We can also study the surface temperatures of the stars in a number of ways. We might take spectra of the stare and classify them according to spectral type. But that would be very time consuming; some clusters contain a thousand stars. We could use a photometer and measure the brightness of the stars through a blue and a yellow filter to find their color. This too would be time consuming. The most efficient way would be to photograph the cluster through the yellow and the blue filters and then determine the colors from the plates. From the colors we could deduce surface temperatures or spectral types.