Colors of Stars in the SDSS
Take a look again at the three stars from the last page, shown at
the right. What colors are the three stars? Are they red, blue, and white?
Or are they orange, turquoise, and cream? What about burnt siena,
cerulean, and periwinkle?
Clearly, "color" as we usually think about it is a subjective idea.
What one person's calls "blue," for example, might be totally
different from what another person calls blue. If astronomers are
going to learn anything about a star from its color, they first
need to have a definition of color that everyone can agree on - a
single measurement that anyone can make to compare the colors of different
In the next Explore exercise, you will look through stars in SkyServer's
database to try to find an objective, mathematical definition of color.
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
When you find a star you are interested in, record its data in this
workbook. Record its position (RA and Dec), color,
and its five magnitudes (u,g,r,i,z). Record data for 10-15 stars. You will use
these data in the next activity.
Launch the Navigation Tool
Download Explore 1 workbook
The workbook is an Excel
spreadsheet. If you do not have Excel, you can open it with
Google Spreadsheets or Open Office.
Explore 2. Now, see if you can discover a pattern in the colors.
Look at the data you have saved in your workbook. The directions here tell you how to
analyze the data in Excel; to analyze the data in other graphing programs, you would follow
similar steps. 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 found.
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?