# 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 stars.

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 magnitudes.

When you find a star you are interested in, record its data in this Excel 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.

The workbook is an Excel spreadsheet. If you do not have Excel, you can open it with Google Spreadsheets.