Colors of Stars in the SDSS

Explore 1. Look through the SDSS 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. A new window will open with a sky globe and two images. The blue-framed image is the Mosaic window, and the red-framed image is the Zoom window. When you click on one of the green or purple stripes in the sky globe (these represent areas of the sky where SkyServer has data), new images will appear in both windows. When you click a point in the Mosaic window, a gray box will appear - the area inside this gray box will appear inside the Zoom window.

Click on the red button labeled "Zoom" to bring the Zoom window to the front. Now, click on any object in the Zoom window. Information on that object will appear at the top of the window. The information box will show the object's sky coordinates (ra and dec), the object type (star, galaxy, or unknown), and the object's magnitudes: u, g, r, i, and z. These five numbers measure how bright the object appears.

When you select a star you are interested in, click "Save this" to save the star in your online notebook. On a separate sheet of paper, record the star's ra, dec, and the color you see. 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 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?