good color-magnitude diagram for the cluster, you can use the location of some feature to help find the distance.

Ask the instructors for details.