Star formation in the outskirts of galaxies
Star formation environments can be roughly grouped into three types, categorized by the density of their gas (or more precisely, the projected "surface" density of the gas, which is easier to determine than the conventional volume density). In moderately high density regions, where the gas is primarily molecular in form rather than atomic, there is a strong correlation between the amount of star formation taking place and the density. This result is the basis for concluding that stars form from molecular material. In very high density regions like those found in merging and starbursting galaxies, the star-formation rates compared to the total mass of available material are even larger. In low-density regions there is little known about correlations between the total gas and star-formation activity.