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Colors of Stars in the SDSS

First, take a look at some of the stars in the SDSS database. The next two Explore exercises will let you examine stars with SkyServer. You will see the colors for yourself, and you will try to discover patterns that could explain why stars come in different colors.

Explore 1. Look through SkyServer's database and find several stars whose colors appear different. Find some blue stars, some red stars, some yellow stars, and some white stars.

To search through the stars, you will use SkyServer's Navigation tool. Click the link below to launch the tool. The tool will open in a new window, which will show a part of the sky. Use the Zoom buttons (the magnifying glasses and blue rectangles) to zoom in our out. Use the NWSE buttons to move around to different parts of the sky.

Click on a star to see its data. The data will appear in a box on the right side of the window - "ra" and "dec" give the star's position, "type" tells you whether it is a star or a galaxy, and u,g,r,i, and z are the star's "magnitudes." You'll learn more about magnitudes later; for now, you'll search for patterns in the magnitudes.

When you find a star you are interested in, save it in your online notebook. To save, click the "Add to Notes" link under the star's data. In addition, record the star's ra and dec on a sheet of paper, and write what color you think it is. After you have selected 10-15 stars, click "Show notes" to see your notebook.

Launch the Navigation Tool

Explore 2. Now, see if you can discover a pattern in the colors. Highlight the table in your online notebook and select Copy from the File menu of your browser. Open a graphing program such as Microsoft Excel and select Paste from the appropriate menu (the Edit menu in Excel) to paste the table into a spreadsheet. The directions below explain the steps you will take to analyze your data with Microsoft Excel; to use other graphing programs, you would follow similar steps.

Look at the notes you made on the colors you saw for each star. Click on the first row of data, then use the tab or right arrow key to move the highlighted box to the column to the right of your data. Type the color of the first star and hit enter. Repeat this process to type in the colors of all the stars you saw.

Now, click on one of the cells in your spreadsheet and select Sort from the Data menu. Sort by u in ascending order. Do you see the colors group into any patterns? Repeat the sort by g, r, i, and z. Do you see any patterns now?

Next, create another column to the right of the colors. Label this column u-g. Click on the u-g column for the row for the first star. Type an equal (=) sign. Click on the box with the first star's u value. Type a minus (-) sign. Click on the box with the first star's g value. Press enter. Then, click the small square in the lower right corner of the cell you just entered and drag down to the last row of data. Excel will automatically repeat the subtraction for the other stars.

Repeat this procedure to get columns for g-r, r-i, and i-z. Now, sort the data by u-g, g-r, r-i, and i-z. What patterns do you see now? What column of data gives you the clearest pattern?