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.
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.
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.