Spiral galaxies are more complex. They consist essentially of two elements: a spherical nucleus surrounded by a disk of matter in which a spiral structure appears. There is a great diversity of form, from a huge nucleus surrounded by small spiral arms to a tiny nucleus with very long arms.
This class of galaxies is further subdivided into two groups: the normal spirals, in which the arms develop directly from the nucleus, and the barred spirals which have a large central bar whose extremities are the starting point of the arms (c. is the case of our Galaxy)
For spiral galaxies, it is not a random internal agitation that prevents gravitational collapse, but an overall rotation of the galaxy. Each star circles around the nucleus and it is this orbital movement that gives rise to a centrifugal force. The global rotation of the galaxy is also responsible for the flattening of the whole and the formation of the disk.
The spirals have stars of all ages and masses, and a large amount of gas and dust. Here too, the two facts are linked because a rich interstellar medium means that there is still enough material to form many stars, hence the presence of young and massive stars. However, this is only true in the spiral arms and for this reason the arms appear brighter and stand out from the rest to give the galaxy its characteristic appearance.
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