In a young region like the so-called Christmas Tree Cluster, stars are still in the process of forming. A star is 'born' once it becomes optically visible (bottom right). During its further evolution, the star contracts and gets smaller in size and hotter until the core temperature is sufficient to start nuclear burning of hydrogen. This marks the end of the stellar childhood phase (bottom left). While the young star evolves from its birth to the beginning of hydrogen burning, its pulsation properties change: the least evolved, i.e., youngest, stars pulsate slower and the most evolved while the oldest stars pulsate faster.