Interstellar dust
 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.
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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.