(matter within a galaxy between star systems)
The interstellar medium (ISM) is
matter such as gas and dust between stars.
(Matter between entire galaxies is called the intergalactic medium.)
Depending upon circumstance, the gas may be
ionized, neutral, or molecular.
Density varies from as little as 1 per
10,000 cm3 to 1,000,000 per cm3
(compared to about 2.5×1019 per cm3
for dry air at sea level).
The ISM's existence was first deduced
through observed absorption lines.
There is a nesting of types of ISM:
- Most ISM: hot and very thin: ionized hydrogen (HII) or neutral atomic hydrogen (HI) at thousands of K (warm neutral medium, WNM, or warm ionized medium, WIM, or coronal gas/hot ionized medium, HIM).
- Cooler, denser region of neutral hydrogen (HI and some CII) at 50-100K (cold neutral medium or CNM, HI region).
- Even cooler, denser subregion: molecular hydrogen (H2, with CI and CO) at 10-20K (molecular cloud, the site of possible star formation).
- Subregion reheated/dissociated/ionized by recently-formed, hot stars, to thousands of K, but still dense: HII, CII, and NII (HII region, modeled by Strömgren sphere).
dark matter halo
dense core mass function (DCMF)
diffuse interstellar bands (DIBs)
diffractive interstellar scintillation (DISS)
extreme ultraviolet (EUV)
star formation feedback
galactic electron density
helium 1083nm line
ionization correction factor (ICF)
intergalactic medium (IGM)
Interstellar Medium Absorption Profile Spectrograph (IMAPS)
interplanetary medium (IPM)
refractive interstellar scintillation (RISS)
submillimeter galaxy (SMG)