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 at the top of
the window - "ra" and "dec" gives 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, save it in your online
notebook. To save, click "Save in Notes" in the right-hand frame of the tool window. 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 Edit 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.
Now sort the data by u-g. Do you see any patterns? Do
certain colors of stars tend to clump together? Create another
column for g-r. Sort by g-r. Do you notice any