Galaxies Teacher Notes

Galaxy M51
Image courtesy the Hubble Heritage Project

This project teaches students about the wide variety of galaxies in the universe. Students develop a classification scheme for galaxies, learn about the different types of galaxies, then learn to classify galaxies using a scheme developed by Edwin Hubble. Interested students can then make color-color diagrams to explore the properties of galaxies.

This project should be done after completing the Color project. If your students have not completed this project, they can still learn about the types of galaxies by completing the first seven pages, up to “The Hubble Tuning Fork.”

For more information on the types of galaxies and how astronomers study them, read the About Astronomy: Galaxies section of SkyServer, or look at one of these suggested references:

Galaxies, by Timothy Ferris

The Color Atlas of Galaxies, by James Wray

Project Goals

By the end of the project, students should be able to:

  • Describe the Hubble Tuning Fork and use it to classify different types of galaxies
  • Explain the characteristics of elliptical, spiral, lenticular, and irregular galaxies
  • Know that galaxies come in clusters, some of which contains thousands of galaxies
  • Identify interacting galaxies in a cluster
  • Classify elliptical and spiral galaxies according to their distinct colors
  • Create color-color diagrams for galaxy clusters, and use them to learn about galaxy characteristics

Background Knowledge

The Galaxies project calls on some of the knowledge that students should have from other projects; the Colors of Stars project is a prerequisite for finishing the Galaxies project.

Students will need to understand how astronomers measure color. To do the Galaxies project’s research challenges, they will also need to know how to make color-color Diagrams, and how to interpret the results.

Students should need to know how to use a computer graphing program such as Microsoft Excel. Alternately, students can plot data points by hand, but will find it quicker and easier with a graphing program.

Project Structure

The first section of the project lets students look at a wide variety of galaxies to try to come up with their own classification scheme. They are repeating the analysis that Edwin Hubble went through in the 1920s when he came up with his tuning fork diagram. It is easy to get bogged down in this section if two students in a group sharply disagree over their classification scheme, so set a firm time limit (maybe 30 minutes) on this section.

The next four pages describe the four basic galaxy types: spiral, elliptical, lenticular and irregular. Let the students read these pages to get familiar with the different types of galaxies. Although there are specific mathematical definitions to determine the ellipticity of the galaxy, they are beyond the scope of this lesson. Similarly, students will estimate the difference between Sa, Sb, and Sc galaxies. You might also wish to point out that the bar in a barred spiral can sometimes be subtle.

The next section introduces the Hubble Tuning Fork. The students look at the same galaxies they classified and classify them with Hubble’s scheme. This section should take about the same amount of time as the first section.

The Galaxy Clusters briefly discusses the fact that galaxies are not evenly distrubuted throughout the universe, but tend to appear together in clusters. Clusters are held together by the mutual gravity between the galaxies. Students will then look at a galaxy cluster, Abell 0957. Students will look at the galaxies in the cluster to determine what common properties they might have.

Galaxy collisions have been observed many times, including several times in the SDSS database. There is an optional section where students can use a Java applet to simulate galaxy collisions. You may choose to do this activity if you have time, or interested students may look at the applet on their own.

The color classification section introduces students to the concept of classifying galaxies by their colors. Since different types of galaxies have different types of stars, you can deduce some of the general properties of galaxies by their colors. Although this technique is not perfect, it is the only way to analyze a database of tens of millions of galaxies.

Exercises 5 and 6 are the Research Challenges for this project. They should not be completed in class. They are self-directeed, open-ended explorations where students look at galaxies in the SDSS database on their own. Interested students can complete one or both Research Challenges.

Questions and Exercises

Questions are designed to get students thinking about the way scientists work. Exercises are designed to get students to explore using SkyServer data to solve problems. For sample solutions to all questions and exercises, email us at
Students should be evaluated based on their written answers to the questions and exercises. You may use our sample scoring rubric or develop your own. If you use our scoring rubric, print out a copy for each student and attach it when you return his or her work.

For specific information on any part of the project, click Next.