Knowing that the color of a star is related to the wavelength of light it gives off lets you understand the meaning of the SDSS’s color filters. Each filter is designed to let in light around a specific wavelength. The filters work by blocking out light at all wavelengths except those around the wavelength they are designed to see. The table below shows the wavelengths at which SDSS’s five filters work the best. The sensitivity of each filter falls off slowly at shorter and longer wavelengths.

Filter Wavelength (Angstroms)
Ultraviolet (u) 3543
Green (g) 4770
Red (r) 6231
Near Infrared (i) 7625
Infrared (z) 9134

If you know the wavelengths of the SDSS filters and the amount of light a star emits in each filter, you could make a crude plot of the amount of light a star emits at different wavelengths. But what if, instead of five filters between 3543 and 9134 Angstroms, you had a hundred filters? >Or a thousand? You would get a clearer, more refined graph of amount of light and wavelength. Eventually, you would be able to see exactly how much light a star emitted at all the wavelengths of the electromagnetic spectrum.

Actually, astronomers do have a tool that lets them see how much light a star gives off as a function of wavelength. You’ll learn about this tool, called a spectrum, later in this project. But for now, think about how such a graph might look.

Question 1. If you made a graph of amount of light as a function of wavelength for some of the red stars you saw in Explore 1, what do you think the graph would look like? What about the blue stars?