Sequential Star Formation in Small Irregular Galaxies

Star formation is not continuous in small irregular galaxies. The galaxy DDO 53 has a size of about 6,500 light years at a distance of 11 million light years. The galaxy GR 8 has a size of about 4,000 light years at a distance of 7.2 million light years. Broadband photometric observations of stars in these galaxies are matched with models based on reduced-metallicity evolutionary tracks of single stars, as well as tracks for close binary stars. Regions within each galaxy are best fit with varying intervals of recent star formation consistent with a pattern of activity that moves from region to region.