Star formation in dwarf irregular galaxies

I have explored the star formation histories of the dwarf irregular galaxies Sextans A and GR 8. I measured photometry of individual stars from images taken by the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. With the photometry I constructed color-magnitude diagrams (CMDs) in the B, V, and I. I investigated the errors in the photometry extraction, and conducted artificial star tests to measure the photometric limits. The high resolution of the Hubble Space Telescope allowed photometric measurements that were far more accurate than ground-based observations. For galaxies at these distances (1-2 Mpc), the accuracy of stellar photometry from ground-based observations is limited by crowding of stellar images.; The high accuracy photometry showed a clear separation of the main sequence from the massive, blue, core He-burning stars (HeB). These are stars in the bluest extent of the so-called "blue-loop" phase of stellar evolution. This is the first time this phase of evolution has been clearly identified in a low metallicity system. The distributions of stars in the CMDs agreed very well with stellar evolution model predictions.; I have used the CMDs to calculate the recent star formation histories of both galaxies. The main sequence luminosity function provided the star formation rate (SFR) over the past {dollar}sim{dollar}50 Myr. I developed a new technique for calculating the SFR from the blue HeB luminosity function. Furthermore, the blue HeB evolutionary phase has a one-to-one relation between age and magnitude. This allowed me to calculate the position, as well as the strength of star formation over the past {dollar}sim{dollar}500 Myr.; The star formation was found in concentrated regions. These regions are of order 100 pc across and last of order 100 Myr. The regions were found near the highest density HI gas. I estimated the gas-to-star conversion efficiency to be 5-10%. The results from GR 8 suggest that the star forming gas clouds may be self-gravitating, and that each cloud can support several star forming episodes. There is an age progression among the star forming regions which suggests that the star formation is propagating through the galaxies.