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Spirals can, in principle, be of 4 types: (1) Transient gravitational instabilities in the gas, causing "flocculent spirals," with too much stability in the stellar disk to give prominent stellar spiral waves. (2) Transient gravitational instabilities in the stars, with the gas adding force and following the stars. The gas and stars move through these transient spirals a little, but not around from arm to arm in a full circle as in idealized global stellar modes and waves. (3) Global stellar waves that are non-steady with a pattern speed that varies with radius and whose patterns wrap up toward the center over time. Stars and gas move through these spirals. (4) Global stellar wave modes that are "standing waves," with a uniform pattern speed between the Lindblad resonances. Gas and stars move through these standing waves with corotation approximately at mid-radius in the spiral.

Young stars concentrate in spiral arms because the gas concentrates there. Spiral arms are dense and promote more gravitational instabilities and cloud collisions than the interarm regions, triggering molecular cloud formation and conglomeration in the arms. The star formation rate per unit area is high in the arms as a result. This excess star formation rate is mostly in proportion to the extra molecular gas column density there, without a significant change in the star formation rate per unit molecular gas mass. The total galactic star formation rate in the main disk is not significantly enhanced by the presence of spiral arms. That is like saying the gas would have formed the same abundance of molecular clouds even without the arms. Outer disks may be different. They may have an excess of total star formation if there are spiral arms there, but this excess has not been observed yet. The difference between inner disks and outer disks is that inner disks are highly molecular and star formation in the gas is virtually saturated. Outer disks are mostly atomic and without star formation, so triggering a higher rate of star formation might be possible with dynamical disturbances.