The figure shows a cluster of stars like the ones analysed in this work. The emitted light loses energy as it escapes the gravitational field of each star. This energy loss manifests itself as a reddening that is larger for main-sequence stars with relatively small radii (right) than for giant-phase stars with larger radii (left). Credit: Gabriel Pérez Díaz (IAC). Background image: ESA/Hubble and NASA. Magnifiers based in an illustration by freepic.es.