Two extra-Galactic surveys are considered. The first takes observations of nearly 200 GMCs across a small sample of local galaxies in the CANON CO survey. In spite of the local nature of the sample, results confirm essential facts about molecular gas distribution in galaxies other than our own, including a confirmed linear relationship between GMC virial mass and CO luminosity, which implies a constant CO-H2 conversion factor and supports a virialization assumption. The second survey, PHANGS-ALMA (Physics at High Angular Resolution in Nearby Galaxies with the Atacama Large Millimetre-submillimetre Array), maps CO emission from galaxies up to 17 Mpc away, with resolutions of 1″–1.5″ encompassing active star-forming galaxies down to total stellar masses ~5 × 109 M☉. Within 11 of those target galaxies considered here, the results offer tens of thousands of measurements at GMCs scales between 20 and 130 pc, comparable to Galactic-scale observations, and one outcome is confirmation of a positive correlation between GMC surface densities and velocity distributions.

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.

The Orion Bar as the canonical high-flux PDR is examined. In addition to a detailed description of the source, the estimation of physical parameters such as ionization fraction and observational indicators such as carbon recombination lines are considered. High-resolution observations point to the sensitivity of carbon chemistry to CR ionization and the apparent merging of C/C+/CO transition and H/H2 transition zones not readily predicted by theory. A wide range of molecular sulphur observations also presents the opportunity to rethink gas–grain reaction networks and model their consequences, with a following chapter looking at the low-flux PDR case of the Horsehead Nebula, through which the sulphur question will be further explored.

Through the emission observations of molecular species in the IRAS2 and IRAS4 locations in NGC 1333 in the Perseus Molecular Cloud (PMC), the distinctions between conditions favouring COMs or WCCC production in the immediate neighbourhoods of low-mass protostars are discussed. The current chemical modelling and that which will follow from accumulating higher-resolution observations using the latest generation of millimetre and submillimetre instrumentation are discussed.

Fourteen super star clusters (SSC) sites are identified in the central bar of the external galaxy NGC 253, and the factors influencing their star-formation efficiency are considered. Molecular emission clearly shows red-shifted emission and blue-shifted absorption line profiles (P-Cygni) characteristic of outflows. While separation of large-scale motions in CO along the line of sight is difficult, CS and HCN are identified as tracing localised and spatially resolved emission within the clusters rather than the foreground gas. The SSCs are shown to fragment into primary clusters surrounded by smaller satellite clusters. From column density and projected sizes, outflow mass and other physical parameters are estimated, and outflow drivers and feedback mechanisms are discussed. The application of chemical clocks, particularly involving sulphur species, is explored and wider molecular comparisons made.

Within the Large Magellanic Cloud, a hot core is observed associated with the embedded high-mass YSO (IRAS 05195˗6911), known as ST16. Comparative observations with molecular abundances typical of Galactic hot cores are discussed, as is the evidence for a rotating protostellar envelope and outflow cavity. A second LMC source, the prominent star-formation region N113, shows centrally focused star formation with associated point-like mid-infrared emission, masers, and compact HII regions superimposed on extended emission. Gas and dust appear compressed by a complex structure of ionized gas bubbles (prominent in Hα detections) engendered by massive stars in several young clusters. In both ST16 and N113 low-metallicity sources, warm dust appears to inhibit COMs formation and survival, while reaction routes appear broadly comparable with Galactic models.

The opening chapter introduces the most significant areas of contemporary research in the molecular astronomy of star formation: prestellar cores, hot cores, hot corinos, accretion, protoplanetary disks, photodissociation regions (PDRs), HII regions, stellar jets, disk winds, outflows, and masers. These sit within the wider considerations of dense molecular clouds on many scales, from the giant molecular clouds (GMCs) to fragments, filaments, and clumps. Our understanding of these molecular cloud environments depends on our understanding of molecular excitation, energy balance, gas and grain surface reaction kinetics, cosmic ray ionization, and photochemistries. Chemical modelling involving both gas-phase and grain-surface reactions is described, as are the observational and analytical essentials of antenna temperature, optical depth, velocity distribution, column density, beam dilution, relative abundance, rotation diagrams, and radiative transfer modelling.

The low-mass star formation Lupus complex sits within the expanding HII shell of the Upper Scorpius OB cluster, with shock impacts triggering multiple star formation. IRAS 15398 in Lupus I-1 is considered as a WCCC source rich in COMs, molecular line emissions allowing distinctions between molecules particularly prevalent in either compact or extended regions. Molecular emissions from close to the protostar as well as from gas spreading in outflow material are involved. Within the latter are found distinguishable localized components (‘blobs’) that show likely shock enhanced chemistry. As is the case for IRAS 16293 and NGC 1333, disk emission is separable from envelope emission through characteristic species and levels of molecular excitation.

Two surveys of high-mass star formation (HMSF) are discussed. One is the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), using the Atacama Pathfinder Experiment (APEX) 12 metre dish. The other uses the Institut de Radioastronomie Millimetrique (IRAM) Northern Extended Millimetre Array (NOEMA). The value of a representative survey of HMSFRs lies in learning what physical and chemical parameters are shared across a variety of sources. The results of statistically large samples of detected, or non-detected, sources such as that of ATLASGAL provide secure data from which to generalise about the typical star-formation process. The results of smaller but still multi-location studies such as the NOEMA sample give us greater specific details, albeit from a self-selecting sample, which may or may not be typical but that we can certainly say are common, at least until future wider surveys demonstrate error.

The three UCHII regions associated with the G34.26+0.15 high-mass star formation complex in Aquila are described, giving evidence for envelope infall, protostellar outflows, expanding ionized gas, and associated molecular hot core chemistry. The prototypical ‘cometary’ UCHII region ‘C’ in G34.26 is one focus, where the interface between ionized hydrogen (HII) and hot molecular core (HMC) gas is well observed and a rich hot core chemistry both detected and modelled in detail. Uncertainties in CH3CN formation, and the displacement of its peak emission from dust and NH3 peaks, are raised in relation to possible photodissociation in the hot core close to the UCHII-C feature.

Sagittarius (Sgr) B2 is the most massive star-forming region in the Galaxy and the canonical HMSFR with probably the richest source of molecules detected to date, not least in the number of COMs recorded. The consequences of a variable and higher-than-standard cosmic ionization rates in this region close to the Galactic centre are discussed. They are seen to have a complex effect on COMs chemistries, offering both an unusual test bed for chemical evolution theory, while not being conditions representative of more widely observed HMSF cores. The particular case of cyanides and isocyanides stands out, and modelling that uses enhanced but extinction-dependent CR ionization rate brings best agreement between model results and observations. Nonetheless, the modelled column densities of some species are much lower than observed, and the physical structure profile of the regions appears to be responsible.

The massive giant molecular cloud (GMC) complex Westerhout 43 (W43) and its subcores are considered, in particular G29.96. HMSF is evident in clusters and the impact of disk winds and outflows on the observable chemistry made clear. Modelling of the hot core COMs abundances matches observations for many key species observed in both this and other Galactic sources. The interaction between an HII region and an associated hot dense core is exemplified in G29.96, in spite of the evident complexity of physical conditions in the surrounding region. As in all studies made through the lens of molecular emission, astronomers are able to probe the physical conditions through trace chemical emissions.