Two HII region surveys are considered. The first is a multi-band survey of over one hundred hypercompact HII (HCHII) candidates using the Jansky-VLA. The second survey, the deep-resolution ALMA Three-millimetre Observations of Massive Star-forming regions (ATOMS-ALMA), studied just under 500 and identified 89 cores that cocoon HCHII or UCHII sources observed in H40α; 32 hot molecular cores (HMCs) showing more than 20 COMs; and 58 HMC candidates not associated with HII regions. The study shows how, in the vicinity of newly formed OB protostars and HII regions at an early stage of evolution, we can begin to understand the dynamics of infall, outflow, and rotational motions, as well as the feedback roles of outflows, stellar winds, and HII regions.

Radiant light is what we see from stars.Radiant light carries the energy outward through the star's photosphere.This chapter gives us the terms and vocabulary to describe radiation processes.

The model photosphere is the core of the theoretical side of our studies.It is a numerical theory from which we compute the theoretical spectrum to be compared to the observations.Our model photospheres are based on hydrostatic equilibrium.To compute such models, we need the temperature distribution, a value for the surface gravity, and the chemical abundances of the electron donors.This chapter reveals how this process is done.

Radiation generated in the deep interior of a star undergoes a diffusion between multiple encounters with the stellar material before it can escape freely into space from the stellar surface. We define the optical depth by the number of mean free paths a photon takes from the center to the surface. This picture of photons undergoing a random walk through the stellar interior can be formalized in terms of a diffusion model for radiation transport in the interior.

Our initial introduction of surface brightness characterized it as a flux confined within an observed solid angle. But actually the surface brightness is directly related to a more general and fundamental quantity known as the “specific intensity.” The light we see from a star is the result of competition between thermal emission and absorption by material within the star.