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
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.
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
Cold clouds of atomic or molecular hydrogen
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
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.