Distribution of the luminosity (magnitude) as a function of the temperature (color) of stars. For a given content of heavy elements (often called “metals” in Astrophysics), this color-magnitude diagram shows particular sequences where the ages of the stars can be identified (vertical axis, the redder the color, the older the star). On the left a color-magnitude diagram not affected by the blurring of observations, where the ages of stars can be determined with satisfactory precision. On the right, the observational blurring makes difficult to determine stellar ages. Credit:  Sebastián Hidalgo. (IAC)