Where is star formation heavily concentrated in spiral galaxies?
In the spiral arms themselves
Star formation within spiral galaxies is not spread uniformly across the entire flat, rotating disk; instead, it is intensely localized and concentrated specifically within the winding structure known as the spiral arms. This concentration is a key defining characteristic of active spiral galaxies when compared to ellipticals. The environment within the arms is uniquely conducive to stellar birth because they act as regions where massive clouds of molecular hydrogen gas are physically compressed. This compression, driven by density waves, raises the density sharply enough to overcome the internal pressure of the gas cloud, initiating gravitational collapse, which ultimately leads to the ignition of nuclear fusion and the birth of a new star. The inter-arm regions, by contrast, show an almost negligible rate of current star formation.
