Select Cluster

The Star Cluster Page

star gauge.

Star Cluster:
Star Cluster

The Instruction / Help Page

Basic Operation

  • Tap the "Select Cluster" button in the upper left to display the control panel.
  • Select a cluster from the available list of star cluster images.
  • An image of the selected cluster will appear on the screen, along with a template to match the color and size of star images.
  • Select a star and slide the cluster image to move the star adjacent to the template, and record the closest matching size (brightness) and color (temperature). Work systematically across your star cluster to record at least 25 stars near the center of the cluster.
  • To get the best match of temperature, try to match the color in the outer fringe of the star image.
  • Record the brightness and temperature in the appropriate box as a dot on a Hertzsprung-Russell chart.
  • Compare your chart to the ones shown on the Hertzsprung-Russell Diagram help page to determine its age.

About Star Clusters

A star cluster is a group of stars that formed together and are held together in a group by their mutual gravity. Stars in clusters are the same age and have the same chemical composition, but form with a range of mass. Typical star clusters form with stars ranging in mass from several to tens of times the mass of our sun down to masses of just a tenth or so of our sun's mass.

Star clusters in our Milky Way galaxy range in age from just a few million years to billions of years. Because more massive stars evolve and die faster than less massive stars, we can determine the age of a star cluster from the most massive stars still remaining in the cluster. Young clusters contain both massive stars and low mass stars, while old clusters contain only low mass stars.

Brightness-Temperature Diagrams

Astronomers graph the brightness of stars in a cluster versus the temperatures of the stars. Astronomers call such a plot a Hertzsprung-Russell diagram. We can determine the brightness of a star from its apparent size in an image. Brighter stars make bigger blobs in an image (even though we can’t actually resolve the actual size of the star). Fainter stars make smaller blobs. Hot stars (T > 10,000 K) show a blue tint, very warm stars (T ~ 7500 K) look white, warm stars (T ~ 5000 K) look yellow, and cool stars (T > 4000 K) look orange or red.

In a Hertzsprung Russell diagram, young star clusters will contain a sequence of hot, bright, blue stars to cool, dim, red stars, while older star clusters will contain only the cooler, dimmer portion of the sequence.

Here you see examples of Hertzsprung-Russell diagrams for young, intermediate age, and old clusters.

Hertzsprung-Russell diagrams

Click here to obtain a printable copy of a Brightness-Temperature chart to estimate the age of your star cluster.

The Images

The images used in this learning activity have been obtained from the National Optical Astronomy Observatory (link to Credits for individual images are shown below.