Molecules in the ISM

Many relatively complex molecules have been identified in the ISM by their spectral lines. Examples include carbon monoxide, table salt, hydrogen cyanide, and water. It is not clear how some of these molecules form, but they are there.

Molecules have weak chemical bonds, so they are easily disrupted by UV light. Thus, molecules are typically found only in the deep, dark recesses of molecular clouds where dust helps protect them from the UV radiation of the ISM.

Interstellar Gas

Of the gas in the ISM, 90 percent by mass is hydrogen, with the remainder mostly helium. This composition is similar to the Sun, except it is depleted in heavier elements like iron and calcium, perhaps because these elements are found preferentially in the dust. The gas appears primarily in two forms

1. Cold clouds of atomic or molecular hydrogen
2. Hot ionized hydrogen near hot young stars or in supernova remnants

The clouds of cold molecular and atomic hydrogen represent the raw material from which stars can be formed. They are found in the disk of the galaxy and may be tens of parsecs across and contain a hundred to a million solar masses of mostly molecular hydrogen at frigid temperatures near 10 K. Although such clouds do not emit visible radiation, they can be detected by their radio frequency emission, particularly from trace molecules (see the right panel). The hot, ionized regions are much more localized, since they require the presence nearby of a hot star. However, they are very visible because of the bright emission nebulae associated with them.

Radiative Cooling of Molecular Clouds

Molecules contribute to the low temperatures of molecular clouds by a process called radiative cooling. They are efficient at absorbing energy from the gas by collisions, and then radiating that energy as IR or RF photons (which are not strongly absorbed by the dust). Since this is a net transfer of energy out of the cloud, it is cooled by this emission.

Part of the ISM has a high enough temperature to emit X-rays. Because this part is a thin gas with a temperature of a million degrees like the Sun's corona, it is called the coronal gas of the interstellar medium. The coronal gas is thought to originate largely from supernova remnants, but gas emission from hot stars may also contribute. It constitutes a significant fraction of the ISM, but estimates vary from 20 percent to 80 percent and so are not very firm.