What material is overwhelmingly concentrated within the spiral arms?
Answer
Molecular clouds, the raw material for new stars
The intense concentration of star formation within the spiral arms is directly attributable to the density of their primary fuel source. While the disk contains gas, the spiral arms act as cosmic traffic jams, compressing the diffuse material into dense molecular clouds. These clouds represent the necessary reservoirs of cold, dense gas, primarily hydrogen and helium, required for gravitational collapse to initiate star birth. Because the arms harbor the immense majority of this molecular cloud material, they serve as the essential incubators for the next generation of suns.
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