A good way to study the properties of many galaxies is by looking at a galaxy cluster. There are many galaxy clusters in the SDSS data, which may contain hundreds or even thousands of galaxies.
The picture at the right shows a famous cluster called Abell 2255. The cluster is named after George Abell, an American astronomer who published a catalog of galaxy clusters in 1958. In this project, you will study the galaxies that make up Abell 2255.
Think about how you know which galaxies are part of Abell 2255, and which are just other galaxies at different distances in the same part of the sky. Click on 15-20 galaxies that you think are part of the cluster, both spirals and ellipticals, and save them in your notebook. You can save the entire notebook to your computer by choosing CSV as the output type and clicking “Export.” You can then open the resulting CSV file in Excel.
How are these galaxies similar? How are they different?
If you have completed the Color project, you are now ready to use Abell 2255 to learn about the nature of galaxies.
Using Galaxy Clusters in Astronomy
In Exercise 4, you looked at only a few galaxies. To draw convincing conclusions about galaxies, you need to examine hundreds or thousands of galaxies – far more than you could look up individually. Therefore, in the next exercise, you will use a search tool to automatically look up information on all the galaxies in Abell 2255.
You may use one of two tools: SkyServer’s Radial Search or SQL Search. The Radial Search tool will return all objects near Abell 2255; you can then use Excel to select only galaxies.
Your teacher may tell you which one to use, or you could pick one yourself. Click one of the links below to learn how to use the tool of your choice. A new page will open in this window. When you are done, you will return to this page to make and analyze your color-color diagram.
Use the Radial Search tool and Excel
Use the SQL Search tool below
Note: Irregular galaxies are difficult to classify by colors and may be scattered on your diagram. But only 3% of observed galaxies are irregular, so this should not be a problem.
SDSS astronomers recently analyzed over 147,000 galaxies and created a diagram similar to the one you made in Exercise 5. If you are interested in some challenging reading, you can download the paper they published here (to see the paper as a .pdf file, click Other Formats, then Download PDF).