Open Clusters Start Page

Figure 1: The open cluster M25.
Credit: Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum Astronomia), Hawaiian Starlight


  • Do you ever wonder about stars?
  • When you look up at the night sky or at the image of M25 above, do you think that they all appear the same?
  • Does it make sense that we might understand more about the properties of stars if we studied groups of them?
  • What do you know about how stars form and what gets left behind once they've formed?
  • How long do you think stars live for and what factors determine this? What properties can be determined and how could you go about this?
  • How can stars be classified according to these properties?
  • What might studying a group of stars (such as in Figure 1) tell you about characteristics of larger populations of stars?

Star clusters provide fantastic (in fact, out of this world) laboratories for us to study populations of stars. This allows us to understand more about the way that stars evolve and what the factors are that influence this evolution. One very effective way that astronomers can do this is in a Colour Magnitude Diagram (or CMD).

Figure 1 is an image of a well known open cluster called M25 (sometimes Messier 25). To extract numerical (i.e. quantitative) data from such a cluster, astronomers use a scientific technique which allows us to measure the brightness of individual stars, known as photometry. An example of free, easy-to-use Windows software for photometry is Makali'i, which is the Hawaiian for the Pleiades, itself a well-known open cluster.

Read an overview of this activity and instructions.

Go straight to the first page to learn more about stars.