In this activity you will use telescope images of two different clusters, an open cluster and a globular cluster. For each cluster there is an image taken through a red filter and an image taken through a blue filter. You can determine the redness or blueness of an individual star by comparing its brightness through one filter to its brightness through the other filter.

By examining the proportion of red and blue stars in a star cluster we can infer something about its age. Compare the color of the stars in the two clusters provided here and hypothesize which cluster is older. Recall the conclusions you made about stars in the activity Changes over Billions of Years: Observing Star Clusters.

Instructions for comparing brightness of stars in each color:

The images are located in the directory your teacher has set up for this unit.

• In HOU-IP, Open the image ngc6939r. This is the red image of the open cluster, NGC6939.
• Open the image ngc6939b. This is the blue image of NGC6939.
• Put the image windows side by side so you can see the same stars in both images.
• Use the cursor to find the (x,y) coordinates of a star that is easily visible in both images.
• Select Auto-Aperture and click the circle on the star. Repeat this to get the brightness of the star in each image. Remember, the Counts for that star appear as the red text on the image and in the Results Box. Notice that the (x,y) coordinates are also provided in the Results Box.
• Repeat this procedure for many stars within each image

1. Record your data in a table such as the one below.

2. Are most of the stars in the cluster blue or red?

________________________________

• Repeat this activity using the red and blue images of the globular cluster, M2, and compare your answers.