Question: Why most of the star forming regions/open clusters are in the periphery of galaxies(in spiral arms)? – Vinod
Answer: Star forming regions are concentrated in parts of galaxies that contain high concentrations of the material from which stars are made: gas and dust. Depending upon the type of galaxy and the kinds of gravitational interactions it might experience, these concentrations of gas and dust can be “pushed” to the point where they collapse to create stars. The spiral arms in spiral galaxies are one type of environment where gravity is pushing gas and dust to form stars more efficiently than in other parts of a spiral galaxy. This is why you see more star forming regions and collections of young stars (open clusters) in spiral arms than in other parts of a spiral galaxy.