The Interstellar Medium
The interstellar medium is the gas and dust filling the space between stars, the reservoir from which stars form and the recipient of the material they return.
Definition
The interstellar medium is the diffuse matter, gas in atomic, molecular, and ionized forms together with dust grains, magnetic fields, and cosmic rays, that occupies the space between the stars of a galaxy and mediates the cycle of star formation and stellar death.
Scope
This area covers the coexisting phases of interstellar gas, from cold molecular clouds to hot ionized plasma; the dust grains that redden and obscure starlight and seed chemistry; the dense molecular clouds where stars are born; and the magnetic fields and cosmic rays that pervade the Galaxy.
Sub-topics
Core questions
- What phases make up the interstellar medium, and how do they coexist?
- How does interstellar dust affect observations and chemistry?
- Where and how does dense gas collapse to form stars?
- What roles do magnetic fields and cosmic rays play in the Galaxy?
Key theories
- Multiphase interstellar medium
- McKee and Ostriker described the interstellar medium as a small number of coexisting phases, cold, warm, and hot, in rough pressure balance and regulated by supernova explosions.
- Gas-dust coupling and chemistry
- Dust grains absorb and scatter starlight, catalyze the formation of molecules such as hydrogen, and lock up heavy elements, making them central to interstellar physics and chemistry.
- Energy balance and feedback
- Heating by starlight and cosmic rays balanced against radiative cooling, together with stirring by supernovae, sets the structure and dynamics of the interstellar medium.
Clinical relevance
The interstellar medium is the cradle of star formation and the conduit of galactic chemical evolution; understanding it is essential for interpreting starlight reddened by dust, for tracing how galaxies recycle their gas, and for studying the conditions that lead to new stars and planetary systems.
History
Spitzer's mid-twentieth-century work laid the physical foundations of the interstellar medium, and the 1977 McKee and Ostriker model framed it as a supernova-regulated multiphase system. Radio, infrared, and ultraviolet observations have since mapped its phases, dust, and magnetic fields in detail.
Key figures
- Bruce Draine
- Christopher McKee
- Jeremiah Ostriker
- Lyman Spitzer
Related topics
Seminal works
- mckee1977
- draine2011
- ferriere2001
Frequently asked questions
- Is the space between stars truly empty?
- No. Although extremely tenuous by everyday standards, the space between stars is filled with gas and dust. Over the vast distances of a galaxy this thin material adds up to a substantial mass and profoundly shapes how we see the cosmos.
- Why does interstellar dust matter if it is such a small fraction of the mass?
- Dust makes up only about one percent of the interstellar mass, but it absorbs and scatters starlight, reddening and dimming distant objects, and its surfaces catalyze the chemistry that builds interstellar molecules, giving it an outsized influence.