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 the 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 they debate does not drag on.

The Hubble Tuning Fork

Let the students read the section and discuss the different types of galaxies. There are formal mathematical definitions for the differences between E0, E1, etc. involving the ellipticity (major axis/minor axis) of the galaxy, but they are beyond the scope of this lesson. Let the students estimate which type of galaxy they have. If they are off by one, it is not a big deal. Some of the elliptical types are diffiuclt to tell apart.

There are also precise definitions for the different types of spiral galaxies. Again, they are beyond the scope of this lesson. Since there are fewer types of spirals and larger differences between them, students will probably find it easier to classify spirals (although the bar can sometimes be sutble!)

Lenticular galaxies are often mistaken for ellipticals. The disk can be subtle, and the galaxies have no distinctive spiral arms.

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 ellipticals 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.

Galaxy Clusters

If your students have not completed the Colors and Spectral Types projects, show them the picture of Abell 2255 and tell them that galaxies are found in clusters. Tell them that studying clusters gives astronomers an easy way to learn about the properties of galaxies, because clusters give astronomers a large statistical sample of galaxies. It’s like taking a poll of people – the more people you ask, the more certain you can be that the responses the people give are like the true average responses in the population.

If students know how to make color-color diagrams, move on to the “Using Clusters in Astronomy” section. SDSS astronomers looked at many galaxies in the Early Data release. After plotting a color-color diagram, they found they could separate the early and late type galaxies based on their colors. Classifying thousands of galaxies by color is much faster than trying to classify the galaxies visually.

In this section, students recreate this diagram for a galaxy cluster. First, they use the Navigation tool to look up colors of a few galaxies. Have them select about ten galaxies, and keep a record of which galaxy types each point on the diagram corresponds to. Next, they use one of two search tools to find information on all the galaxies in Abell 2255. The Radial Search tool is a little simpler, but students must take the extra step of filtering out stars with Excel (which will teach them valuable technology skills). The SQL Search tool is a little more complicated, but it allows students to do sophisticated astronomy research on SkyServer. Which tool you have students use is up to you. Click here for more information on the Abell catalog of galaxy clusters.

Many of the galaxies students see are background (behind the cluster) or foreground (in front of the cluster) galaxies. In Exercise 3, ask students how they know which galaxies are actually part of the cluster – cluster galaxies have similar sizes, magnitudes, and colors. In Exercise 4, we are concerned about the properties of large numbers of galaxies, so using foreground and background galaxies that come from the MAST tool is all right.

After drawing the separator line, students will see the galaxies separated into ellipticals and spirals. Instead of trying to count the numbers of galaxies, they can use the Sort feature (under the Data menu in Excel) to sort the data according to u-r values. They can then see the 2.2 separator clearly.

Students may also need to change the scale to see the graph clearly. A few outliers can give the graph an inappropriate scale.

Galaxy Spectra

The galaxies in this section are arranged from the bluest galaxies to the reddest. The blue galaxies have lots of hydrogen emission lines characteristic of star-forming regions of gas and dust called HII regions. As the students go down the list, the hydrogen lines will diminish and lots of absorption lines will become visible including hydrogen, sodium, and the H and K calcium lines.

Galaxy Collisions

This section lets students see some of the spectacular galaxy collisions the SDSS has captured. Remember 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. 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. Many fields of science include models as part of their research.

Conclusion

Exercise 8 is a difficult final challenge. There are many different effects that students could see. They may see the galaxies reddened as the redshift increases. Students may also see different ratios of elliptical and spiral galaxies. They need to remember that the separator line may move at different redshifts as the spectra shift!