The Disk
Opposing to the bulge, the disks of the spiral
galaxies are rich in gas and dust clouds and in open clusters that behave as
factories that produce young stars with a high metallic content. These stars belong to the so-called population I
(contrarily to the older stars of the bulge and the halo, which belong to the population II). In the Milky Way, this
disk extends for 80 000 light years. The thickness of the stellar disk
diminishes from the centre towards the outskirts of the galaxies. However, the
gaseous and dusty disk, which is larger than the stellar disk, is only about
700 light years thick close to the centre but 3000 light years thick at the
exterior edges.
Composition
Neutral Hydrogen
One of the main components of the disk is the neutral hydrogen (1 proton
and 1 electron), detected through its emissions at the 21-cm wavelength line,
which take place when it changes from a high energy level to a low energy
level. This material is mainly distributed along the spiral arms and in a ring
located about 16.000 light years from the galactic centre.
Molecular Clouds
Other important components of the disk are the molecular clouds. They
may reach dimensions of 3000 light years and, opposing to what succeeds with
the neutral (atomic) hydrogen, they strongly prevail in the internal regions of
the disk.
Inside them there may be found simple molecules as the oxydryl (OH), the
carbon monoxide (CO) and other more complex, frequently organic, as the formic acid (HCOOH) or the methilic alcohol (CH3OH).
The most frequent is the molecular hydrogen (H2).
It is thought that the dusty cocoons protect the gases from the
ultraviolet radiation that dissociate the molecules. They also provide binders
to which the gas atoms may be attached and be kept at comparatively low speeds.
So, protected from the radiation and kept in low energy states, the hydrogen
atoms have the chance to fuse themselves into the molecules that are found
inside the clouds, under the dust and atomic gas layers.
On the other hand, the dust grains are thought to be composed by a solid
core of silicon carbide surrounded by amorphous coal layers. They are also
observed in these particles water ice, methane and carbon oxide layers.
M16: open cluster in Aquila (H. Frommert, C. Kronberg)
Molecular gas cloud inside the Aquila cluster
The Spiral Arms
The spiral arms are density waves (gravitational traps) that sweep the
galaxy and force the matter that they intercept to softly keep the pace with
the density wave. This phenomenon provides a slight increase of the matter
density in the arms' region, which works as a stimulant for the production of new stars.
They are, for this reason, particularly rich in nebulae and open
clusters containing high quantities of newborn stars. These stars, which
frequently display large dimensions and an intense blue
brightness, are precisely those who give the arms their known luminosity.
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