This brief brings together the theoretical aspects of star formation and
ionized regions with the most up-to-date simulations and observations.
Beginning with the basic theory of star formation, the physics of
expanding HII regions is reviewed in detail and a discussion on how a
massive star can give birth to tens or hundreds of other stars follows.
The theoretical description of star formation is shown in simplified and
state-of-the-art numerical simulations, describing in a more clear way
how feedback from massive stars can trigger star and planet formation.
This is also combined with spectacular images of nebulae taken by
talented amateur astronomers. The latter is very likely to stimulate the
reader to observe the structure of nebulae from a different point of
view, and better understand the associated star formation therein.