The interstellar medium (or ISM) is the matter and radiation that exists in the space between stars and planets and associated systems in a galaxy and contains gas as ions, atoms and molecules as well as dust and cosmic rays, and electromagnetic radiation/
The interstellar medium distinguished by the state of the matter - ionic, atomic, or molecular, and the temperature and its density. Each of these exerts a thermal pressure maintaining an equilibrium but magnetic fields and turbulent motions also provide pressure and are typically more important dynamically than the thermal pressure is.
The interstellar medium is extremely dilute. In cool, dense regions it, is primarily in molecular form and reaches number densities of
molecules per
In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as
ions per
compared with roughly
molecules perfor liquid water. 99% of the medium is gas by mass in any form, and 1% is dust. Of the gas, 89% of atoms are hydrogen and 9% helium, with 2% of atoms being heavier elements called "metals" in astronomical lingo. The hydrogen and helium are a result of primordial nucleosynthesis, while the heavier elements in are primarily a result of enrichment in the process of stellar evolution.
Stars form in the densest regions of the medium called molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae when stars end their lives.
The table shows properties of the interstellar medium of the Milky Way.
Component | Fractional | Scale Height | Temperature | Density | State of hydrogen | Primary observational techniques |
|---|---|---|---|---|---|---|
Molecular clouds | < 1% | 80 | 10—20 | 102—106 | molecular | Radio and infrared molecular emission and absorption lines |
Cold Neutral Medium (CNM) | 1—5% | 100—300 | 50—100 | 20—50 | neutral atomic | H I 21 cm line absorption |
Warm Neutral Medium (WNM) | 10—20% | 300—400 | 6000—10000 | 0.2—0.5 | neutral atomic | H I 21 cm line emission |
Warm Ionized Medium (WIM) | 20—50% | 1000 | 8000 | 0.2—0.5 | ionized | Hα emission and pulsar dispersion |
H II regions 0 regions of space containing hughly ionized Hydrogen | < 1% | 70 | 8000 | 102—104 | ionized | Hα emission and pulsar dispersion |
Coronal gas | 30—70% | 1000—3000 | 106—107 | 10−4—10−2 | ionized | X-ray emission; absorption lines of highly ionized metals, primarily in the ultraviolet |