What critical ingredient separates spiral arms as primary stellar birth sites from other disk regions?
Answer
Compression of diffuse gas and dust by the density wave
The key distinction making the spiral arms the primary sites for current star birth, even though they exist within the larger Galactic Disk, is the mechanism of material concentration. If the gas were spread smoothly throughout the disk, star formation would occur at a low, constant rate everywhere. Instead, the gravitational influence of the spiral density wave sweeps up the otherwise diffuse disk material—gas and dust—and clumps it together. This sustained compression is the critical ingredient that forces the material to achieve the necessary density for gravitational collapse and subsequent ignition, leading to intense, episodic bursts of star birth confined to the arm lanes.
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