Star formation in low brightness galaxies and in the extended gaseous disks of normal galaxies

We analyze the available observational data on the radial distribution of gas and young stellar populations in the disks of low surface brightness (LSB) galaxies and in the outer regions or the extended disks of normal brightness (HSB) galaxies. These cases involve star formation under special conditions of low volume and surface gas density. There is no well-defined boundary between these subgroups of galaxies that we consider, but in non-dwarf LSB galaxies the rate of current star formation within the wide range of radial distances appears to be higher compared to the outer disks of most of HSB galaxies at similar values of the surface gas density. The factors that could stimulate the compression of the rarefied gas at the periphery of galaxies are briefly discussed. Attention is drawn to the idea that the densities of LSB disks estimated from their brightness may be underestimated.