Heraclitus already knew: “Everything flows”. This seemed to be contradicted by the starry sky, which always remained the same for humans, as it was considered the epitome of the truly unchanging until modern times. It was only with the advent of astrophysics in the nineteenth century that people began to realize that the diversity of stars, determined by their luminosity and temperature, is in reality the image of a mixture of stars of different ages and masses that populate our Milky Way today.