Phases of the Interstellar Medium
Interstellar gas exists in several coexisting phases, spanning cold dense clouds, warm diffuse gas, and a hot tenuous plasma, in approximate pressure balance.
Definition
The phases of the interstellar medium are the thermally distinct states of interstellar gas, cold and warm neutral atomic gas, warm ionized gas, and a hot ionized plasma, that coexist at comparable pressures because of the balance between heating and cooling.
Scope
This topic covers the cold and warm neutral atomic gas, the warm and hot ionized gas including HII regions and the hot bubbles carved by supernovae, the thermal instability that drives gas into distinct phases, and the rough pressure equilibrium that links them.
Core questions
- What are the main phases of interstellar gas and their temperatures and densities?
- Why does the gas separate into distinct phases rather than a single uniform medium?
- How do HII regions and hot supernova bubbles arise?
- How is approximate pressure balance maintained among the phases?
Key theories
- Thermal instability and two stable phases
- Field and collaborators showed that the balance of heating and cooling makes intermediate-density gas thermally unstable, so neutral gas settles into coexisting cold dense and warm diffuse phases at similar pressure.
- The three-phase model
- McKee and Ostriker added a pervasive hot phase created by supernova blast waves, giving a picture of cold, warm, and hot components in dynamic, pressure-regulated coexistence.
Clinical relevance
The phase structure of the interstellar medium governs where gas can cool and collapse to form stars, how supernova energy is dissipated, and how observable tracers such as neutral hydrogen and ionized gas emission are interpreted.
History
The 1969 work of Field, Goldsmith, and Habing established that thermal instability divides neutral gas into two stable phases. McKee and Ostriker's 1977 three-phase model incorporated the hot, supernova-driven component, and subsequent multiwavelength surveys mapped these phases throughout the Galaxy.
Key figures
- George Field
- Christopher McKee
- Jeremiah Ostriker
- Harm Habing
Related topics
Seminal works
- field1969
- mckee1977
- ferriere2001
Frequently asked questions
- Why does interstellar gas split into hot and cold phases?
- At certain densities the way the gas cools makes intermediate states unstable: gas tends to drift toward either a cold dense state or a warm diffuse one. With supernovae adding a hot component, the medium ends up as several coexisting phases at similar pressure.
- What is an HII region?
- An HII region is a zone of interstellar gas ionized by the ultraviolet light of hot young stars. The ionized hydrogen glows, producing the bright nebulae often seen around regions of recent star formation.