Interstellar Medium (ISM) - Gas and Dust

Carina Nebula ImageCarina Nebula

Interstellar Medium:


Extinction (including red shift) of Electromagnetic Radiation
Polarization of light


Radio Emission (molecules)
Radio Absorption (21 cm)
Optical Absorption
Type of ISM Comment Density
Temperature (K) Mass (MSun) Size (ly)
Cold Atomic Atomic Hydrogen
In Equilibrium
10-20 50 1000 30
mean separation 500ly
Warm Atomic Atomic Hydrogen
In equilibrium
0.1 5000 This and Cold make 50% of all ISM Mass Small Fraction of Volume
Coronal Gas Typical of Solar Area
Heated by Supernova
0.01 1,000,000 very small 50% of all volume
Molecular Clouds H2 not detectible
CO radiation used to map locations
Not in gravitational equilibrium
1000-10,000 ~10 ~<1,000,000
50% of ISM mass!
(< 1% of space)
HII Regions associated with Molecular Clouds
Hot and expanding
100-1000 10,000 Small fraction Small fraction of volume