The Interstellar Medium and H II Regions
The interstellar medium, or ISM, is the matter between stars. It holds gas, dust, cosmic rays, and magnetic fields. The gas is mostly hydrogen. Helium and trace heavier elements sit with it. Density is low. One centimetre can hold less than one atom in the thinnest zones. Even so, the ISM shapes star birth and galaxy light.
The ISM is not one uniform fluid. It has phases. Cold molecular clouds hide H2 and form stars. A cold neutral phase holds atomic hydrogen. A warm neutral phase fills much of a galaxy’s disk. Warm ionised gas surrounds hot stars. A hot, thin phase comes from supernova shocks. These phases exchange mass and energy. Turbulence and magnetic fields help mix them.
Dust is a small fraction by mass. It still matters. Dust grains extinct starlight. They also cool gas and catalyse H2 on their surfaces. Infrared maps therefore trace both dust and hidden star-forming sites.
An H II region is a volume of ionised hydrogen. A hot O or early B star floods nearby gas with ultraviolet photons. Those photons strip electrons from hydrogen. The gas then glows. Recombination and forbidden-line emission produce the familiar red and pink nebulae. The Orion Nebula is a nearby example.
The Strömgren sphere gives a simple model. It estimates how far ionising photons travel before the gas soaks them up. Real nebulae are messier. Density clumps, dust, and stellar winds break the neat sphere. Champagne flows can burst ionised gas out of a molecular cloud. A blister geometry can form on a cloud’s edge.
H II regions are plasma. Electrons and ions move together under electromagnetic forces. Electron temperature often sits near 10,000 K. Density varies from tens to thousands of particles per cubic centimetre in bright cores. Shock fronts and ionisation fronts sweep through the gas. They can compress nearby molecular material and trigger new stars.
Spectra reveal conditions. Hydrogen Balmer lines trace recombination. Oxygen and nitrogen forbidden lines help estimate temperature and density. Radio continuum emission from free electrons maps the ionised volume even through dust. Observers combine optical, infrared, and radio data for a fuller picture.
These nebulae also enrich the ISM. Massive stars shed winds. They later explode as supernovae. Heavy elements then mix into the surrounding gas. Later clouds inherit that metal content. In that way, H II regions mark both current star formation and chemical recycling.
H II regions are beacons across galaxies. Their luminosity helps count massive stars. Their size and structure show how feedback clears gas. They also test models of radiative transfer and plasma cooling. Study of the ISM and H II regions therefore links atomic physics, plasma physics, and galaxy evolution.