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 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
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
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?