Investigating Astronomy logo
Home  |  About IA  |  For Families  |  For Teachers
Doing Science

Unit Five: Investigating Stars
Doing Science

2. Open Star Clusters: Distances and Ages

Open star clusters are groups of stars containing a few hundred to a few thousand stars. Evidence indicates that all the stars in any given cluster were all born at about the same time from the same interstellar cloud of gas and dust. Consequently, they are all about the same age and composition, properties that make them ideal for studying stellar evolution. In addition, as members of a single cluster, they are all about the same distance from us, so their relative properties like brightness and diagnostic tools like their HR diagram can be measured fairly easily using CCD images and public domain software. Astronomers at the National Optical Astronomy Observatory (NOAO) are conducting a project studying open clusters that you can join.

Go to the NOAO open cluster web site at:, and scroll down the page to the introduction to the project. Click on Open Clusters background. There you will find links to the Sloan Digital Sky Survey HR Diagram Project, and descriptive materials, image data, and directions for data reduction for the NOAO project. Do a complete analysis of at least one open cluster, including measurement of magnitudes of blue and visual images, creation of a data table for the stars in the cluster, plotting of the HR diagram, and calculating the distance to the cluster.

Your Report:

  1. Prepare a poster to resent your observations and conclusions. Include your images, data tables, HR diagram, and calculations.
  2. Obtain access to a telescope with a CCD camera and color filters and take your own images of an open cluster, and perform the same analysis as you did for the cluster in #1. A list of open clusters is found at: Additional descriptive information about open clusters is given at the top of the page. Links to individual clusters with information including coordinates (RA and Dec) and images aiding identification are found near the bottom of the page.