What structure propagation mechanism defines the spiral arms dynamically?
Answer
Density waves where interstellar material is compressed
The spiral arms are not static, rigid structures but rather dynamic phenomena described as density waves. These waves move through the galactic disk material, and their defining characteristic is the compression they exert on the interstellar gas and dust located there. This compression is the crucial physical mechanism that clumps the diffuse material together, raising the local density to the threshold required to initiate gravitational collapse and subsequent star formation. This process is what concentrates stellar births along the winding lanes we observe.
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