Clusters

In this exercise we will see how red and blue stars in clusters can easily be detected by observing the cluster through a red filter and through a blue filter.

Exercise:

Blue Image of Stellar Cluster -- represents an image of a star cluster taken with a blue filter (only blue light hits the detector).

Red Image of Stellar Cluster -- represents an image of a star cluster taken with a red filter (only red light hits the detector).

  1. Click on the above two links to open two separate windows. This is the same CCD simulation applet that we used back in the early part of the term.

  2. For each image, take a 100 second exposure and use 20x20 box apertures.

  3. Compare the two images. The numbers under each star are the same.

    However, a star that is very red, would appear much brighter (deposit more energy on the detector) on the red image than on the blue image. Typically such stars would have surface temperatures of 4000K or less.

    Similarly, a very blue star will appear to be much brighter on the blue image than on the red image. Typically such stars would have surface temperatures of 10,000K or more.

    Stars that are between 4,000 and 10,000 degrees have neutral colors and their brightnesses will appear to be about the same on both the red and blue images.

  4. Identify, by star number, whether the star is red, blue or neutral in color, by comparing the image size/brightness on the respective red and blue filter images. Remember the star brightness = green box mean - red box mean.

    Note, the positions of each star are the same on the blue detector and the red detector. So your comparing image brightnesses on each of the detectors for the same star. Stars whose images appear approximately the same in each filter would not be either very red or very blue.

    In rough terms, the ratio of blue stars to red stars would be an indicator of cluster age. The less blue stars there are, the older the cluster.

    Question:

    You observe a stellar cluster and make an HR diagram of the stars in it. From that you notice that the most massive star that is still on the main sequence is 3 solar masses. From your knowledge of estimating stellar lifetimes, what is the approximate age of these stars in this cluster?