About 99% of the interstellar medium is in a gaseous state, with hydrogen making up 90% of the atoms. About half of this gas is tied up in interstellar gas clouds which have different properties depending on the temperature of the gas.

The Rosette Nebula. Interstellar gas clouds that have been heated to several thousand Kelvin can be seen as emission nebulae such as this.
Credit: AAO/ROE

In the coldest and densest regions of the interstellar medium we find clouds whose cores contain molecular gases, primarily molecular hydrogen (H₂) gas. Molecular gases can only be found under these conditions since very little energy is required to break the molecules apart. A small increase in the gas temperature of the cloud will cause the molecules to dissociate, as will starlight if it is able to penetrate deep enough into the cloud to be absorbed by the molecules. These molecular clouds have temperatures of only about 10 Kelvin and have a high concentration of dust grains to protect the molecular gases at their centre from incoming photons.

If the gas cloud is not quite cold or dense enough for hydrogen molecules to survive, we end up with a cloud of neutral hydrogen atoms. These clouds tend to have temperatures of around 100 Kelvin and are commonly named HI clouds, since astronomers often refer to neutral hydrogen as HI (pronounced H-one).

Occasionally gas clouds are found close to a very hot star which heats the gas to about 10,000 Kelvin. The radiation from the star also ionises the hydrogen which later emits light at a wavelength of 656.3 nm as it recaptures an electron and returns to its lowest energy state. These gas clouds result in one of the most common forms of emission nebulae and are usually referred to as HII regions, again following the naming convention astronomers use for ionised hydrogen (HII, pronounced H-two).