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Galaxies
 Teacher's Guide
     - Goals
     - Background
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Galaxies

This project teaches students about the wide variety of galaxies in the universe. Students learn how to classify galaxies using the scheme developed by Edwin Hubble. They will also use a color-color diagram to explore the properties of galaxies. Students will look at the spectra of a wide variety of galaxies to see how spectral differences show up in the visual images of galaxies.

This project should be done after completing the Color and Spectral Types projects. If your students have not completed these projects, they can still learn about the types of galaxies by completing the first three pages, up to "Characteristics of Galaxies."

For more information on types of galaxies and on how astronomers study galaxies, 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, and irregular galaxies
  • Know that galaxies come in clusters, some of which contains thousands of galaxies
  • Create color-color diagrams for galaxy clusters
  • Classify elliptical and spiral galaxies according to their distinct colors
  • Identify some of the most common spectral lines in galaxies
  • Use galactic spectral lines to deduce the most common star types present in the galaxies

Background Knowledge

The Galaxies project calls on some of the knowledge that students should have from other projects; the Colors and Spectral Types projects are prerequisites for finishing the Galaxies project.

Students will need to understand how astronomers measure color. They will need to know how to make color-color Diagrams, and how to interpret the results.

Students will also need to have a basic understanding of spectra. The spectra of galaxies are really just the combination of spectra from many stars, so students should recognize many of the same spectral lines.

Students will need to know how to use some computer graphing program such as Microsoft Excel. They will be walked through a simple query of the data using a search tool, so basic computer skills are important.

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 classificatin scheme, so set a firm time limit (maybe 30 minutes) on this section.

The next section introduces the Hubble Tuning Fork. The students then classify the same galaxies according to Hubble's scheme. It should take about the same amount of time as the first section.

Making a color-color diagram for the galaxy cluster Abell 2255 should be relatively straightforward. Students have a choice of two search tools, both of which will return results in one to three minutes. Most students will have a color-color diagram in about 30 minutes.

Sketching the separator line should be easy for students who have completed Algebra I. The line will separate the galaxies into ellipticals and spirals very quickly.

Looking at the spectra will probably take about an hour. The students should try to correlate the spectra they see to they galaxies they are looking at.

You may wish to use the GalCrash Java applet to teach students more about galaxy interactions. This section should take about one hour. If you are short on time, you may learn the program yourself and simply demonstrate it to your students - or, you may skip the program section altogether.

The research challenge should not be done during class. Interested students should do the exercise on their own; it has lots of subtleties and can take several hours. Students will need to learn how to look up data on a galaxy cluster and eliminate galaxies not in the cluster. They will also need to consider other issues, such as how many galaxies of each type the cluster contains, to get any meaningful results.

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 answers to all questions and exercises, click here. To see the answers, you must have the Adobe Acrobat PDF viewer installed on your computer. Acrobat is available for free download at Adobe's web site.

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.