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The blue-compact-dwarf (BCD) galaxy IZw18 is of special interest to studies of galactic chemical evolution by virtue of its ionized interstellar medium having the lowest O/H ratio of any known star-forming galaxy. Therefore, it is quite possibly the best opportunity to study stars forming from isolated H{\sc i} clouds relatively unpolluted by the metal-rich ejecta from stars in a nearby galaxy. We report the results of an imagery and UV spectroscopy study of this system using the Hubble Space Telescope in Cycle 4. WFPC2 observations were made in the B-V-R wide-bands and the emission lines of H$\alpha$ and [O{\sc iii}] 5007\AA\ (limiting magnitude: V$\approx$28). The main body is resolved into over a hundred very young OB stars. The NE part consists of a dense cluster (d$\sim$100pc) surrounded by a ring of ionized gas, presumably being blown out by the collective stellar winds of the cluster stars. The SE part of the main body consists of several massive hot stars surrounded by small H{\sc ii} regions indicative of just formed stars. A second elongated component $\sim$1.5kpc to the NW of the main body is resolved into stars and contains a faint extended H{\sc ii} region. The brightest stars in this component are $\sim3^m$ fainter that those in the main body. If this system is at the same distance as the main body of IZw18, then it may be a prior episode of star formation from the intergalactic H{\sc i} clouds believed to be associated with IZw18. We present color-magnitude diagrams of the stars in both components and discuss their luminosity and mass functions.
We also present UV FOS spectra and derive an accurate carbon abundance, C/O $= 0.020 \pm 0.08$, for an H{\sc ii} region in the main body. While this C/O value is lower than in the sun or Orion Nebula, it is higher than previously found for three other very metal-poor irregulars (Garnett et al. 1995, ApJ, in press). Scenarios for the formation and chemical evolution of the IZw18 system are discussed in the light of these new observations and results. Supported in part by AURA/STScI for GO program 5434.
