Record this for the distance modulus in Table 1. To convert this into the distance of the cluster, we must invert the distance modulus equations. There is another thing that we can learn from these color-magnitude diagrams, after the age and distance of the cluster. 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.