Teacher's Guide to Specific Sections

Classifying Galaxies

Let the students try to classify galaxies on their own. Remind them that there may be subtle differences in galaxies. Most students will recognize the difference between elliptical and spiral galaxies, but not everyone will make subdivisions. Ask if there are more subtle distinctions between galaxies that might make a more useful classification system.

When students compare their classification schemes, they will probably find some similarities and some differences. Put a firm time limit on this section so debate does not drag on.

Spiral, Elliptical, Lenticular, and Irregular Galaxies

These pages outline some of the distinguishing characteristics of the galaxies that make up the Hubble tuning fork. Allow students to read through these pages carefully so they can properly classify galaxies later on. You may wish to find other pictures of galaxies for students. You can find many great galaxy pictures on the Internet, from sites such as the Famous Places page of SkyServer or the Hubble Space Telescope Archive.

Astronomers have precise definitions for the subcategories of both spiral and elliptical galaxies. However, these definitions are beyond the scope of this lesson.

Some lenticular galaxies are very difficult to tell from elliptical galaxies. Even the best of astronomers can be fooled from time to time.

Make sure students realize that the terms “early” and “late” galaxies have nothing whatsoever to do with the galaxies’ ages. Hubble used the terms because he thought that galaxies evolved from elleipticals to spirals on the tuning fork. He turned out to be wrong, but the terms have been kept for historical reasons.

The terms are actually quite confusing because elliptical galaxies have used up all their gas and have more old stars than spirals. Another confusing aspect is that S0 and Sa galaxies are frequently grouped with early galaxies. The terms “early” and “late” galaxies do not mark a clean split between spiral and elliptical galaxies.

The Hubble Tuning Fork

The diagram represents how Hubble saw the galaxies. Emphasize to students that he went through the same proecess that they went through in Exercise 1, and the Hubble Tuning Fork was the classification system he developed.

A tuning fork is a device used to find the correct pitch of a musical note. It looks like a fork with two prongs. Hubble put elliptical galaxies on the “handle” of the diagram and spiral and barred sprial galaxies on the two “prongs”. Lenticular galaxies “link” spiral and elliptical galaxies. The tuning fork is a useful visual representation of the galaxy classification scheme, but has no real physical meaning.


Astronomers have known for a long time that galaxies tend to appear in large groups and clusters. Clusters of clusters form superclusters, separated by great voids. One of the primary goals of the SDSS is to explore the large scale structure of the universe, so galaxy clusters are an important aspect of research by SDSS astronomers. You may wish to have students look at some galaxy clusters in the SDSS database so they get a better idea of what they look like. A list of several clusters can be found on the Conclusion page of this project.

Galaxy Collisions

This section lets students see some of the spectacular galaxy collisions the SDSS has captured. Remind students that even when galaxies collide, the stars in them are so far apart that they rarely collide. When a small galaxy collides with a large galaxy, the small galaxy is frequently swallowed. This process is thought to lead to the giant elliptical galaxies we see.

You may want students to learn a little about how galaxies collide using the GalCrash Java applet to model the collisions of galaxies. The link at the bottom of the page leads to a set of supplemental exercises that use GalCrash. You should try the applet out before your class does the project, so you can guide them in using it.

Tell students that GalCrash is a very simple program that models galaxy collisions. Teach them the idea of a scientific simulation – a simple model that incorporates only the most basic features of a real situation, but can produce results similar to those observed in nature. Real galaxies contain billions of stars. A simulation involving billions of stars would not be able to run on a personal computer, so they use a few thousand stars. Many fields of science include models as part of their research.

Other Ways to Classify Galaxies

SDSS astronomers looked at many galaxies in the early data release and learned to classify them based on color. They found that a u-r color of 2.22 separated early galaxies from late galaxies (you can read the entire paper here).

Their method is not perfect because there is contamination from other galaxy types. However, the method can be used to pefrorm statistical analysis of groups and clusters of galaxies where visual classification would be impractical.

In this color classification scheme, S0 and Sa galaxies are considered early galaxies; Sb, Sc and Irregular galaxies are considered late galaxies. Students will probably find some galaxies that do not fit this pattern. Students need to realize that this classification scheme works well statiistically, but there are individual galaxies whose types may not match their expected color.


The research challenges, Exercises 5 and 6, should be completed outside of class. Exercise 5 involves making color-color diagrams for galaxies. Students can use these diagrams look for other patterns of galaxy distribution. Similar types of galaxies should lie in similar areas on all color-color diagrams. By labeling the points with galaxy type (or using Excel to color code different galaxy types), students may come up with other ways of predicting galaxy type based on color.

Exercise 6 allows students to look at galaxy clusters. There are a couple of differences they might notice. One is the evolution of colors vs redshift. Galaxy clusters at different redshifts may lie in slightly different regions of color space. The second difference depends on the shape of the clusters. Spherical clusters (also called regular clusters) tend to have an extremely high percentage of early galaxies. Irregular clusters tend to have a more even mix of early and late galaxies.

To see a list of Standards this project addresses, click Next.