Look again at the data box on the right side of the window.
The ra and dec tell you where in the sky this object is. The type tells you what the SDSS computer program thinks the object is. (The program isn’t always right!) u,g,r,i, and z tell you the object’s magnitude through each of the five filters.
The star shown to the left is the star whose data are displayed above. What color does it look like to you?
Astronomers measure color with numbers: differences in magnitude between filters. For example, one color is the difference between the star’s u and g values. The star above has a u-g color of 19.10 – 18.04 = 1.06.
The color u-g = 1.06 means the star gives off more green light than ultraviolet light (remember that the magnitude scale runs backwards). You could also measure the star’s g-r color, which would be 18.04 – 18.29 = -0.25. The star gives off more green light than red light.
Now that you know a little about the colors of objects, let’s see what types of objects you will be searching for. Click Next to find out.