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A set of software tools has been developed for the IRAF/STSDAS environment to derive the physical conditions in a low-density (nebular) gas given appropriate diagnostic emission line ratios; and line emissivities given appropriate emission line uxes, the electron temperature (T e) and density (N e). The package is based on the ve-level atom program developed by De Robertis, Dufour and Hunt (1987), but it includes diagnostics from a greater set of ions and emission lines, most particularly those in the satellite ultraviolet that are now observable. Two of the applications make use of a 3-zone nebular model to derive T e and N e simultaneously in separate zones of low-, intermediate-, and high-ionization. These applications are useful for calculating nebular densities and temperatures directly from the traditional diagnostic line ratios, either to provide some reasonable input parameters for a more complicated physical model, or to calculate ionic abundances (or other quantities) within some simplifying assumptions. Examples of the utility of these diagnostics for real nebulae are presented.

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We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space T elescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O B [0.8 at log O/H \ [4.0 to log C/O B [0.1 at log O/H \ [3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, conÐrming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemicalevolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much Ñatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important e †ect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

We have used the Hubble Space Telescope to obtain photometric data of the resolved stars in the nearby (D= 1.4 Mpc) dwarf irregular galaxy Sextans A (DDO 75, A 1008-04). The data consist of WFPC2 images in 3 bands: F439W (1 hour), F555W (30 minutes), and F814W (30 minutes). We constructed very accurate color-magnitude diagrams (CMDs) in V and I down to a limiting magnitude of 26 in V. The CMDs show several clearly separated populations that align well with stellar evolution model predictions for a low metallicity system. We use the MS stars to calculate the star formation history (SFH) over the past 100 Myr. We also present a method for extracting the SFH from a second population, the blue He-buming (HeB) stars, independent of the MS. These are stars in the bluest part of the so-called "blue-loop" phase. This is the first time these stars have been unambiguously identified in a low metallicity system. This method has the potential to determine the SFH over the past 1 Gyr, although photometric errors in the present data limit the range to 600 Myr. We have combined the spatial density distribution of the blue HeB stars with the star formation rate (SER) calculations to determine the behavior of the star formation in both space and time. In the past 50 Myr, Sextans A has had an average SER (5000± 1500 M 0 Myr-1 kpc-2 assuming a Salpeter IMF) that is-20 times that of the average SER over the history of the galaxy (-310 M 0 Myr-1 kpc-2 assuming an 11 Gyr lifetime). This current activity is highly concentrated in a young region in the Southeast roughly 25 pc across. This coincides with the brightest HII regions and the highest column density of H I. This one region contains half of all the current star formation activity within our field of view. Between the ages of 100 and 600 Myr, the star formation is roughly constant at-2000±500 M 0 Myr-1 kpc-2 , still well above the lifetime average for the galaxy. The fluctuations during this time are spatially correlated. There are regions with a factor of-5 enhancement in SFR compared to the full field of view (25%-30%