This last quantity doesn't have much to do with the cluster itself however, but rather it tells us about what it is between us and the cluster. That is, the interstellar dust . Interstellar dust acts in similar ways to dust in our planet's atmosphere. You have probably noticed how the sun or bright full moon can appear orange or even red at sunset or sunrise. The reason is that at those times you are looking through a lot of dust in the air.
Dust tends to absorb and selectively scatter blue light more effectively than red.
The red light can pass by the dust grains more easily than the blue, so we end up receiving a higher fraction of the red light that is emitted than of the blue. We can make an estimate of how much reddening has occurred by using the spectral type to calculate an intrinsic, un-reddened color index. This should make sense to you, in light of the reddening effects of dust. Record the color excess in the Table 1. Extinction Stars also appear dimmer due to interstellar dust.