Sagitarius is probably the most dense cloud in our milkyway. Lots of molecules are found in this cloud. (Royal Observatory Edinburgh)
Interstellar matter distinguishes its presence in obvious ways. We can observe them as shining nebula, like the Orion nebula, that mainly consist of hydrogen atoms that are ionized by the radiation of young, hot stars.
Also they are visible as dark, black spots in the sky, dark silhouettes a midst the stars.
Atom and molecular clouds.
Interstellar matter consists mainly of hydrogen (90%) and helium (approx. 10%). Only one out of one thousand particles in the clouds contain other elements, like carbon, nitrogen and oxygen. The dust particles, that form only a small part of the cloud, consist mainly of silicates (sand like matter) and graphite which are often surrounded by a small layer of ice.
The dark interstellar clouds are the most dense and are cold concentrations of interstellar gas. They are this dark because they contain dust particles that extinguish the light of the stars that they lay behind. In these dark clouds most molecules are found, besides the dust. Because of the presence of molecules these clouds are called molecular clouds, to distinguish them from ordinary atom clouds.
Often the dark clouds are joined with luminous nebula, since from molecular clouds new stars are formed. These young stars emit lots of ultraviolet radiation that light up the surrounding gas.
The new stars that are formed from molecular clouds are called second generation stars. Because of the molecules and more complex atoms, often astroids and planets come into being around these stars. Our Sun is an example of a second generation star with a planetary system orbiting it.
Giant Molecular Clouds.
Giant Molecular Clouds, that contain more than one million sun masses of matter, are the heaviest objects in our Milkyway. The total mass of molecular clouds is comparable with the mass of the atom clouds that are observable through the 21-cm radiation of the hydrogen atom.
Together they hold between five to ten percent of the mass of the stars in the Milkyway. Although astronomers speak about 'dense' clouds, these areas are very thin, the average density in interstellar clouds is much lower than that of the best vacuum we can create on Earth. Besides the temperature of these clouds is enormously low, only a few degrees above the absolute zero point. Chemical activity seems impossible under these conditions, still we find a rich variety of chemical compounds.
Observation of molecules.
In the so called diffusion clouds we can see the atoms and molecules through the absorbtion lines at visible and ultraviolet wavelengths in the spectrum of a star in the background.
These absorbtion lines are very slim because of the very low temperature of the cloud. In this way it is possible to determine exactly at which wavelengths molecules in the cloud absorb the starlight.
Between 1937 and 1941 by means of this technique the first interstellar molecules CN, CH and CH+ were detected.
With dark clouds this way of detection is not applicable, since visible light cannot penetrate them.
Here the molecules are determined via molecular emmision lines. Through collision with other particles molecules reach a higher energy level. When it eventually falls back again to a lower level it emits a photon (light particle).This photon is detectable with a radio or microwave telescope.
The most frequent molecule H2 however is very hard to determine, the emission lines are too weak to detect. The second most fequent molecule after H2 is carbon-monoxide, CO, which emits very strong radiation. The ratio in the quantities of these molecules in interstellar matter is rather constant so detecting the quantity of CO in a cloud gives a good indication of the quantity of H2.
Nowadays about 80 different molecules are found in the universe, amongst which also rather large ones like ethanol (CH3CH2OH).
The chemical cycle.
The interstellar material forms a dynamic reservoir from which stars are formed and to which they return their remnants. The cycle starts with the most diffused atom clouds, the interstellar cirrus. These are mainly composed of atoms and contain only very few molecules.
In the next phase the atom cloud develops into diffused molecular clouds, where simple molecules like H2, CO, CH, CH+, C2, CN and OH frequently occur. These clouds evolve to more dense clouds, then the most dense parts are contracting, which eventually leads to a new star formation.
Here two different possibilities can be distinguished: the formation of stars with low density (comparable with that of the Sun) and the formation of stars with a much larger mass.
When low mass stars are formed the temperature remains rather low. In clouds of very low temperatures and large density the molecules condense at the surface of the dust particles. These finally turn up at the planets and comets that are formed in the nebula near the newly born star.
After its long lifetime the star ultimately becomes a red giant, at which stage they loose a part of their mass. They shed mass in due of peels that expand in space. From these peels new dust particles are formed, in which large molecular elements are present.
The lifecycle of clouds that form larger stars is much faster. In these clouds the temperatures are higher (50 to 200 kelvin). When the clouds are near a young heavy star the chemical reactions take place under influence of ultraviolet light and shockwaves, here new molecular combinations are formed. After a short while the heavy stars develop into supernovas where the matter is expelled and blasted into space.
Now the chemical cycle is finished, the freshly formed elements and dust particles reassemble to diffused clouds and the process restarts.